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Oikopleura dioica is a planktonic tunicate (Appendicularia class) found extensively across the marine waters of the globe. The genome of a single male individual collected from Okinawa, Japan was sequenced using the single-molecule PacBio Hi-Fi method and assembled with NOVOLoci. The mitogenome is 39,268 bp long, featuring a large control region of around 22,000 bp. We annotated the proteins atp6, cob, cox1, cox2, cox3, nad1, nad4, and nad5, and found one more open reading frame that did not match any known gene. This study marks the first complete mitogenome assembly for an appendicularian, and reveals that A and T homopolymers cumulatively account for nearly half of its length. This reference sequence will be an asset for environmental DNA and phylogenetic studies. [ABSTRACT FROM AUTHOR]

Abstract Background The larvacean Oikopleura dioica is an abundant tunicate plankton with the smallest (65–70 Mbp) non-parasitic, non-extremophile animal genome identified to date. Currently, there are two genomes available for the Bergen (OdB3) and Osaka (OSKA2016) O. dioica laboratory strains. Both assemblies have full genome coverage and high sequence accuracy. However, a chromosome-scale assembly has not yet been achieved. Results Here, we present a chromosome-scale genome assembly (OKI2018_I69) of the Okinawan O. dioica produced using long-read Nanopore and short-read Illumina sequencing data from a single male, combined with Hi-C chromosomal conformation capture data for scaffolding. The OKI2018_I69 assembly has a total length of 64.3 Mbp distributed among 19 scaffolds. 99% of the assembly is contained within five megabase-scale scaffolds. We found telomeres on both ends of the two largest scaffolds, which represent assemblies of two fully contiguous autosomal chromosomes. Each of the other three large scaffolds have telomeres at one end only and we propose that they correspond to sex chromosomes split into a pseudo-autosomal region and X-specific or Y-specific regions. Indeed, these five scaffolds mostly correspond to equivalent linkage groups in OdB3, suggesting overall agreement in chromosomal organization between the two populations. At a more detailed level, the OKI2018_I69 assembly possesses similar genomic features in gene content and repetitive elements reported for OdB3. The Hi-C map suggests few reciprocal interactions between chromosome arms. At the sequence level, multiple genomic features such as GC content and repetitive elements are distributed differently along the short and long arms of the same chromosome. Conclusions We show that a hybrid approach of integrating multiple sequencing technologies with chromosome conformation information results in an accurate de novo chromosome-scale assembly of O. dioica’s highly polymorphic genome. This genome assembly opens up the possibility of cross-genome comparison between O. dioica populations, as well as of studies of chromosomal evolution in this lineage.

The larvacean Oikopleura dioica is a cosmopolitan planktonic chordate and is closely related to vertebrates. It is characterized by a tadpole‐shaped morphology with notochord flanked by muscle in the tail and brain on the dorsal side, a short life cycle of five days, a compact genome of approximately 56 Mb, a simple and transparent body with a small number of cells (~4000 in functional juveniles), invariant embryonic cell lineages, and fast development that ensures complete morphogenesis and organ formation 10 h after fertilization. With these features, this marine chordate is a promising and advantageous animal model in which genetic manipulation is feasible. In this review, we introduce relevant resources and modern techniques that have been developed: (1) Genome and transcriptomes. Oikopleura dioica has the smallest genome among non‐parasitic metazoans. Its genome databases have been generated using three geographically distant O. dioica populations, and several intra‐species sequence differences are becoming evident; (2) Functional genetic knockdown techniques. Comprehensive screening of genes is feasible using ovarian microinjection and double‐strand DNA‐induced gene knockdown; and (3) Live imaging of embryos and larvae. Application of these techniques has uncovered novel aspects of development, including meiotic cell arrest, left–right patterning, epidermal cell patterning, and mouth formation involving the connection of ectoderm and endoderm sheets. Oikopleura dioca has become very useful for developmental and evolutionary studies in chordates. [ABSTRACT FROM AUTHOR]

Background: The larvacean Oikopleura dioica is an abundant tunicate plankton with the smallest (65–70 Mbp) non-parasitic, non-extremophile animal genome identified to date. Currently, there are two genomes available for the Bergen (OdB3) and Osaka (OSKA2016) O. dioica laboratory strains. Both assemblies have full genome coverage and high sequence accuracy. However, a chromosome-scale assembly has not yet been achieved. Results: Here, we present a chromosome-scale genome assembly (OKI2018_I69) of the Okinawan O. dioica produced using long-read Nanopore and short-read Illumina sequencing data from a single male, combined with Hi-C chromosomal conformation capture data for scaffolding. The OKI2018_I69 assembly has a total length of 64.3 Mbp distributed among 19 scaffolds. 99% of the assembly is contained within five megabase-scale scaffolds. We found telomeres on both ends of the two largest scaffolds, which represent assemblies of two fully contiguous autosomal chromosomes. Each of the other three large scaffolds have telomeres at one end only and we propose that they correspond to sex chromosomes split into a pseudo-autosomal region and X-specific or Y-specific regions. Indeed, these five scaffolds mostly correspond to equivalent linkage groups in OdB3, suggesting overall agreement in chromosomal organization between the two populations. At a more detailed level, the OKI2018_I69 assembly possesses similar genomic features in gene content and repetitive elements reported for OdB3. The Hi-C map suggests few reciprocal interactions between chromosome arms. At the sequence level, multiple genomic features such as GC content and repetitive elements are distributed differently along the short and long arms of the same chromosome. Conclusions: We show that a hybrid approach of integrating multiple sequencing technologies with chromosome conformation information results in an accurate de novo chromosome-scale assembly of O. dioica's highly polymorphic genome. This genome assembly opens up the possibility of cross-genome comparison between O. dioica populations, as well as of studies of chromosomal evolution in this lineage. [ABSTRACT FROM AUTHOR]

Planktonic Oikopleura dioica respond almost instantly to opportunistic algal blooms, but it is unknown whether the population increase can change from an exponential to a logistic model to avoid a final sudden collapse. To test the hypothesis that intraspecific competition regulates the intrinsic rate of natural increase (r), density-dependent effects on growth and reproduction performance were investigated in O. dioica via laboratory incubations. Over a large food concentration range, batch maturation was observed above the per capita food supply (PFS) of 8.1 µgC/ind. in 4.5 d. Somatic growth was saturated beyond this PFS value, whereas gonad length increased continuously. Below this threshold, individuals reached small body and gonad lengths, and maturation was rarely observed during the incubation period. The gonad/body volume and maturation ratios also increased with the PFS. Instead of the food concentration, the r values were regulated by competition pressure via variability in maturation duration and the proportion of mature individuals in the cohorts. When the minimum food demand was satisfied in the designated generation time, the r value tended to be regulated by the spawning proportion in the population. Otherwise, prolonged development duration and decreased r values were expected. [ABSTRACT FROM AUTHOR]

Unfertilized eggs of most animals are arrested at a certain point in the meiotic cell cycles. Reinitiation of meiosis and the start of embryogenesis are triggered by fertilization. This arrest is essential for preventing parthenogenetic activation and for promoting proper initiation of development by fertilization. In the larvacean Oikopleura dioica , which is a simple model organism for studies of chordate development, the unfertilized egg is arrested at metaphase of meiosis I. We show here that protein phosphatase 2A (PP2A) is essential for maintenance of meiotic arrest after spawning of oocytes. Knockdown (KD) of the maternal PP2A catalytic subunit, which was found in functional screening of maternal factors, caused unfertilized eggs to spontaneously release polar bodies after spawning, and then start pseudo-cleavages without fertilization, namely, parthenogenesis. Parthenogenetic embryos failed to undergo proper mitosis and cytokinesis because of lack of a centrosome, which is to be brought into the egg by a sperm. Activation of the KD oocytes was triggered by possible rise of ambient and intracellular pH upon their release from the gonad into seawater at spawning. Live recording of intracellular calcium level of the KD oocytes indicated that the pH rise caused an aberrant Ca2+ burst, which mimicked the Ca2+ burst that occurs at fertilization. Then, the aberrant Ca2+ burst triggered meiosis resumption through Calcium/calmodulin-dependent protein kinase (CaMK II). Therefore, PP2A is essential for maintenance of meiotic arrest and prevention of parthenogenesis by suppressing the aberrant Ca2+ burst at spawning. • PP2A is essential for maintenance of meiotic arrest in O. dioica oocytes. • Knock down of PP2Ac resulted in parthenogenesis after spawning. • Ambient pH rise at spawning promoted aberrant Ca2+ burst in PP2Ac-KD oocytes. • The aberrant Ca2+ burst led oocytes to reinitiate meiosis and pseudo-cleavages via CaMK II. • O. dioica ' s mechanism of meiotic arrest is more similar to that of vertebrates than ascidians. [ABSTRACT FROM AUTHOR]

Planktonic organisms feed while suspended in water using various hydrodynamic pumping strategies. Appendicularians are a unique group of plankton that use their tail to pump water over mucous mesh filters to concentrate food particles. As ubiquitous and often abundant members of planktonic ecosystems, they play a major role in oceanic food webs. Yet, we lack a complete understanding of the fluid flow that underpins their filtration. Using high-speed, high-resolution video and micro particle image velocimetry, we describe the kinematics and hydrodynamics of the tail in Oikopleura dioica in filtering and free-swimming postures. We show that sinusoidal waves of the tail generate peristaltic pumping within the tail chamber with fluid moving parallel to the tail when filtering. We find that the tail contacts attachment points along the tail chamber during each beat cycle, serving to seal the tail chamber and drive pumping. When we tested how the pump performs across environmentally relevant temperatures, we found that the amplitude of the tail was invariant but tail beat frequency increased threefold across three temperature treatments (5°C, 15°C and 25°C). Investigation into this unique pumping mechanism gives insight into the ecological success of appendicularians and provides inspiration for novel pump designs.

Oikopleura dioica , the only gonochoric species among appendicularians, has a spematozoon with a mid-piece and a conspicuous acrosome that, during fertilisation, undergoes a reaction forming an acrosomal process. To provide more insight into the spermatogenesis of a holoplanktonic tunicate species that completes its life cycle in three to five days, changes in the testis during individual growth have been examined. Spermatogenesis has been subdivided into seven stages based on ultrastructural features during the formation and organisation of the male gonad and the relationships between its macroscopic anatomy and the events of sperm differentiation. Gametes undergo highly synchronised differentiation due to the presence of widespread syncytial structures. Both meiosis and spermiogenesis are brief, and the passage from spermatocytes to spermatids involves a progressive segregation of the germ cells from the syncytial mass with the formation of large cytoplasmic bridges and volume reduction for nucleus compacting and cytoplasmic material changing. The nucleus is small and penetrated anteriorly by a complex acrosome and posteriorly by the distal centriole and part of the flagellum. In spermatids, the single, large mitochondrion appears laterally to the nucleus, and finally, in spermatozoa, it migrates into the mid-piece, wrapping the proximal portion of the axoneme. Because this mitochondrial position is reached only in the late phases of spermatogenesis, it suggests that appendicularians have derived oligopyrenic sperms in which the small nucleus results from adaptation to the assembly of numerous spermatozoa inside the narrow space of the testis compacted in the genital cavity. The formulation of a staging system of gonad development in a model tunicate species known for having the most compacted genome in chordates led to a comparison of histological observations with recent molecular data, improving the characterisation of its biology and life cycle in light of evolutionary implications. • Spermatogenesis is proposed as a tool for staging male individuals of O. dioica. • Spermatogenesis is highly synchronised by means of a syncytial cytoplasm. • Mitotic phase (stages 1–3) is brief and generates a primary spermatocyte pool. • Meiotic phase (stage 4) produces haploid spermatids segregating from syncytium. • Spermiogenetic phase occurs during stages 5–7 with volume increase of the testis. [ABSTRACT FROM AUTHOR]

Mechanisms for morphogenetic processes that generate complex patterns in a reproducible manner remain elusive. Live imaging provides a powerful tool to record cell behaviors. The appendicularian, Oikopleura dioica , is a planktonic tunicate that has a rapid developmental speed, small number of cells (less than 3500 cells in a juvenile), and a transparent body. The trunk epidermis, called the oikoplastic epithelium (OE), has elaborate cellular arrangements showing a complex pattern to secrete so-called “house” made of extracellular components. The OE is characterized by invariant number, size, and shape of the monolayer epithelial cells. Pattern formation is achieved during 5 h of larval development without growth of the body, making this a suitable system for live imaging of a two-dimensional (2D) sheet. First, we subdivided the OE and defined several domains by cellular resolution, and systematically gave names to the constituent cells, since there is no variation among individuals. Time-lapse imaging of the epidermal cells revealed region-specific pattern formation processes. Each identified domain served as a compartment into which distribution of descendant cells of founder cells is restricted. Regulation of orientation, timing, and the number of rounds of cell divisions, but not cell death and migration, was a critical mechanism for determination of final cell arrangement and size. In addition, displacement of epithelial sheet plates was observed in the Eisen domain. Stem-cell-like cell divisions, whereby large mother stem cells generate a chain of small daughter cells, were involved in formation of the Nasse region and ventral sensory organ. These are the first examples of this kind of stem-cell-like cell division in deuterostomes. Furthermore, labeling of the left or right blastomere of the two-cell-stage embryo, which roughly gives rise to the left or right side of the body, respectively, revealed that the boundary of the descendant cells does not match with the midline of the trunk epidermis. Left and right descendants largely invade into the opposite side in an invariant way, suggesting the possibility that specification of the OE cell identities may occur later in development, most probably around hatching, and depending on cell position in the OE epithelial sheet. These detailed descriptions of OE patterning processes provide basic and essential information to analyze further cell behaviors in the generation of elaborate and intricate but stereotyped 2D cellular patterns in this advantageous model system for developmental and cell biological studies in chordates. [ABSTRACT FROM AUTHOR]

Background: Peroxisomes are ubiquitous eukaryotic organelles that compartmentalize a variety of metabolic pathways that are primarily related to the oxidative metabolism of lipids and the detoxification of reactive oxygen species. The importance of peroxisomes is underscored by serious human diseases, which are caused by disorders in peroxisomal functions. Some eukaryotic lineages, however, lost peroxisomes. These organisms are mainly anaerobic protists and some parasitic lineages including Plasmodium and parasitic platyhelminths. Here we performed a systematic in-silico analysis of peroxisomal markers among metazoans to assess presence of peroxisomes and peroxisomal enzymes. Results: Our analyses reveal an obvious loss of peroxisomes in all tested flukes, tapeworms, and parasitic roundworms of the order Trichocephalida. Intriguingly, peroxisomal markers are absent from the genome of the free-living tunicate Oikopleura dioica, which inhabits oxygen-containing niches of sea waters. We further map the presence and predicted subcellular localization of putative peroxisomal enzymes, showing that in organisms without the peroxisomal markers the set of these enzymes is highly reduced and none of them contains a predicted peroxisomal targeting signal. Conclusions: We have shown that several lineages of metazoans independently lost peroxisomes and that the loss of peroxisomes was not exclusively associated with adaptation to anaerobic habitats and a parasitic lifestyle. Although the reason for the loss of peroxisomes from O. dioica is unclear, organisms lacking peroxisomes, including the free-living O. dioica, share certain typical r-selected traits: high fecundity, limited ontogenesis and relatively low complexity of the gene content. We hypothesize that peroxisomes are generally the first compartment to be lost during evolutionary reductions of the eukaryotic cell. [ABSTRACT FROM AUTHOR]

The appendicularian, Oikopleura dioica , is a planktonic chordate. Its simple and transparent body, invariant cell lineages and short life cycle of 5 days make it a promising model organism for studies of chordate development. Here we describe the cell migration that occurs during development of the O. dioica larva. Using time-lapse imaging facilitated by florescent labeling of cells, three cell populations exhibiting long-distance migration were identified and characterized. These included (i) a multinucleated oral gland precursor that migrates anteriorly within the trunk region and eventually separates into the left and right sides, (ii) endodermal strand cells that are collectively retracted from the tail into the trunk in a tractor movement, and (iii) two subchordal cell precursors that individually migrate out from the trunk to the tip of the tail. The migration of subchordal cell precursors starts when all of the endodermal strand cells enter the trunk, and follows the same path but in a direction opposite to that of the latter. Labeling of these cells with a photoconvertible fluorescent protein, Kaede, demonstrated that the endodermal strand cells and subchordal cell precursors have distinct origins and eventual fates. Surgical removal of the trunk from the tail demonstrated that the endodermal strand cells do not require the trunk for migration, and that the subchordal cell precursors would be attracted by the distal part of the tail. This well-defined, invariant and traceable long-distance cell migration provides a unique experimental system for exploring the mechanisms of versatile cell migration in this simple organism with a chordate body plan. [ABSTRACT FROM AUTHOR]

Tunicates have been extensively studied because of their crucial phylogenetic location (the closest living relatives of vertebrates) and particular developmental plan. Recent genome efforts have disclosed that tunicates are also remarkable in their genome organization and molecular evolutionary patterns. Here, we review these latter aspects, comparing the similarities and specificities of two model species of the group: Oikopleura dioica and Ciona intestinalis. These species exhibit great genome plasticity and Oikopleura in particular has undergone a process of extreme genome reduction and compaction that can be explained in part by gene loss, but is mostly due to other mechanisms such as shortening of intergenic distances and introns, and scarcity of mobile elements. In Ciona, genome reorganization was less severe being more similar to the other chordates in several aspects. Rates and patterns of molecular evolution are also peculiar in tunicates, being Ciona about 50% faster than vertebrates and Oikopleura three times faster. In fact, the latter species is considered as the fastest evolving metazoan recorded so far. Two processes of increase in evolutionary rates have taken place in tunicates. One of them is more extreme, and basically restricted to genes encoding regulatory proteins (transcription regulators, chromatin remodeling proteins, and metabolic regulators), and the other one is less pronounced but affects the whole genome. Very likely adaptive evolution has played a very significant role in the first, whereas the functional and/or evolutionary causes of the second are less clear and the evidence is not conclusive. The evidences supporting the incidence of increased mutation and less efficient negative selection are presented and discussed. [ABSTRACT FROM PUBLISHER]

Background: Hair cells are vertebrate secondary sensory cells located in the ear and in the lateral line organ. Until recently, these cells were considered to be mechanoreceptors exclusively found in vertebrates that evolved within this group. Evidence of secondary mechanoreceptors in some tunicates, the proposed sister group of vertebrates, has recently led to the hypothesis that vertebrate and tunicate secondary sensory cells share a common origin. Secondary sensory cells were described in detail in two tunicate groups, ascidians and thaliaceans, in which they constitute an oral sensory structure called the coronal organ. Among thaliaceans, the organ is absent in salps and it has been hypothesised that this condition is due to a different feeding system adopted by this group of animals. No information is available as to whether a comparable structure exists in the third group of tunicates, the appendicularians, although different sensory structures are known to be present in these animals. Results: We studied the detailed morphology of appendicularian oral mechanoreceptors. Using light and electron microscopy we could demonstrate that the mechanosensory organ called the circumoral ring is composed of secondary sensory cells. We described the ultrastructure of the circumoral organ in two appendicularian species, Oikopleura dioica and Oikopleura albicans, and thus taxonomically completed the data collection of tunicate secondary sensory cells. To understand the evolution of secondary sensory cells in tunicates, we performed a cladistic analysis using morphological data. We constructed a matrix consisting of 19 characters derived from detailed ultrastructural studies in 16 tunicate species and used a cephalochordate and three vertebrate species as outgroups. Conclusions: Our study clearly shows that the circumoral ring is the appendicularian homologue of the coronal organ of other tunicate taxa. The cladistic analysis enabled us to reconstruct the features of the putative ancestral hair cell in tunicates, represented by a simple monociliated cell. This cell successively differentiated into the current variety of oral mechanoreceptors in the various tunicate lineages. Finally, we demonstrated that the inferred evolutionary changes coincide with major transitions in the feeding strategies in each respective lineage [ABSTRACT FROM AUTHOR]

The appendicularian Oikopleura dioica is a chordate that has a remarkably simple adult body with small cell number. Its transparency, stereotyped cell lineages, short life cycle, and small genome make it a promising new experimental model of chordate developmental biology. However, the functions of its various genes are still poorly understood due to lack of a tool for suppression of gene expression. Here, we applied a double-stranded RNA (dsRNA)-based-RNA interference (RNAi) method in O. dioica. For introducing dsRNA into eggs and embryos, we injected dsRNAs into the ovary. dsRNA, which is specific to EGFP or mCherry mRNA, decreased the exogenous mRNA-derived fluorescence in both eggs and embryos. dsRNA specific to the Brachyury gene of O. dioica, which is a homologous gene of a key notochord transcriptional factor in ascidians, triggered degradation of endogenous Brachyury mRNA and induced malformation or loss of the notochord in the tail. This effect was Brachyury sequence specific, as three dsRNAs covering different sequences produced the same phenotype. The result is in accordance with its expression site and also with the key regulatory function of Brachyury in notochord formation in other chordates. RNAi in O. dioica would be a useful tool for gaining insight into the oogenesis and early developmental processes in chordates. [ABSTRACT FROM AUTHOR]

All tunicates produce a cellulosic extracellular matrix external to the epidermis; this is the most remarkable character of the subphylum (Tunicata = Urochordata). In Ascidiacea and Thaliacea, two of the three classes of Tunicata, cellulose is the main component of the tunic matrix that covers the whole body. In Appendicularia, however, the animals do not possess a tunic, and instead secrete a feeding apparatus called a "house" that is composed of cellulose. In the appendicularian Oikopleura dioica, we found a unique intracellular matrix in the apical cytoplasm of the adult tail epidermis that appears to form a tough shell supporting the tail. Although some experimental data suggest that this matrix may contain cellulose, the composition is still unknown. The significance of the tunicate capacity for cellulose synthesis is briefly discussed. [ABSTRACT FROM AUTHOR]

The native form of cellulose is a fibrillar composite of two crystalline phases, the triclinic I and monoclinic I allomorphs. Allomorph ratios are species-specific, and this gives rise to natural structural variations in cellulose crystals. However, the mechanisms contributing to crystal formation remain unknown. We show that the two crystalline phases of cellulose are tailored to distinct structures during different developmental stages of the tunicate chordate Oikopleura dioica. Larval cellulose consisting of I allomorph constitutes the body cuticle fin, whereas adult cellulose consisting of I allomorph frames a mucous filter-feeding device, the 'house.' Both structures are secreted from the epidermis in accordance with the mutually exclusive expression patterns of two distinct putative cellulose synthase genes. We discuss a possible linkage between structural variations of the crystalline phases of cellulose and the underlying evolutionary genetics of cellulose biosynthesis. [ABSTRACT FROM AUTHOR]

The article discusses a research study on heritability of Oikopleura dioica and the genetic correlation of mophological and life history traits. Collected were parent Oikopleura dioica at El Musel harbor in northern Spain from April to May and October 2005 and used the half-sib, dam-sire breeding design for examination of quantitative genetic variation patterns of traits and morphology determination through lateral view measurements of their trunk length, tail length and gonad length to determine total body size. Results showed variability with high coefficients of variation (CVs) for life history traits and less CVs on morphological traits while phenotypic correlations of longer-trunked mature appendicularians showed bigger feeding structures.

Phylogenomics has recently revealed that tunicates represent the sister-group of vertebrates in the newly defined clade Olfactores. However, phylogenomic and comparative genomic studies have also suggested that tunicates are characterized by an elevated rate of molecular evolution and a high degree of genomic divergence. Despite the recurrent interest in the group, the picture of tunicate peculiar evolutionary dynamics is still fragmentary, as it mainly lies in studies focusing on only a few model species. In order to expand the available genomic data for the group, we used the high-throughput 454 technology to sequence the partial transcriptome of a previously unsampled tunicate, Microcosmus squamiger. This allowed us to get further insights into tunicate-accelerated evolution through a comparative analysis based on pertinent phylogenetic markers, i.e., a core of 35 housekeeping genes conserved across bilaterians. Our results showed that tunicates evolved on average about two times faster than the other chordates, yet the degree of this acceleration varied extensively upon genes and upon lineages. Appendicularia and Aplousobranchia were detected as the most divergent groups which were also characterized by highly heterogeneous substitution rates across genes. Finally, an estimation of the d/ d ratio in three pairs of closely related taxa within Olfactores did not reveal strong differences between the tunicate and vertebrate lineages suggesting that for this set of housekeeping genes, the accelerated evolution of tunicates is plausibly due to an elevated mutation rate rather than to particular selective effects. [ABSTRACT FROM AUTHOR]

The article features a study that aims to compare the dynamics of zooplankton in the St. Lucia estuary in South Africa at close-mouthed and open-mouthed conditions. The study took samples and conducted quarterly surveys at five stations in the lake bed from February 2006 until November 2008. It says that during the closed-mouth phase 70 per cent of the estuary is dry and has a high salinity level, and previously recorded aquatic plants decreased. It also says that fish larva and the prawn Penaeus indicus re-entered the estuary during the open-mouth phase but when the estuary was re-closed zooplankton grazers disappeared and turnicate Oikopleura dioica colonized the opening. It adds that further studies are needed to understand the effect of climate and environmental changes on the estuary.

Abstract: A model has been developed to simulate the growth of an individual appendicularian (Oikopleura dioica) from egg to spawning. This model uses a new set of experimental data presented in a companion paper in this volume and estimates growth rates and generation times as well as clearance rates, house and fecal pellet production and reproductive capacity at different temperatures and food concentrations. According to the model outputs, the weight of a single house represents 11.5% of the individual''s total weight, a lower value than previously estimated. The relative weight of one fecal pellet varies as a function of food concentration. The model also confirms that the minimum food concentration for growth is about 20–30 µg C l−1 and that growth is maximal for a 100 µg C l−1 feeding concentration. The limits of the growth optimum in relation to food concentration and temperature can be considered as a first description of the fundamental ecological niche of appendicularians. This model can be used both for predicting (1) the impact of in situ observed populations on the pico- and nanoseston and (2) the production of large aggregates. These two pieces of information are often needed for large-scale biogeochemical models. [Copyright &y& Elsevier]

Abstract: Three aspects of the appendicularian O. dioica''s ecophysiology were measured here: 1) morphological parameters over a wide range of appendicularian sizes, including mature animals in order to document the morphological characteristics inducing reproduction; 2) clearance rate and assimilation efficiency using feeding incubations with different algal concentrations and 3) the effect of food concentration on growth, mortality and reproduction. The relationship between the body carbon weight and the clearance rate follows a power function, with an exponent of 0.91 (±0.07). The rate of particles retention increases with the food concentration following a Michaelis–Menten relationship (half-saturation constant = 151±22 µg C l−1, maximum clearance rate = 12±1 µg C µg C−1 d−1). The carbon assimilation efficiency decreases with the increasing food concentration. As a result, appendicularian growth which is limited in concentrations lower than 50 µg C l−1 is saturated above 100 µg C l−1. In immature animals the gonad represents less than 30% of the body volume whereas in mature individuals, its volume varies between 50% and 87% (mean 63%) suggesting that gonad/total volume ratio can be used as indicator of the maturation stages. The gonad weight in mature animals represents 70.3 (±4.6)% of the total body carbon weight. Two major maturity stages can explain the changes in energy allocation: i) the somatic growth, when less energy is invested in gonad growth when compared to the rest of the body and ii) the maturation phase where most of the assimilated matter is invested in gonad maturation. This process is rapid, lasting only few hours. For this reason we measured completely mature organisms that are generally not measured during the experimental work with appendicularians. In food-limited conditions, the gonad maturation process starts with smaller individuals and ends with smaller reproductive animals having the same gonad to total volume ratio than in unlimited food conditions. The results obtained in this study were used to model the life cycle of O. dioica (see Lombard, F., Sciandra, A. and Gorsky, G., 2009-this volume. Appendicularian ecophysiology. II. Modeling nutrition, metabolism, growth and reproduction of the appendicularian Oikopleura dioica.). [Copyright &y& Elsevier]

A key problem in understanding deuterostome evolution has been the origin of the chordate body plan. A biphasic life cycle with a sessile adult and a free-swimming larva is traditionally considered ancestral in chordates with subsequent neotenic loss of the sessile adult stage. Molecular phylogenies challenged this view, suggesting that the primitive life cycle in chordates was entirely free-living as in modern day larvaceans. Here, we report the precise cell lineage and fate map in the normal embryo of the larvacean Qikopleura dioica, using 4D microscopy technique and transmission electron microscopy. We document the extraordinary rapidity of cleavage and morphogenetic events until hatching and demonstrate that-compared with ascidians-fate restriction occurs considerably earlier in O. dioica and that clonal organization of the cell lineage is more tightly coupled to tissue fate. We show that epidermal cells in the trunk migrate through 90°, reminiscent of events in ascidian metamorphosis and that the axis of bilateral symmetry in the tail rotates in relation to the trunk. We argue that part of the tail muscle cells are ectomesodermal, because they are more closely associated with prospective epidermis than with other tissues in the cell lineage. Cladistic comparison with other deuterostomes suggests that these traits are derived within tunicates strengthening the hypothesis that the last common ancestor of tunicates had a sessile adult and thus support traditional morphology-derived scenarios. Our results allow hypothesizing that molecular developmental mechanisms known from ascidian models are restricted to fewer, yet identifiable, cells in O. dioica. [ABSTRACT FROM AUTHOR]

The appendicularian, Oikopleura dioica is a chordate. Its life cycle is extremely short—approximately 5 days—and its tadpole shape with a beating tail is retained throughout entire life. The tadpole hatches after 3 h of development at 20°C. Here, we describe the cleavage pattern and morphogenetic cell movements during gastrulation and neurulation. Cleavage showed an invariant pattern. It is basically bilateral but also shows various minor left–right asymmetries starting from the four-cell stage. We observed two rounds of unequal cleavage of the posterior-vegetal B-line cells at the posterior pole. The nature of the unequal cleavages is reminiscent of those in ascidian embryos and suggests the presence of a centrosome-attracting body, a special subcellular structure at the posterior pole. The representation of the cell division pattern in this report will aid the identification of each cell, a prerequisite for clarifying the gene expression patterns in early embryos. Gastrulation started as early as the 32-cell stage and progressed in three phases. By the end of the second phase at the 64-cell stage, every vegetal cell had ingressed into the embryo, and animal cells had covered the entire embryo by epiboly. There was no archenteron formation. In the anterior region, eight A-line cells were aligned as a 2 × 4 array along the anterior–posterior axis and become internalized during the 64-cell stage. This process was considered to correspond to neurulation. The simple and accelerated development of Oikopleura, nevertheless giving rise to a conserved chordate body plan, is advantageous for studying developmental mechanisms using molecular and genetic approaches and makes this animal the simplest model organism in the phylum Chordata. [ABSTRACT FROM AUTHOR]