Search

Sequencing and comparative analyses of genomes from multiple vertebrates are providing insights about the genetic basis for biological diversity. To date, these efforts largely have focused on eutherian mammals, chicken, and fish. In this article, we describe the generation and study of genomic sequences from noneutherian mammals, a group of species occupying unusual phylogenetic positions. A large sequence data set (totaling > 5 Mb) was generated for the same orthologous region in three marsupial (North American opossum, South American opossum, and Australian tammar wallaby) and one monotreme (platypus) genomes. These ancient mammalian genomes are characterized by unusual architectural features with respect to G + C and repeat content, as well as compression relative to human. Approximately 14% and 34% of the human sequence forms alignments with the orthologous sequence from platypus and the marsupials, respectively; these numbers are distinctly lower than that observed with nonprimate eutherian mammals (45-70%). The alignable sequences between human and each marsupial species are not completely overlapping (only 80% common to all three species) nor are the platypus-alignable sequences completely contained within the marsupial-alignable sequences. Phylogenetic analysis of synonymous coding positions reveals that platypus has a notably long branch length, with the human-platypus substitution rate being on average 55% greater than that seen with human-marsupial pairs. Finally, analyses of the major mammalian lineages reveal distinct patterns with respect to the common presence of evolutionarily conserved vertebrate sequences. Our results confirm that genomic sequence from noneutherian mammals can contribute uniquely to unraveling the functional and evolutionary histories of the mammalian genome. comparative genomics | genome sequencing | genome analysis | phylogenetics | mammalian evolution

Evolutionary interactions among insect herbivores and plant chemical defenses have generated systems where plant compounds have opposing fitness consequences for host plants, depending on attack by various insect herbivores. This interplay complicates understanding of fitness costs and benefits of plant chemical defenses. We are studying the role of the glucosinolate-myrosinase chemical defense system in protecting Arabidopsis thaliana from specialist and generalist insect herbivory. We used two Arabidopsis recombinant inbred populations in which we had previously mapped QTL controlling variation in the glucosinolate-myrosinase system. In this study we mapped QTL controlling resistance to specialist (Plutella xylostella) and generalist (Trichoplusia ni) herbivores. We identified a number of QTL that are specific to one herbivore or the other, as well as a single QTL that controls resistance to both insects. Comparison of QTL for herbivory, glucosinolates, and myrosinase showed that T. ni herbivory is strongly deterred by higher glucosinolate levels, faster breakdown rates, and specific chemical structures. In contrast, P. xylostella herbivory is uncorrelated with variation in the glucosinolate-myrosinase system. This agrees with evolutionary theory stating that specialist insects may overcome host plant chemical defenses, whereas generalists will be sensitive to these same defenses.

Research has been conducted on the male body size evolution due to natural and sexual selection in insects. The relationship between the nuptial gift size and the male body size has been investigated in the ground cricket populations and the results indicate that gifts indirectly affect the male size evolution.

Research has been conducted on the social insect queen multiple mating. The hypothesis that the queens mate multiply in manipulating sex ratios to their advantage in spite of mortality risk has been investigated and the results indicate that sex-ratio manipulation theory is unable to explain the multiple mating evolution in these insects.

Significant investigators and aspects in the past century of insect paleontology are briefly reviewed. Despite the pervasive influence of the paleoentomologist Willi Hennig in systematic biology, the study of fossil insects remains more descriptive than most other paleontological areas. Hypotheses are reviewed on relationships and chronologies of early divergences in insects (Paleozoic, Lower Mesozoic), particularly living and extinct orders of the lower pterygotes and putative monophyly of the Paleoptera (Odonata + Ephemeroptera). The Dictyoptera (Mantodea, Isoptera, Blattaria) illustrate relationships and discrepencies between stratigraphic record and phylogenetic relationships. Future directions in the field are suggested.

Selection on female color polymorphism has been studied in the damselfly Ischnura graellsii with molecular evidence for the selection found. It appears that in some I. graellsii populations, at least, density-dependent mechanisms may help prevent loss of the polymorphism. This does explain similarity in morph frequencies among populations. Five populations were studied.

Female mating frequency as a correlated response has been studied by looking at selection on age at reproduction in Drosophila melanogaster and fertility in the female. Results show that response to selection on age at reproduction has involved changes in scheduling of female reproductive behavior.

Genetic evidence has been found for assortative mating between 13-year cicadas and sympatric '17-year cicadas with 13-year life cycles.' It supports allochronic speciation theories. A previously unsuspected broad zone of overlap was found. Recent derivation of all northern 13-year cicadas from the 17-year ones seems to have occurred via life-cycle switching.

Color-pattern and body-size genetics in the polymorphic mimetic diadem butterfly, Hypolimnas misippus (L.), are discussed. The butterfly shows some anomalies in the presumed Batesian mimetic relationship with the distasteful danaine, Danaus chrysippus, and it has been shown that the A locus, probably a supergene with at least four loci linked in coupling, segregates for body size and color pattern. Multiple origins, not selective maintenance, is likely in the evolution of the mimicry.

One of the rare examples of a single major gene underlying a naturally occurring behavioral polymorphism is the foraging locus of Drosophila melanogaster. Larvae with the rover allele, [for.sup.R], have significantly longer foraging path lengths on a yeast paste than do those homozygous for the sitter allele, [for.sup.S]. These variants do not differ in general activity in the absence of food. The evolutionary significance of this polymorphism is not as yet understood. Here we examine the effect of high and low animal rearing densities on the larval foraging path-length phenotype and show that density-dependent natural selection produces changes in this trait. In three unrelated base populations the long path (rover) phenotype was selected for under high-density rearing conditions, whereas the short path (sitter) phenotype was selected for under low. density conditions. Genetic crosses suggested that these changes resulted from alterations in the frequency of the [for.sup.S] allele in the low-density-selected lines. Further experiments showed that density-dependent selection during the larval stage rather than the adult stage of development was sufficient to explain these results. Density-dependent mechanisms may be sufficient to maintain variation in rover and sitter behavior in laboratory populations.

The muscle of Lawrence (MOL) is a bilaterally symmetrical muscle spanning the tergite of the fifth abdominal segment of adult male Drosophila melanogaster. It is not, however, a general feature of male-specific development within the subfamily Drosophilinae. Of 95 species surveyed within this subfamily, 67 exist with no MOL at all. By drawing comparisons with published cladograms of species relatedness, three conclusions regarding the evolutionary history of the MOL are made: (i) The MOL predates the major radiations of the genus Drosophila, given its presence in earlier-branching Chymomyza and Scaptodrosophila; the MOL has been subsequently excluded in at least one present species of each of these two primitive genera. (ii) Within the genus Drosophila the MOL is present sporadically in the radiation of the subgenus Sophophora, showing repetitive loss even in very close evolutionary lineages. (iii) The MOL may have been entirely excluded from the prolific radiation of the subgenus Drosophila. Thus the MOL shows a uniquely incongruous pattern of presence or absence relative to accepted drosophilid phylogeny.

Anonymous nuclear DNA restriction fragment polymorphisms were employed to characterize the array of multilocus genotypes present within a well-studied hybrid zone between two eastern North American field crickets Gryllus pennsylvanicus and Gryllus firmus. Analyses based on four diagnostic anonymous nuclear markers indicated that hybrid zone populations in Connecticut contain few F1 hybrids and that nonrandom associations persist among nuclear gene markers, between nuclear and cytoplasmic markers, and between molecular markers and morphology.

The evolutionary consequences of density-dependent natural selection on growth rate and body size in Drosophila melanogaster were studied. D. melanogaster populations subjected to extreme larval crowding (CU) have been previously observed to have evolved higher larval feeding rates than their corresponding controls (UU). The CU populations showed a higher growth rate during the postcritical period of larval life than UU populations, but the sustained differences in weight did not translate into the adult stage.

There are two theories on the origin of insect wings. The first theory proposes that wings evolved by modification of limb branches present in multibranched ancestral appendages and functioned as gills. The other theory holds that wings evolved as novel outgrowths of the body wall, not directly related to any pre-existing limbs. A study based on gene expression patterns has provided support for the first theory.

The evolution of insect mouthparts closely follows the environmental changes occurring over several periods of planetary history. Their geochronological development, driven by food partitioning, includes piercing-and-sucking, fluid- and solid-feeding, fluid and aquatic particle-feeding, and surface-fluid-feeding. This data is important in plotting ecological and phenetic attributes onto baseline phylogenies.

The females of the Zimbabwe isofemale Drosophila melanogaster populations prefer to mate with males of their kind rather than the males from other continents. The males also prefer females of their own kind, but their preference is less strong. Thus, Zimbabwe populations are probably developing into a new species. The speciation is in the incipient stage as the genetic determinants in the males and females are polymorphic, the F1 and F2 hybrids are not sterile and the divergence of the X chromosome on the basis of mating behavior is low.

Insects and their hostplants represent the major part of terrestrial diversity, yet we are just beginning to understand why there are so very many species. By far the most influential model of insect/plant diversification has been Ehrlich and Raven's (1964) hypothesis of insect/plant coevolution. While the coevolution model was based on macroevolutionary patterns in plant defenses and hostplant affiliations, most of the subsequent work has been on its possible ecological and genetic mechanisms, with relatively little systematic scrutiny of the evolutionary patterns Ehrlich and Raven described. We explore the possible roles insect/plant interactions may play in the long-term evolution of insect and plant lineages, and review some of the evidence on whether or not insects and plants have exerted reciprocal influences on each other's diversification. Insects and plants have diversified over roughly the same time intervals, and many insect host-affiliations are evolutionarily conserved, thus reflecting long-term, phylogenetic history. Rather than accumulating herbivores at a rate proportional to their geographic area of distribution or biomass, some plant groups pose apparent chemical barriers to potential herbivore colonists, and seem accessible to relatively few insect lineages, possibly preadapted by use of chemically similar or related hostplants. Evolutionary innovations in plant defenses and in insect feeding habits seem to have spurred their respective adaptive radiations, thus ecological opportunity may influence long-term evolutionary success. The greater diversity of insects and plants in the tropics, compared to the temperate zone, probably reflects the greater age of tropical habitats as well as climatic barriers that limit successful invasion of the temperate zone to just those primitively tropical groups able to evolve strategies for both overwintering and use of temperate resources. Though evidence is still sparse, successful invasion of the temperate zone may promote subsequent radiations of both insects and plants. We conclude that much of the available evidence from systematics is consistent with Ehrlich and Raven's suggestion that much of insect and plant diversification has been spurred by a series of ecological opportunities over evolutionary time.

Evolutionary trees for the human and Drosophila homeobox genes were constructed by the neighbor-joining method combined with bootstrapping resampling. The resulting trees showed that the Antennapedia-type homeobox genes have evolved from three precursors which diverged early in metazoan evolution. Gene duplications in all three of these precursors occured during the evolution of insects and vertebrates.

Insect-fungus interactions range from agonistic to mutualistic, and include several spectacular examples of complex symbioses. A potential benefit of mycophagy (the ingestion of fungal tissue) is the augmentation of digestive capacity by the ingestion of fungal enzymes that remain active in the gut following ingestion. Cellulose digestion is mediated by ingested fungal enzymes in the wood-boring larvae of cerambycid beetles and siricid woodwasps, in detritus-feeding stonefly nymphs, and in the workers of fungus-growing termites. In this paper I discuss a plausible scenario for the evolution of stable symbiotic insect-fungus associations, in which the augmentation of digestive capacity through the ingestion of fungal enzymes is an important factor leading to the establishment of interdependence between the interacting partners in a mutualism. Ingested fungal enzymes play a different role in the mutualistic association of the attine ants and their symbiotic fungi. Analyses of the associations of the siricid woodwasps, fungus-growing termites, and fungus-growing ants with their symbiotic fungi permit the testing of Law's (1985) predictions concerning the consequences of evolution in a mutualistic environment. As predicted, the rate of speciation has been slower in the protected partner than in the host partner, selection has favored asexual reproduction in the protected partner, and, at least in the attine ant-fungus symbiosis, the protected partner exhibits a low degree of specificity toward different host species. Insect-fungus interactions provide rich material for the study of both mechanistic and theoretical aspects of mutualism.

Many species of insects exhibit wing dimorphism, one morph having fully developed wings and the other morph having reduced wings and being incapable of flight. These wing dimorphisms provide visible manifestations of migratory polymorphisms. Since winged individuals do not, in principle, have to fly, the existence of forms with reduced wings suggests that there is a tradeoff between flight capability and other fitness components. Comparisons of the life histories of the fully winged and wing reduced morphs demonstrate that this tradeoff is most commonly expressed as a decreased in the age of first reproduction and increased fecundity in the morph with reduced wings. Given these tradeoffs, the evolution of wing dimorphism will depend upon its genetic basis, including correlations with other life history components. A review of the recent literature suggests that the heritability of wing morphology is high, and we suggest that this high heritability could be maintained, in part, by antagonistic pleiotropy. In dimorphic species, the winged morph ie generally considered to be the migrant form. However, there are significant correlations, both within and among species, between the proportion of winged individuals, the proportion of winged individuals with functional flight muscles, and the flight propensity of those individuals. This suggests that the proportion of winged individuals and the propensity of the winged morph to migrate are intimately connected at both the physiological and population level. Therefore, the study of the evolution of wing dimorphism is important not only in its own right but also as a model of how migratory propensity evolves in monomorphically winged species.

The lifetime of egg maturation in insects is evaluated. The ovigeny index and initial egg load as measures of fitness and of resource allocation are focused.

Advanced eusociality sometimes is given credit for the ecological success of termites, ants, some wasps, and some bees. Comprehensive study of bees fossilized in Baltic amber has revealed an unsuspected middle Eocene (ca. 45 million years ago) diversity of eusocial bee lineages. Advanced eusociality arose once in the bees with significant post-Eocene losses in diversity, leaving today only two advanced eusocial tribes comprising less than 2% of the total bee diversity, a trend analogous to that of hominid evolution. This pattern of changing diversity contradicts notions concerning the role of eusociality for evolutionary success in insects. Apidae | Baltic amber | Hominidae | Paleogene

The prairie mole cricket (Gryllotalpa major) is a burrowing insect native to the tallgrass prairies of the southern United States. The male produces a loud, long-range calling song at sunset in April and May by rubbing the plectrum of one wing along the file of the other wing. The sound produced is a series of chirps. The wing morphology of G. major was compared with five other mole cricket species found in North America: N. hexadactyla, G. gryllotalpa, S. abbreviatus, S. vicinus, S. borellii. The overall wing structures among the crickets were similar, yet calling songs differ in that G. major and N. hexadactyla chirp while the others trill. S. abbreviatus does not have a calling song. The type of calling song may reflect an adaptation that promotes being heard in a particular habitat and may influence the type of mating system of a species. In addition, DNA sequence data were compared for the same species. A morphological phylogeny and a molecular phylogeny for the Gryllotalpidae were constructed to determine if the different species express a similar trait because they inherited it from a common ancestor or because they acquired it independently. The phylogenies account for the history of character change and allow for hypotheses to be developed regarding the course of the mole crickets' evolution and the correlation of this with behavior traits.

Hovering insects possess the highest mass-specific metabolic rate ever measured and are therefore an excellent model to study the evolution of metabolic pathways involved in energy production. We conducted a survey of the maximal activity of several enzymes as indicators of glycolytic capacity (Hexokinase, Phosphofructokinase), redox balance (Glycerol 3-phosphate dehydrogenase), and mitochondrial metabolism (Citrate synthase, Cytochrome c oxidase) in 19 species (4 genera) of Euglossine bees of varying size (varying from 50 mg to 1 g). A strong negative allometric relationship was observed for the activity of Hexokinase. The slope was similar to that of the oxygen consumption rate previously published for 9 species of Euglossine bees, which highlights the importance (maybe high control coefficient) of Hexokinase in these insects. Citrate synthase activity was also related to body mass, however, other factors (e.g. phylogeny) influenced the relationship. We analyzed data using phylogeneticaly independent contrasts and discuss the metabolic and evolutionary implications of these results.

The Acp26Aa and Acp26Ab genes that code for male accessory gland proteins are tandemly arranged in the species of the Drosophila melanogaster complex. An [approximately]1.6-kb region encompassing both genes has been sequenced in 10, 24, and 18 lines from Spain, Ivory Coast, and Malawi, respectively; the previously studied 10 lines from North Carolina have also been included in the analyses. A total of 110 nucleotide and 4 length polymorphisms were detected. Silent variation for the whole Acp26A region was slightly higher in African than in non-African populations, while for both genes nonsynonymous variation was similar in all populations studied. based on Fat estimates no major genetic differentiation was detected between East and West Africa, while in general non-African populations were strongly differentiated from both African populations. Comparison of polymorphism and divergence at synonymous and nonsynonymous sites revealed that directional selection acting on amino acid replacement changes has driven the evolution of the Acp26Aa protein in the last 2.5 myr.

Paleontological studies indicate that insects have dominated regions marked by fauna since the late Paleozoic age. The timespan includes the evolutionary transformations of many life forms, including whales and dinosaurs. Evidence of the dominance and diversity of insects from an early age is being uncovered and will be discussed at the Fourth International Congress of Dipterology in Oxford, England in Sep 1998.

Modern stoneflies offer possible evidence of the evolution of insect wings from the articulated gills of water-dwelling ancestors. Stoneflies of the genus Leuctra are observed to use a form of locomotion that is mechanically halfway between skimming and flying. They are sometimes found to be able to ascend above the water surface and actually fly at an air temperature of 12 degrees centigrade. The stoneflies' different forms of surface locomotion suggest the occurrence of gradual improvements in aerodynamic locomotion while aquatic organisms evolved into flying insects.

All insects exhibit similar stages of embryonic development despite their different adult morphologies and the varying environments that they occupy. Genetic and molecular studies explain the development of the germ-band stage embryo of the Dipteran Drosophila melanogaster. Examination of different species constituting insect phylogeny will help understand the evolutionary mechanism of insects.

Insects and vertebrates exhibit similarities in immune defenses despite the differences in their respective organ systems. The immunity system of both insects and vertebrates depend on rapid forces such as natural or innate immunity. Insect embryos in egg-shells communicate with their immediate surroundings through their immunity system, and the main axis of the embryo is aligned correctly with those of the egg-shell.