Search

As the avian embryo grows and develops within the egg, its metabolic rate gradually increases. Obligate avian brood-parasitic birds lay their eggs in the nests of other species to avoid the costs of parental care, and all but one of these brood-parasitic species are altricial at hatching. Yet the chicks of some altricial brood-parasitic species perform the physically demanding task of evicting, stabbing or otherwise killing host progeny within days of hatching. This implies a need for high metabolic rates in the embryo, just as precocial species require. Using flow-through respirometry in situ , we investigated embryonic metabolic rates in diverse avian brood parasite lineages which either kill host offspring (high virulence) or share the nest with host young (low virulence). High-virulence brood parasite embryos exhibited higher overall metabolic rates than both non-parasitic (parental) species and low-virulence parasites. This was driven by significantly elevated metabolic rates around the halfway point of incubation. Additionally, a fine-scale analysis of the embryos of a host-parasitic pair showed faster increases in metabolic rates in the parasite. Together these results suggest that the metabolic patterns of the embryos of high-virulence parasites facilitate their early-life demands.

The conditions an organism experiences during development can modify how they plastically respond to short-term changes in their environment later in life. This can be adaptive because the optimal average trait value and the optimal plastic change in trait value in response to the environment may differ across different environments. For example, early developmental temperatures can adaptively modify how reptiles, fish and invertebrates metabolically respond to temperature. However, whether individuals within populations respond differently (a prerequisite to adaptive evolution), and whether this occurs in birds, which are only ectothermic for part of their life cycle, is not known. We experimentally tested these possibilities by artificially incubating the embryos of Pekin ducks (Anas platyrhynchos domesticus) at constant or variable temperatures. We measured their consequent heart rate reaction norms to short-term changes in egg temperature and tracked their growth. Contrary to expectations, the early thermal environment did not modify heart rate reaction norms, but regardless, these reaction norms differed among individuals. Embryos with higher average heart rates were smaller upon hatching, but heart rate reaction norms did not predict subsequent growth. Our data also suggests that the thermal environment may affect both the variance in heart rate reaction norms and their covariance with growth. Thus, individual avian embryos can vary in their plasticity to temperature, and in contrast to fully ectothermic taxa, the early thermal environment does not explain this variance. Because among-individual variation is one precondition to adaptive evolution, the factors that do contribute to such variability may be important., Competing Interests: Competing interests The authors declare no competing or financial interests., (© 2024. Published by The Company of Biologists Ltd.)

Living birds (Aves) have bodies substantially modified from the ancestral reptilian condition. The avian pelvis in particular experienced major changes during the transition from early archosaurs to living birds 1,2 . This stepwise transformation is well documented by an excellent fossil record 2-4 ; however, the ontogenetic alterations that underly it are less well understood. We used embryological imaging techniques to examine the morphogenesis of avian pelvic tissues in three dimensions, allowing direct comparison with the fossil record. Many ancestral dinosaurian features 2 (for example, a forward-facing pubis, short ilium and pubic 'boot') are transiently present in the early morphogenesis of birds and arrive at their typical 'avian' form after transitioning through a prenatal developmental sequence that mirrors the phylogenetic sequence of character acquisition. We demonstrate quantitatively that avian pelvic ontogeny parallels the non-avian dinosaur-to-bird transition and provide evidence for phenotypic covariance within the pelvis that is conserved across Archosauria. The presence of ancestral states in avian embryos may stem from this conserved covariant relationship. In sum, our data provide evidence that the avian pelvis, whose early development has been little studied 5-7 , evolved through terminal addition-a mechanism 8-10 whereby new apomorphic states are added to the end of a developmental sequence, resulting in expression 8,11 of ancestral character states earlier in that sequence. The phenotypic integration we detected suggests a previously unrecognized mechanism for terminal addition and hints that retention of ancestral states in development is common during evolutionary transitions., (© 2022. The Author(s), under exclusive licence to Springer Nature Limited.)

Some evidence shows that body mass index in humans and extreme weights in animal models, including avian species, are associated with low in vitro fertilization, bad oocyte quality, and embryo development failures. Adipokines are hormones mainly produced and released by white adipose tissue. They play a key role in the regulation of energy metabolism. However, they are also involved in many other physiological processes including reproductive functions. Indeed, leptin and adiponectin, the most studied adipokines, but also novel adipokines including visfatin and chemerin, are expressed within the reproductive tract and modulate female fertility. Much of the literature has focused on the physiological and pathological roles of these adipokines in ovary, placenta, and uterine functions. The purpose of this review is to summarize the current knowledge regarding the involvement of leptin, adiponectin, visfatin, and chemerin in the oocyte maturation, fertilization, and embryo development in both mammals and birds.

The periodic color motifs such as the spots or stripes that adorn the coat of vertebrates have served as emblematic systems in empirical and theoretical studies of pattern formation, because they vary extensively between taxa but often have conserved orientation and are highly reproducible within species. Two major patterning theories have been proposed, namely instructional signaling, in which positional information is encoded as a program, and self-organization, in which position is spontaneously acquired within the developing tissue. We review here recent empirical evidence that supports both theories in vertebrates: with the advent of new molecular techniques and functional approaches, researchers nowadays take advantage of natural populations of mammals, birds and fish species, closely-related to model organisms and varying in periodic patterns. As a whole, results strongly suggest that instruction and self-organization act in combination in space and time. The orientation and reproducibility of periodic patterns relies on initial foundations provided by early developmental landmarks while their periodicity and natural variation are shaped by late-acting self-organizing processes., (Copyright © 2019 Elsevier Inc. All rights reserved.)

Public experimental embryology opens a relationship between an embryo and an amateur transgenic designer. Artists produce real-world effects by forcing hereditary aesthetics on developing bodies. This lab was meant to aid in public understanding of the relationship between transgenics and aesthetics. How do we to take an active and hands-on tactical stance on the role of hereditary designer and how does this help in public analysis of the bioethics of genetic engineering. Through naming and funeral rites, we assign the embryos an uncertain amount of clout or cultural worth. This lab is an example of how to understand the relationship between institutional oversight in pre-animal experimentation, embryonic dignity, and the problem of humane sacrifice. The intention is to make a hands-on wet bioart lab meant to aid in public comprehension of the range of politics and responsibilities involved in play at the level of heredity. The Developmental Biology and Transgenic Avian Embryology Bioart Wet Lab was held in Gorlaeus Laboratories, LIC, University of Leiden, Leiden, Netherlands, 2007.

Cellular senescence was traditionally considered a stress response that protected the organism by limiting the proliferation of damaged and unwanted cells. However, the recent identification of developmentally-programmed cellular senescence during embryo development has changed our view of the process. There are now a number of examples of developmental senescence in evolutionary distant organisms ranging from mammals to fish, showing senescence at various sites during specific time windows of development. Developmental senescence shares many features with stress-induced senescence but also present some specific characteristics. The different examples of developmental senescence provide evidence of the diverse functions contributed by senescence and represent an opportunity to learn more about this process. Also, the existence of senescence during embryogenesis opens the possibility of identifying human developmental syndromes caused by alterations in this response. Studying in more detail this process will expand our understanding of cellular senescence and could offer new insights into the cause of human pathologies., (Copyright © 2019 Elsevier Inc. All rights reserved.)

Natural nests of egg-laying birds and reptiles exhibit substantial thermal variation, at a range of spatial and temporal scales. Rates and trajectories of embryonic development are highly sensitive to temperature, favouring an ability of embryos to respond adaptively (i.e. match their developmental biology to local thermal regimes). Spatially, thermal variation can be significant within a single nest (top to bottom), among adjacent nests (as a function of shading, nest depth etc.), across populations that inhabit areas with different weather conditions, and across species that differ in climates occupied and/or nest characteristics. Thermal regimes also vary temporally, in ways that generate differences among nests within a single population (e.g. due to seasonal timing of laying), among populations and across species. Anthropogenic activities (e.g. habitat clearing, climate change) add to this spatial and temporal diversity in thermal regimes. We review published literature on embryonic adaptations to spatio-temporal heterogeneity in nest temperatures. Although relatively few taxa have been studied in detail, and proximate mechanisms remain unclear, our review identifies many cases in which natural selection appears to have fine-tuned embryogenesis to match local thermal regimes. Developmental rates have been reported to differ between uppermost versus lower eggs within a single nest, between eggs laid early versus late in the season, and between populations from cooler versus warmer climates. We identify gaps in our understanding of thermal adaptations of early (embryonic) phases of the life history, and suggest fruitful opportunities for future research.

For many, if not all, air-breathing vertebrates, breathing-like movements begin while the embryo is still ensconced in an aqueous environment. This is because primordial regions of the CNS become spontaneously active during early gestation and then must functionally transform and specialize once air breathing commences. The degree to which the embryonic ventilatory control system is established and competent at birth is variable, however, even between different components of the respiratory system. Moreover, the embryological experiences of an individual can also affect the outcomes and responsiveness of ventilation to respiratory stimuli and these details have major clinical implications. The broad field of respiratory neurobiology still has much to learn about the ontogeny of breathing control systems, and the oviparity of birds provides a unique model to examine how early rhythms transform day-to-day as they become functional. This hybrid review and research article will highlight the contributions of birds to the study of breathing control during early development. We will detail what is currently known about the onset and maturation of respiratory rhythm generation and also provide novel data about the development of central chemosensitivity. Finally, we will review data regarding the development of peripheral afferent inputs during early development and discuss whole-animal reflex responsiveness to common respiratory stimuli, both chronic and acute, during the incubation period and following hatching., (Copyright © 2019. Published by Elsevier B.V.)

Somites are epithelial segments of the paraxial mesoderm. Shortly after their formation, the epithelial somites undergo extensive cellular rearrangements and form specific somite compartments, including the sclerotome and the myotome, which give rise to the axial skeleton and to striated musculature, respectively. The dynamics of somite development varies along the body axis, but most research has focused on somite development at thoracolumbar levels. The development of tail somites has not yet been thoroughly characterized, even though vertebrate tail development has been intensely studied recently with respect to the termination of segmentation and the limitation of body length in evolution. Here, we provide a detailed description of the somites in the avian tail from the beginning of tail formation at HH-stage 20 to the onset of degeneration of tail segments at HH-stage 27. We characterize the formation of somite compartment formation in the tail region with respect to morphology and the expression patterns of the sclerotomal marker gene paired-box gene 1 (Pax1) and the myotomal marker genes MyoD and myogenic factor 5 (Myf5). Our study gives insight into the development of the very last segments formed in the avian embryo, and provides a basis for further research on the development of tail somite derivatives such as tail vertebrae, pygostyle and tail musculature., (© 2019 Anatomical Society.)

Early embryonic cells are capable of acquiring numerous developmental fates until they become irreversibly committed to specific lineages depending on intrinsic determinants and/or regional interactions. From fertilization to gastrulation, such pluripotent cells first increase in number and then turn to undergoing differentiation. Mechanisms regulating pluripotency in each species attract great interest in developmental biology. Also, outlining the evolutionary background of pluripotency can enhance our understanding of mammalian pluripotency and provide a broader view of early development of vertebrates. Here, we introduce integrative models of pluripotent states in amniotes (mammals, birds and reptiles) to offer a comprehensive overview of widely accepted knowledge about mammalian pluripotency and our recent findings in non-mammalian amniotes, such as chicken and gecko. In particular, we describe 1) the IL6/Stat3 signaling pathway as a positive regulator of naive pluripotency, 2) Fgf/Erk signaling as a process that prepares cells for differentiation, 3) the role of the interactions between these two signaling pathways during the transition from pluripotency to differentiation, and 4) functional diversification of two transcription factors, Class V POUs and Nanog. In the last section, we also briefly discuss possible relationships of unique cell cycle properties of early embryonic cells with signaling pathways and developmental potentials in the pluripotent cell states., (Copyright © 2019 Elsevier Inc. All rights reserved.)

The crested ibis (Nipponia nippon) is endangered worldwide. Although a series of conservation measures have markedly increased the population size and distribution area of these birds, the high mortality of embryos and nestlings considerably decreases the survival potential of this bird species. High-throughput sequencing technology was utilized to compare whole genomes between ten samples from dead crested ibises (including six dead embryos and four dead nestlings aged 0-45 days) and 32 samples from living birds. The results indicated that the dead samples all shared the genetic background of a specific ancestral subpopulation. Furthermore, the dead individuals were less genetically diverse and suffered higher degrees of inbreeding compared with these measures in live birds. Several candidate genes (KLHL3, SETDB2, TNNT2, PKP1, AK1, and EXOSC3) associated with detrimental diseases were identified in the genomic regions that differed between the alive and dead samples, which are likely responsible for the death of embryos and nestlings. In addition, in these regions, we also found several genes involved in the protein catabolic process (UBE4A and LONP1), lipid metabolism (ACOT1), glycan biosynthesis and metabolism (HYAL1 and HYAL4), and the immune system (JAM2) that are likely to promote the normal development of embryos and nestlings. The aberrant conditions of these genes and biological processes may contribute to the death of embryos and nestlings. Our data identify congenital factors underlying the death of embryos and nestlings at the whole genome level, which may be useful toward informing more effective conservation efforts for this bird species., (© The Author(s) 2019. Published by Oxford University Press on behalf of the Society for Molecular Biology and Evolution.)

Variation in digit number has occurred multiple times in the history of archosaur evolution. The five digits of dinosaur limbs were reduced to three in bird forelimbs, and were further reduced in the vestigial forelimbs of the emu. Regulation of digit number has been investigated previously by examining genes involved in anterior-posterior patterning in forelimb buds among emu (Dromaius novaehollandiae), chicken (Gallus gallus) and zebra finch (Taeniopygia guttata). It was described that the expression of posterior genes are conserved among these three birds, whereas expression of anterior genes Gli3 and Alx4 varied significantly. Here we re-examined the expression pattern of Gli3 and Alx4 in the forelimb of emu, chicken and zebra finch. We found that Gli3 is expressed in the anterior region, although its range varied among species, and that the expression pattern of Alx4 in forelimb buds is broadly conserved in a stage-specific manner. We also found that the dynamic expression pattern of the BMP antagonist Gremlin1 (Grem1) in limb buds, which is critical for autopodial expansion, was consistent with the digital pattern of emu, chicken and zebra finch. Furthermore, in emu, variation among individuals was observed in the width of Grem1 expression in forelimb buds, as well as in the adult skeletal pattern. Our results support the view that the signalling system that regulates the dynamic expression of Grem1 in the limb bud contributes substantially to variations in avian digital patterns.

DDT remains in use for malaria control in South Africa. We quantified DDTs in aquatic bird eggs from the highly biodiverse northern KwaZulu-Natal, a province of South Africa where DDT has been used for more than 80 years for malaria control. Pelican eggs had the highest ΣDDT concentration (7200 ng/g lipid mass; lm), Little Egret eggs had 6900 ΣDDT lm, African Openbill eggs had 3400 ng/g lm ΣDDT, and White-breasted Cormorant had 2400 ng/g lm. All species had non-significantly different mean concentrations of o,p'-DDT, p,p'-DDT, and ΣDDT, but with significant differences for p,p-DDE, o,p'-DDD, p,p'-DDD, %DDT, %DDD, and %lipid. The thinnest pelican eggshell (0.40 mm) had a ΣDDT concentration of 3300 ng/g lm.; the thickest shell (0.96 mm) had the lowest ΣDDT concentration at 29 ng/g lm; a 58% difference. Linear regressions of concentrations with shell thickness for the pelican eggs were significant for p,p'-DDE and p,p'-DDD, indicating risk of reproductive impairment. Compositional profiles indicate different food webs for the different species. DDT concentrations were lower than from another DDT-sprayed locality in South Africa, possible linked to differences in hydrology and rainfall. We conclude that significant ecotoxic threats associated with DDT remain in this area, and possibly threatens birds from less polluted areas. Our findings suggest continued negative human health and environmental impacts from DDT. There is an urgency to move away from DDT as quickly as possible; alternatively, to implement practices that prevent emissions of DDT to the environment while protecting human life., (Copyright © 2019 Elsevier Ltd. All rights reserved.)

The evolution of flight in birds involves (i) decoupling of the primitive mode of quadrupedal locomotor coordination, with a new synchronized flapping motion of the wings while conserving alternating leg movements, and (ii) reduction of wing digits and loss of functional claws. Our observations show that hoatzin nestlings move with alternated walking coordination of the four limbs using the mobile claws on their wings to anchor themselves to the substrate. When swimming, hoatzin nestlings use a coordinated motion of the four limbs involving synchronous or alternated movements of the wings, indicating a versatile motor pattern. Last, the proportions of claws and phalanges in juvenile hoatzin are radically divergent from those in adults, yet strikingly similar to those of Archaeopteryx. The locomotor plasticity observed in the hoatzin suggests that transitional forms that retained claws on the wings could have also used them for locomotion.

The embryonic metabolism of the saurischian dinosaur Troodon formosus and the ornithischian dinosaurs Protoceratops andrewsi and Hypacrosaurus stebingeri have been determined by using a mass growth model based on conservation of energy and found to be very similar. Embryonic and ontogenetic growth metabolisms are also evaluated for extant altricial birds, precocial birds, mammals, and crocodylians to examine for trends in the different groups of animals and to provide a context for interpreting our results for nonavian dinosaurs. This analysis reveals that the embryonic metabolisms of these nonavian dinosaurs were closer to the range observed in extant crocodylians than extant birds. The embryonic metabolisms of nonavian dinosaurs were in the range observed for extant mammals of similar masses. The measured embryonic metabolic rates for these three nonavian dinosaurs are then used to calculate the incubation times for eggs of 22 nonavian dinosaurs from both Saurischia and Ornithischia. The calculated incubation times vary from about 50 days for Archaeopteryx lithographica to about 150 days for Alamosaurus sanjuanensis.

Maternal effects can adaptively modulate offspring developmental trajectories in variable but predictable environments. Hormone synthesis is sensitive to environmental factors, and maternal hormones are thus a powerful mechanism to transfer environmental cues to the next generation. Birds have become a key model for the study of hormone-mediated maternal effects because the embryo develops outside the mother's body, facilitating the measurement and manipulation of prenatal hormone exposure. At the same time, birds are excellent models for the integration of both proximate and ultimate approaches, which is key to a better understanding of the evolution of hormone-mediated maternal effects. Over the past two decades, a surge of studies on hormone-mediated maternal effects has revealed an increasing number of discrepancies. In this review, we discuss the role of the environment, genetic factors and social interactions in causing these discrepancies and provide a framework to resolve them. We also explore the largely neglected role of the embryo in modulating the maternal signal, as well as costs and benefits of hormone transfer and expression for the different family members. We conclude by highlighting fruitful avenues for future research that have opened up thanks to new theoretical insights and technical advances in the field. This article is part of the theme issue 'Developing differences: early-life effects and evolutionary medicine'.

Feathers are arranged in a precise pattern in avian skin. They first arise during development in a row along the dorsal midline, with rows of new feather buds added sequentially in a spreading wave. We show that the patterning of feathers relies on coupled fibroblast growth factor (FGF) and bone morphogenetic protein (BMP) signalling together with mesenchymal cell movement, acting in a coordinated reaction-diffusion-taxis system. This periodic patterning system is partly mechanochemical, with mechanical-chemical integration occurring through a positive feedback loop centred on FGF20, which induces cell aggregation, mechanically compressing the epidermis to rapidly intensify FGF20 expression. The travelling wave of feather formation is imposed by expanding expression of Ectodysplasin A (EDA), which initiates the expression of FGF20. The EDA wave spreads across a mesenchymal cell density gradient, triggering pattern formation by lowering the threshold of mesenchymal cells required to begin to form a feather bud. These waves, and the precise arrangement of feather primordia, are lost in the flightless emu and ostrich, though via different developmental routes. The ostrich retains the tract arrangement characteristic of birds in general but lays down feather primordia without a wave, akin to the process of hair follicle formation in mammalian embryos. The embryonic emu skin lacks sufficient cells to enact feather formation, causing failure of tract formation, and instead the entire skin gains feather primordia through a later process. This work shows that a reaction-diffusion-taxis system, integrated with mechanical processes, generates the feather array. In flighted birds, the key role of the EDA/Ectodysplasin A receptor (EDAR) pathway in vertebrate skin patterning has been recast to activate this process in a quasi-1-dimensional manner, imposing highly ordered pattern formation., Competing Interests: The authors have declared that no competing interests exist.

Interpreting the relative impact of cell autonomous patterning versus extrinsic microenvironmental influence on cell lineage determination represents a general challenge in developmental biology research. In the embryonic heart, this can be particularly difficult as regional differences in the expression of transcriptional regulators, paracrine/juxtacrine signaling cues, and hemodynamic force are all known to influence cardiomyocyte maturation. A simplified method to alter a developing cardiomyocyte's molecular and biomechanical microenvironment would, therefore, serve as a powerful technique to examine how local conditions influence cell fate and function. To address this, we have optimized a method to physically transplant juvenile cardiomyocytes into ectopic locations in the heart or the surrounding embryonic tissue. This allows us to examine how microenvironmental conditions influence cardiomyocyte fate transitions at single cell resolution within the intact embryo. Here, we describe a protocol in which embryonic myocytes can be isolated from a variety of cardiac sub-domains, dissociated, fluorescently labeled, and microinjected into host embryos with high precision. Cells can then be directly analyzed in situ using a variety of imaging and histological techniques. This protocol is a powerful alternative to traditional grafting experiments that can be prohibitively difficult in a moving tissue such as the heart. The general outline of this method can also be adapted to a variety of donor tissues and host environments, and its ease of use, low cost, and speed make it a potentially useful application for a variety of developmental studies.

During development, the vertebrate embryo undergoes significant morphological changes which lead to its future body form and functioning organs. One of these noticeable changes is the extension of the body shape along the antero-posterior (A-P) axis. This A-P extension, while taking place in multiple embryonic tissues of the vertebrate body, involves the same basic cellular behaviors: cell proliferation, cell migration (of new progenitors from a posterior stem zone), and cell rearrangements. However, the nature and the relative contribution of these different cellular behaviors to A-P extension appear to vary depending upon the tissue in which they take place and on the stage of embryonic development. By focusing on what is known in the neural and mesodermal tissues of the bird embryo, I review the influences of cellular behaviors in posterior tissue extension. In this context, I discuss how changes in distinct cell behaviors can be coordinated at the tissue level (and between tissues) to synergize, build, and elongate the posterior part of the embryonic body. This multi-tissue framework does not only concern axis elongation, as it could also be generalized to morphogenesis of any developing organs.

The increased correlation between maternal cadmium (Cd) exposure and restricted fetal growth has become a matter of global concern. Because dietary nitrate has been demonstrated to offer a range of beneficial vascular effects, we attempt to unveil the effect of one such nitrate-rich food, beetroot, on recovering Cd-induced fetal growth retardation. Using chick embryos as a model, our results confirm that retarded growth is a result of exposure to Cd at an early stage of development and that Cd hinders vasculogenesis and angiogenesis. We find that beetroot juice (BRJ) recovers fetal growth retardation effects of Cd via its vasodilatory effect and promotes embryonic angiogenesis. In conclusion, this study confirms that BRJ reduces the rate of congenital malformations with Cd exposure in utero and suggests the importance of dietary supplements for pregnant women.

Radical transformation of the skull characterizes bird evolution. An increase in the relative size of the brain and eyes was presumably related to the loss of two bones surrounding the eye, the prefrontal and postorbital. We report that ossification centres of the prefrontal and postorbital are still formed in bird embryos, which then fuse seamlessly to the developing nasal and frontal bones, respectively, becoming undetectable in the adult. The presence of a dinosaur-like ossification pattern in bird embryos is more than a trace of their evolutionary past: we show how persistent modularity of ossification centres has allowed for evolutionary re-organization of skull architecture in evolution. Our findings also demonstrate that enigmatic mesodermal cells forming the posterior region of the avian frontal correspond to the ossification centre of the postorbital, not the parietal, and link its failure to develop into an adult bone to its incorporation into the expanded braincase of birds.

The study of oogenesis and early development of frogs belonging to the family Hemiphractidae provide important comparison to the aquatic development of other frogs, such as Xenopus laevis, because reproduction on land characterizes the Hemiphractidae. In this review, the multinucleated oogenesis of the marsupial frog Flectonotus pygmaeus (Hemiphractidae) is analyzed and interpreted. In addition, the adaptations associated with the incubation of embryos in the pouch of the female marsupial frog Gastrotheca riobambae (Hemiphractidae) and the embryonic development of this frog are summarized. Moreover, G. riobambae gastrulation is compared with the gastrulation modes of Engystomops randi and Engystomops coloradorum (Leptodactylidae); Ceratophrys stolzmanni (Ceratophryidae); Hyalinobatrachium fleischmanni and Espadarana callistomma (Centrolenidae); Ameerega bilinguis, Dendrobates auratus, Epipedobates anthonyi, Epipedobates machalilla, Epipedobates tricolor, and Hyloxalus vertebralis (Dendrobatidae); Eleutherodactylus coqui (Terrarana: Eleutherodactylidae), and X. laevis (Pipidae). The comparison indicated two modes of frog gastrulation. In X. laevis and in frogs with aquatic reproduction, convergent extension begins during gastrulation. In contrast, convergent extension occurs in the post-gastrula of frogs with terrestrial reproduction. These two modes of gastrulation resemble the transitions toward meroblastic cleavage found in ray-finned fishes (Actinopterygii). In spite of this difference, the genes that guide early development seem to be highly conserved in frogs. I conclude that the shift of convergent extension to the post-gastrula accompanied the diversification of frog egg size and terrestrial reproductive modes., (Copyright © 2018 Elsevier B.V. All rights reserved.)

A list of papers and books of the late Lev V. Beloussov was compiled and is available in Word and EndNote Supplements. The breadth of his work is briefly described., (Copyright © 2018 Elsevier B.V. All rights reserved.)

Parthenogenesis or 'virgin birth' is embryonic development in unfertilized eggs. It is a routine means of reproduction in many invertebrates. However, even though parthenogenesis occurs naturally in even more advanced vertebrates, like birds, it is mostly abortive in nature. In fact, multiple limiting factors, such as delayed and unorganized development as well as unfavorable conditions developing within the unfertilized egg upon incubation, are associated with termination of progressive development of parthenogenetic embryos. In birds, diploid parthenogenesis is automictic and facultative producing only males. However, the mechanisms controlling parthenogenesis in birds are not clearly elucidated. Additionally, it appears from even very recent research that these mechanisms may hinder the normal fertilization process and subsequent embryonic development. For instance, virgin quail and turkey hens exhibiting parthenogenesis have reduced reproductive performance following mating. Also, genetic selection and environmental factors, such as live virus vaccinations, are known to trigger the process of parthenogenesis in birds. Therefore, parthenogenesis has a plausible negative impact on the poultry industry. Hence, a better understanding of parthenogenesis and the mechanisms that control it could benefit commercial poultry production. In this context, the aim of this review is to provide a complete overview of the process of parthenogenesis in birds., (© 2018 Society for Reproduction and Fertility.)

The toothless beak of modern birds was considered as an adaption for feeding ecology; however, several recent studies suggested that developmental factors are also responsible for the toothless beak. Neontological and palaeontological studies have progressively uncovered how birds evolved toothless beaks and suggested that the multiple occurrences of complete edentulism in non-avian dinosaurs were the result of selection for specialized diets. Although developmental biology and ecological factors are not mutually exclusive, the conventional hypothesis that ecological factors account for the toothless beak appears insufficient. A recent study on dinosaur incubation period using embryonic teeth posited that tooth formation rate limits developmental speed, constraining toothed dinosaur incubation to slow reptilian rates. We suggest that selection for tooth loss was a side effect of selection for fast embryo growth and thus shorter incubation. This observation would also explain the multiple occurrences of tooth loss and beaks in non-avian dinosaur taxa crownward of Tyrannosaurus Whereas our hypothesis is an observation without any experimental supports, more studies of gene regulation of tooth formation in embryos would allow testing for the trade-off between incubation period and tooth development., (© 2018 The Author(s).)

Organizers, which comprise groups of cells with the ability to instruct adjacent cells into specific states, represent a key principle in developmental biology. The concept was first introduced by Spemann and Mangold, who showed that there is a cellular population in the newt embryo that elicits the development of a secondary axis from adjacent cells. Similar experiments in chicken and rabbit embryos subsequently revealed groups of cells with similar instructive potential. In birds and mammals, organizer activity is often associated with a structure known as the node, which has thus been considered a functional homologue of Spemann's organizer. Here, we take an in-depth look at the structure and function of organizers across species and note that, whereas the amphibian organizer is a contingent collection of elements, each performing a specific function, the elements of organizers in other species are dispersed in time and space. This observation urges us to reconsider the universality and meaning of the organizer concept., Competing Interests: Competing interestsThe authors declare no competing or financial interests., (© 2018. Published by The Company of Biologists Ltd.)

The orderly formation of the avian feather array is a classic example of periodic pattern formation during embryonic development. Various mathematical models have been developed to describe this process, including Turing/activator-inhibitor type reaction-diffusion systems and chemotaxis/mechanical-based models based on cell movement and tissue interactions. In this paper we formulate a mathematical model founded on experimental findings, a set of interactions between the key cellular (dermal and epidermal cell populations) and molecular (fibroblast growth factor, FGF, and bone morphogenetic protein, BMP) players and a medially progressing priming wave that acts as the trigger to initiate patterning. Linear stability analysis is used to show that FGF-mediated chemotaxis of dermal cells is the crucial driver of pattern formation, while perturbations in the form of ubiquitous high BMP expression suppress patterning, consistent with experiments. Numerical simulations demonstrate the capacity of the model to pattern the skin in a spatial-temporal manner analogous to avian feather development. Further, experimental perturbations in the form of bead-displacement experiments are recapitulated and predictions are proposed in the form of blocking mesenchymal cell proliferation., (Copyright © 2017 Elsevier Ltd. All rights reserved.)

Obligate brood parasites have evolved unusually thick-shelled eggs, which are hypothesized to possess a variety of functions such as resistance to puncture ejection by their hosts. In this study, we tested the hypothesis that obligate brood parasites lay unusually thick-shelled eggs to retain more heat for the developing embryo and thus contribute to early hatching of parasite eggs. By doing so, we used an infrared thermal imaging system as a non-invasive method to quantify the temperature of eggshells of common cuckoos (Cuculus canorus) and their Oriental reed warbler (Acrocephalus orientalis) hosts in an experiment that artificially altered the duration of incubation. Our results showed that cuckoo eggshells had higher temperature than host eggs during incubation, but also less fluctuations in temperature during incubation disturbance. Therefore, there was a thermal and hence a developmental advantage for brood parasitic cuckoos of laying thick-shelled eggs, providing another possible explanation for the unusually thick-shelled eggs of obligate brood parasites and earlier hatching of cuckoo eggs compared to those of the host.

Pluripotency defines the ability of a cell to self-renew and to differentiate into all embryonic lineages both in vitro and in vivo. This definition was first established mainly with the mouse model and the establishment of mouse embryonic stem cells (ESCs) in the 1980's and extended later on to other species including non-human primates and humans. Similarly, chicken ESCs were derived and established in vitro from pregastrulating embryos leading to cells with unique properties at molecular, epigenetic and developmental levels. By comparing the properties of murine, mammalian and avian ESCs and of the more recently discovered induced pluripotential stem (iPS)-derived cells generated in all of these species, avian specificities start to emerge including specific molecular genes, epigenetic mark profiles and original developmental properties. Here, we present common, but also avian-specific elements that contribute to defining avian pluripotency.

Reconciling the origins of morphological diversity with the deep homology of underlying mechanisms is a question fundamental to the goals of evolutionary developmental biology ("evo-devo" or EDB). In this paper I argue that differing research agendas in evolutionary and developmental biology have hindered how we address this question, but that the limb provides ideal "common ground" for their fuller integration. To support this idea, I review two previous analyses of limb variation in mammal, bird, and reptile taxa that offer complementary approaches to explaining diversity. Specifically, I present evidence suggesting that: (1) a shared genetic architecture affects the pattern of between limb developmental integration, while their functional dissociation is linked to both increased phenotypic evolvability and diversity of interlimb proportions, and (2) within limb proportional diversity is biased such that proximal and distal segments function as tradeoffs while the middle segment is more conservative, a signal that is both evident from early in morphogenesis and suggestive of an "inhibitory cascade" model of limb proximo-distal axis development. In the first case, shared genetic mechanisms predict both observed developmental integration between limbs and patterns of clade-specific diversity. In the second case, underappreciated patterns of phenotypic diversity suggest novel insights into the underlying developmental mechanisms by which variation is generated. These studies show how insights from both evolutionary and developmental biology of the limb may be used to generate novel testable hypotheses into the origins of diversity that are broadly applicable to the integration of EDB., (© The Author 2017. Published by Oxford University Press on behalf of the Society for Integrative and Comparative Biology. All rights reserved. For permissions please email: journals.permissions@oup.com.)

Embryonic muscular activity (EMA) is involved in the development of several distinctive traits of birds. Modern avian diversity and the fossil record of the dinosaur-bird transition allow special insight into their evolution. Traits shaped by EMA result from mechanical forces acting at post-morphogenetic stages, such that genes often play a very indirect role. Their origin seldom suggests direct selection for the trait, but a side-effect of other changes such as musculo-skeletal rearrangements, heterochrony in skeletal maturation, or increased incubation temperature (which increases EMA). EMA-shaped traits like sesamoids may be inconstant, highly conserved, or even disappear and then reappear in evolution. Some sesamoids may become increasingly influenced in evolution by genetic-molecular mechanisms (genetic assimilation). There is also ample evidence of evolutionary transitions from sesamoids to bony eminences at tendon insertion sites, and vice-versa. This can be explained by newfound similarities in the earliest development of both kinds of structures, which suggest these transitions are likely triggered by EMA. Other traits that require EMA for their formation will not necessarily undergo genetic assimilation, but still be conserved over tens and hundreds of millions of years, allowing evolutionary reduction and loss of other skeletal elements. Upon their origin, EMA-shaped traits may not be directly genetic, nor immediately adaptive. Nevertheless, EMA can play a key role in evolutionary innovation, and have consequences for the subsequent direction of evolutionary change. Its role may be more important and ubiquitous than currently suspected., (© The Author 2017. Published by Oxford University Press on behalf of the Society for Integrative and Comparative Biology. All rights reserved. For permissions please email: journals.permissions@oup.com.)

The cerebral cortex covers the rostral part of the brain and, in higher mammals and particularly humans, plays a key role in cognition and consciousness. It is populated with neuronal cell bodies distributed in radially organized layers. Understanding the common and lineage-specific molecular mechanisms that orchestrate cortical development and evolution are key issues in neurobiology. During evolution, the cortex appeared in stem amniotes and evolved divergently in two main branches of the phylogenetic tree: the synapsids (which led to present day mammals) and the diapsids (reptiles and birds). Comparative studies in organisms that belong to those two branches have identified some common principles of cortical development and organization that are possibly inherited from stem amniotes and regulated by similar molecular mechanisms. These comparisons have also highlighted certain essential features of mammalian cortices that are absent or different in diapsids and that probably evolved after the synapsid-diapsid divergence. Chief among these is the size and multi-laminar organization of the mammalian cortex, and the propensity to increase its area by folding. Here, I review recent data on cortical neurogenesis, neuronal migration and cortical layer formation and folding in this evolutionary perspective, and highlight important unanswered questions for future investigation., Competing Interests: Competing interestsThe author declares no competing or financial interests., (© 2017. Published by The Company of Biologists Ltd.)

Variation in regional identity, patterning, and structure of epidermal appendages contributes to skin diversity among many vertebrate groups, and is perhaps most striking in birds. In pioneering work on epidermal appendage patterning, John Saunders and his contemporaries took advantage of epidermal appendage diversity within and among domestic chicken breeds to establish the importance of mesoderm-ectoderm signaling in determining skin patterning. Diversity in chickens and other domestic birds, including pigeons, is driving a new wave of research to dissect the molecular genetic basis of epidermal appendage patterning. Domestic birds are not only outstanding models for embryonic manipulations, as Saunders recognized, but they are also ideal genetic models for discovering the specific genes that control normal development and the mutations that contribute to skin diversity. Here, we review recent genetic and genomic approaches to uncover the basis of epidermal macropatterning, micropatterning, and structural variation. We also present new results that confirm expression changes in two limb identity genes in feather-footed pigeons, a case of variation in appendage structure and identity., (Copyright © 2017 The Authors. Published by Elsevier Inc. All rights reserved.)

The exact genetic mechanism regulating avian gonadal sex differentiation has not been completely resolved. The most likely scenario involves a dosage mechanism, whereby the Z-linked DMRT1 gene triggers testis development. However, the possibility still exists that the female-specific W chromosome may harbor an ovarian determining factor. In this study, we provide evidence that the universal gene regulating gonadal sex differentiation in birds is Z-linked DMRT1 and not a W-linked (ovarian) factor. Three candidate W-linked ovarian determinants are HINTW, female-expressed transcript 1 (FET1), and female-associated factor (FAF). To test the association of these genes with ovarian differentiation in the chicken, we examined their expression following experimentally induced female-to-male sex reversal using the aromatase inhibitor fadrozole (FAD). Administration of FAD on day 3 of embryogenesis induced a significant loss of aromatase enzyme activity in female gonads and masculinization. However, expression levels of HINTW, FAF, and FET1 were unaltered after experimental masculinization. Furthermore, comparative analysis showed that FAF and FET1 expression could not be detected in zebra finch gonads. Additionally, an antibody raised against the predicted HINTW protein failed to detect it endogenously. These data do not support a universal role for these genes or for the W sex chromosome in ovarian development in birds. We found that DMRT1 (but not the recently identified Z-linked HEMGN gene) is male upregulated in embryonic zebra finch and emu gonads, as in the chicken. As chicken, zebra finch, and emu exemplify the major evolutionary clades of birds, we propose that Z-linked DMRT1, and not the W sex chromosome, regulates gonadal sex differentiation in birds., (Copyright © 2017 Endocrine Society.)

An avian uropygial gland is located on the mid-dorsum of the tail, and is the only external gland found in birds. Most studies have focused on the function, gross anatomy and chemical nature of this gland, with little research on its ontogeny. The purpose of this study was to examine the development of this gland in a series of Laysan Albatross (Phoebastria immutabilis) embryos. Specimens were examined anatomically and histologically. It was found that grooves preceded glandular development by many stages. The embryogenesis of the uropygial gland was divided into 6 phases: preinception, groove inception, mesodermal separation, migrating mesodermal cells, oval shaped "depressions", constriction and finally glandular inception. No other gland is known to develop similarly, though there may be parallels with femoral gland development. In comparison to other bird species, the length of the development period in the Albatross, as well as other compounding factors, make it difficult to determine the significance of these observations. The development of a mesodermal band, soon to be a connective tissue capsule, is more complex than originally described in ducks. Thus, the unique nature of this gland is established, but the significance of the observations required further studies into uropygial gland development. Anat Rec, 300:1420-1428, 2017. © 2017 Wiley Periodicals, Inc., (© 2017 Wiley Periodicals, Inc.)

A central question in the evolution of brain development is whether species differ in rates of brain growth during fetal neurogenesis. Studies of neonatal data have found allometric evidence for brain growth rate differences according to physiological variables such as relative metabolism and placental invasiveness, but these findings have not been tested against fetal data directly. Here, we measure rates of exponential brain growth acceleration in 10 eutherian mammals, two marsupials, and two birds. Eutherian brain acceleration exhibits minimal variation relative to body and visceral organ growth, varies independently of correlated growth patterns in other organs, and is unrelated to proposed physiological constraints such as metabolic rate or placental invasiveness. Brain growth rates in two birds overlap with eutherian variation, while marsupial brain growth is exceptionally slow. Peak brain growth velocity is linked in time with forebrain myelination and eye opening, reliably separates altricial species born before it from precocial species born afterwards, and is an excellent predictor of adult brain size ( r 2 = 0.98). Species with faster body growth exhibit larger relative brain size in early ontogeny, while brain growth is unrelated to allometric measures. These findings indicate a surprising conservation of brain growth rates during fetal neurogenesis in eutherian mammals, clarify sources of variation in neonatal brain size, and suggest that slow body growth rates cause species to be more encephalized at birth., (© 2017 The Author(s).)

Birds are an extensively specialized animal group with unique anatomical, physiological and ecological characteristics. Sophisticated social behaviors and remarkable cognitive abilities are present in several avian lineages, driven by their enlarged brains and intricate neural networks. These unique traits could be a result of adaptive evolution under the wide range of environmental constraints; however, the intrinsic mechanisms of avian brain development and evolution remain unclear. Here, we introduce recent findings regarding developmental aspects of avian brain organization and neuronal networks for specific avian behaviors, which provide an insight into the link between the evolution of brain development and complex cognitive functions., (© 2017 Japanese Society of Developmental Biologists.)

The embryonic metabolisms of the ornithischian dinosaurs Protoceratops andrewsi and Hypacrosaurus stebingeri have been determined and are in the range observed in extant reptiles. The average value of the measured embryonic metabolic rates for P. andrewsi and H. stebingeri are then used to calculate the incubation times for 21 dinosaurs from both Sauischia and Ornithischia using a mass growth model based on conservation of energy. The calculated incubation times vary from about 70 days for Archaeopteryx lithographica to about 180 days for Alamosaurus sanjuanensis. Such long incubation times seem unlikely, particularly for the sauropods and large theropods. Incubation times are also predicted with the assumption that the saurischian dinosaurs had embryonic metabolisms in the range observed in extant birds.

One of the most amazing transitions and innovations during the evolution of mammals was the formation of a novel jaw joint and the incorporation of the original jaw joint into the middle ear to create the unique mammalian three bone/ossicle ear. In this review, we look at the key steps that led to this change and other unusual features of the middle ear and how developmental biology has been providing an understanding of the mechanisms involved. This starts with an overview of the tympanic (air-filled) middle ear, and how the ear drum (tympanic membrane) and the cavity itself form during development in amniotes. This is followed by an investigation of how the ear is connected to the pharynx and the relationship of the ear to the bony bulla in which it sits. Finally, the novel mammalian jaw joint and versatile dentary bone will be discussed with respect to evolution of the mammalian middle ear.This article is part of the themed issue 'Evo-devo in the genomics era, and the origins of morphological diversity'., (© 2016 The Authors.)

Birds stand out from other egg-laying amniotes by producing relatively small numbers of large eggs with very short incubation periods (average 11-85 d). This aspect promotes high survivorship by limiting exposure to predation and environmental perturbation, allows for larger more fit young, and facilitates rapid attainment of adult size. Birds are living dinosaurs; their rapid development has been considered to reflect the primitive dinosaurian condition. Here, nonavian dinosaurian incubation periods in both small and large ornithischian taxa are empirically determined through growth-line counts in embryonic teeth. Our results show unexpectedly slow incubation (2.8 and 5.8 mo) like those of outgroup reptiles. Developmental and physiological constraints would have rendered tooth formation and incubation inherently slow in other dinosaur lineages and basal birds. The capacity to determine incubation periods in extinct egg-laying amniotes has implications for dinosaurian embryology, life history strategies, and survivorship across the Cretaceous-Paleogene mass extinction event., Competing Interests: The authors declare no conflict of interest.

Background: Syngnathia is a congenital craniofacial disorder characterized by bony or soft tissue fusion of upper and lower jaws. Previous studies suggested some causative signals, such as Foxc1 or Bmp4, cause the disruption of maxillomandibular identity, but their location and the interactive signals involved remain unexplored. We wanted to examine the embryonic origin of syngnathia based on the assumption that it may be located at the separation between the maxillary and mandibular processes. This region, known as the maxillomandibular junction (MMJ), is involved in segregation of cranial neural crest-derived mesenchyme into the presumptive upper and lower jaws., Results: Here we investigated the role of Fgf, Bmp, and retinoid signaling during development of MMJ in chicken embryos. By changing the levels of these signals with bead implants, we induced syngnathia with microstomia on the treated side, which showed increased Barx1 and neural cell adhesion molecule (NCAM) expression. Redistribution of proliferating cells was also observed at the proximal region to maxillary and mandibular arch around MMJ., Conclusions: We propose that interactive molecular signaling by Fgfs, Bmps, and retinoids around MMJ is required for normal separation of the maxilla and mandible, as well as the proper positioning of beak commissure during early facial morphogenesis. Developmental Dynamics 246:28-40, 2017. © 2016 Wiley Periodicals, Inc., (© 2016 Wiley Periodicals, Inc.)

A combination of both in vivo and in vitro techniques is invaluable for studying semaphorin signaling in the avian central nervous system. Here we describe how both types of approaches can be used to compliment each other in order to unravel the role that semaphorins play during embryonic development and elucidate the functional consequences of semaphorin knockdown using RNA interference vectors. We describe and discuss specifically the use of in ovo electroporation and primary oculomotor neuron culture to identify the role of semaphorins in oculomotor neuron migration and assess functional consequences of semaphorin disruption in this system.

Reptiles and birds have been highlighted as excellent experimental models for the study of developmental biology; however, due to technical limitations in cellular analysis, dynamics of neural stem/progenitor cells of these animals remain unclear. In this chapter, we introduce the protocols for neurosphere culture and ex ovo embryonic culture of developing reptilian and avian embryos, which are modified from the method originally established for rodent embryos. Applications of these techniques provide powerful strategies for the study of comparative neural development of amniotes.

Bird incubation is subdivided into two phases: differentiation (embryonic phase) and growth (fetal phase). Most birds have a relatively short incubation period (20-30 days) with the phase transition occurring midway through the incubation period. The Laysan albatross (Phoebastris immutabilis) is a large pelagic bird with a long incubation period. The purpose of this study was to document the differentiation phase with the aim of ascertaining the impact of a lengthened incubation on embryonic development. Eighty-two previously collected albatross embryos were examined, measured, and staged. The albatross was found to develop more slowly than smaller birds, with a rate similar to other long-incubating birds. Legs and wings grow at similar rates but exhibit variation in growth among their anatomical components. While the albatross embryos shared some morphological stages with chickens, they were more similar to ducks and pelicans. Special features of the albatross not shared with the Gallianserae (chickens and ducks) included an alligator-like curved tail, narial tubes, and a cloacal bulge. Further examination of other larger pelagic birds with long incubation periods are needed to determine the uniqueness of the Laysan albatross embryonic development. Although much embryonic phase growth was documented in the postnatal period, little is known about the later, fetal phase in Laysan albatross. Future studies should involve examination of later (post day 32) fetuses. J. Morphol. 277:1231-1249, 2016. © 2016 Wiley Periodicals, Inc., (© 2016 Wiley Periodicals, Inc.)

In the field of germline development in amniote vertebrates, primordial germ cell (PGC) specification in birds and reptiles remains controversial. Avians are believed to adopt a predetermination or maternal specification mode of PGC formation, contrary to an inductive mode employed by mammals and, supposedly, reptiles. Here, we revisit and review some key aspects of PGC development that channelled the current subdivision, and challenge the position of birds and reptiles as well as the 'binary' evolutionary model of PGC development in vertebrates. We propose an alternative view on PGC specification where germ plasm plays a role in laying the foundation for the formation of PGC precursors (pPGC), but not necessarily of PGCs. Moreover, inductive mechanisms may be necessary for the transition from pPGCs to PGCs. Within this framework, the implementation of data from birds and reptiles could provide new insights on the evolution of PGC specification in amniotes., (© 2016 The Authors BioEssays Published by WILEY Periodicals, Inc.)

We present a detailed study of the formation of the amniotic sac in the avian embryo, and a comparison with the crocodile amniotic sac. We show that the amniotic sac forms at a circular line of stiffness contrast, separating rings of cell domains. Cells align at this boundary, and this in turn orients and concentrates the tension forces. The tissue fold which forms the amniotic sac is locked exactly along this line due to the colocalization of the stiffness contrast and of the tensile force. In addition, the tensile force plays a regenerative role when the amniotic sac is cut. The fold forming the ventral side of the embryo displays the same characteristics. This work shows that amniote embryogenesis consists of a cascade of buckling events taking place at the boundaries between regions of differing mechanical properties. Hence, amniote embryogenesis relies on a simple and robust biomechanical scheme used repeatedly, and selected ancestrally.

The upper jaw in vertebrates forms from several prominences that arise around the stomodeum or primitive mouth. These prominences undergo coordinated growth and morphogenesis to fuse and form the face. Undirected, regionalized cell proliferation is thought to be the driving force behind the morphogenesis of the facial prominences. However, recent findings suggest that directed cell behaviors in the mesenchyme (e.g., directed cell division, directed cell movement, convergent extension) might be required for successful face formation. Here we discuss the evidence for this view and how directed behaviors may interact with the basement membrane to regulate morphogenesis of the facial region. We believe that future research in these largely unexplored areas could significantly impact our understanding of facial morphogenesis., (© 2016 Wiley Periodicals, Inc.)

Birds have a distally reduced, splinter-like fibula that is shorter than the tibia. In embryonic development, both skeletal elements start out with similar lengths. We examined molecular markers of cartilage differentiation in chicken embryos. We found that the distal end of the fibula expresses Indian hedgehog (IHH), undergoing terminal cartilage differentiation, and almost no Parathyroid-related protein (PTHrP), which is required to develop a proliferative growth plate (epiphysis). Reduction of the distal fibula may be influenced earlier by its close contact with the nearby fibulare, which strongly expresses PTHrP. The epiphysis-like fibulare however then separates from the fibula, which fails to maintain a distal growth plate, and fibular reduction ensues. Experimental downregulation of IHH signaling at a postmorphogenetic stage led to a tibia and fibula of equal length: The fibula is longer than in controls and fused to the fibulare, whereas the tibia is shorter and bent. We propose that the presence of a distal fibular epiphysis may constrain greater growth in the tibia. Accordingly, many Mesozoic birds show a fibula that has lost its distal epiphysis, but remains almost as long as the tibia, suggesting that loss of the fibulare preceded and allowed subsequent evolution of great fibulo-tibial disparity., (© 2016 The Author(s). Evolution published by Wiley Periodicals, Inc. on behalf of The Society for the Study of Evolution.)