Fetal nutrition in lecithotrophic squamate reptiles: Toward a comprehensive model for evolution of viviparity and placentation

The primary pattern of embryonic nutrition for squamate reptiles is lecithotrophy; with few exceptions, all squamate embryos mobilize nutrients from yolk. The evolution of viviparity presents an opportunity for an additional source of embryonic nutrition through delivery of uterine secretions, or placentotrophy. This pattern of embryonic nutrition is thought to evolve through placental supplementation of lecithotrophy, followed by increasing dependence on placentotrophy. This review analyzes the relationship between reproductive mode and pattern of embryonic nutrition in three lecithotrophic viviparous species, and oviparous counterparts, for concordance with a current model for the evolution of viviparity and placentation. The assumptions of the model, that nutrients for oviparous embryos are mobilized from yolk, and that this source is not disrupted in the transition to viviparity, are supported for most nutrients. In contrast, calcium, an essential nutrient for embryonic development, is mobilized from both yolk and eggshell by oviparous embryos and reduction of eggshell calcium is correlated with viviparity. If embryonic fitness is compromised by disruption of a primary source of calcium, selection may not favor evolution of viviparity, yet viviparity has arisen independently in numerous squamate lineages. Studies of fetal nutrition in reproductively bimodal species suggest a resolution to this paradox. If uterine calcium secretion occurs during prolonged intrauterine egg retention, calcium placentotrophy evolves prior to viviparity as a replacement for eggshell calcium and embryonic nutrition will not be compromised. This hypothesis is integrated into the current model for evolution of viviparity and placentation to address the unique attributes of calcium nutrition. The sequence of events requires a shift in timing of uterine calcium secretion and the embryonic mechanism of calcium retrieval to be responsive to calcium availability. Regulation of uterine calcium secretion and the mechanism of embryonic uptake of calcium are important elements to understanding evolution of viviparity and placentation. J. Morphol., 2013. © 2013 Wiley Periodicals, Inc.     

Evolution of placentation among squamate reptiles: recent research and future directions

Squamate reptiles are uniquely suited to study of evolution of reproductive mode and pattern of embryonic nutrition. Viviparous species have evolved from oviparous ancestors on numerous occasions, patterns of nutritional provision to embryos range widely from lecithotrophy, at one end of a continuum, to placentotrophy at the other, and structure and function of the maternal-embryonic relationship is highly constrained resulting in parallel evolutionary trajectories among taxa. Embryos of oviparous species primarily receive nourishment from yolk, but also mobilize a significant quantity of calcium from the eggshell. Most viviparous species also are predominantly lecithotrophic, yet all viviparous species are placentotrophic to some degree. Similarities in embryonic development and nutritional pattern between oviparous species and most viviparous species suggest that the pattern of nutrition of oviparous squamates is an exaptation for the evolution of viviparity and that placentotrophy and viviparity evolve concomitantly. The few species of squamates that rely substantially on placentotrophy have structural modifications of the interface between the embryo and mother that are interpreted as adaptations to enhance nutritional exchange. Recent studies have extended understanding of the diversity of embryonic nutrition and placental structure and have resulted in hypotheses for transitions in the evolution of placentotrophy, yet data are available for few species. Indirect tests of these hypotheses, by comparison of structural-functional relationships among clades in which viviparity has evolved, awaits further study of the reproductive biology of squamates.     

Vertebral evolution and the diversification of squamate reptiles

Taxonomic, morphological, and functional diversity are often discordant and independent components of diversity. A fundamental and largely unanswered question in evolutionary biology is why some clades diversify primarily in some of these components and not others. Dramatic variation in trunk vertebral numbers (14 to >300) among squamate reptiles coincides with different body shapes, and snake-like body shapes have evolved numerous times. However, whether increased evolutionary rates or numbers of vertebrae underlie body shape and taxonomic diversification is unknown. Using a supertree of squamates including 1375 species, and corresponding vertebral and body shape data, we show that increased rates of evolution in vertebral numbers have coincided with increased rates and disparity in body shape evolution, but not changes in rates of taxonomic diversification. We also show that the evolution of many vertebrae has not spurred or inhibited body shape or taxonomic diversification, suggesting that increased vertebral number is not a key innovation. Our findings demonstrate that lineage attributes such as the relaxation of constraints on vertebral number can facilitate the evolution of novel body shapes, but that different factors are responsible for body shape and taxonomic diversification.     

Implantation, development of the fetal membranes, and placentation in the captive black mastiff bat, Molossus ater

Uterine events during pregnancy were examined histologically in laboratory-bred black mastiff bats (Molossus ater) as part of an effort to develop this species as a model for studies of the factors controlling trophoblastic growth. Embryos entered the uterus at the morula stage and in most cases shed their zonae pellucidae reasonably intact, apparently as blastocyst expansion occurred. Implantation was superficial and observed to occur only in the right uterine horn. During implantation to the endometrium by both blastocyst expansion and closure of the uterine lumen. A decidual reaction was evident at an early stage of uterine epithelial displacement and spread rapidly through the endometrium. Initial trophoblastic proliferation occurred along the uterine lumen and into the glands, while its invasion of the endometrial stroma was delayed. Although one or several primordial cavities have been observed to develop within the epiblast during implantation, these subsequently opened to a trophoepiblastic cavity, and the definitive amnion was formed by folding. A choriovitelline placenta was present briefly at thesomite stage, but disappeared as the exocoelom enlarged and the yolk sac collapsed. The latter persisted through pregnancy, however, as a glandular-appearing body. As the yolk sac retracted from the chorion, it was replaced by allantoic mesoderm, creating a diffuse labyrinthine endotheliodichorial placenta. This was prominent during mid-gestation, but was supplanted by the discoidal hemochorial placenta as the major site of feto-maternal exchange during late pregnancy.     

Climatic correlates of live-bearing in squamate reptiles

Summary We use a stepwise multiple regression procedure to correlate geographic patterns in the distribution of live-bearing reptilian species with patterns in climatic variables, in both Australia and North America. Previous authors have interpreted reptilian live-bearing as an evolutionary adaptation to cold climates. Our results indicate that environmental temperature and irradiance measures are no more highly correlated with the percent live-bearing species than are measures of precipitation, evaporation and humidity. We conclude that, except in very cold environments in North America, environmental temperatures seem to play little role in the relative success of live-bearing versus egg-laying reptilian reproductive strategies. It appears from previous work that reptilian live-bearing evolves mainly, or exclusively, because of the advantage it confers in enabling successful reproduction in cold climates. The present study suggests that the subsequent radiation of live-bearing reptilian species may be due to entirely different selective forces.     

A developmental synapomorphy of squamate reptiles

The reptilian clade Squamata is defined primarily by osteological synapomorphies, few of which are entirely unambiguous. Studies of developing squamate eggs have revealed a uniquely specialized feature not known to occur in any other amniotes. This feature—the yolk cleft/isolated yolk mass complex—lines the ventral hemisphere of the egg. During its formation, extraembryonic mesoderm penetrates the yolk and an exocoelom (the yolk cleft [YC]) forms in association with it, cutting off a thin segment of yolk (the “isolated yolk mass” [IYM]) from the main body of the yolk. The YC–IYM complex has been observed and described in more than 65 squamate species in 12 families. In viviparous species, it contributes to the “omphaloplacenta,” a type of yolk sac placenta unique to squamates. The only squamates known to lack the IYM are a few highly placentotrophic skinks with minuscule eggs, viviparous species in which it clearly has been lost. Given its absence in mammals, chelonians, crocodylians, and birds, the YC–IYM complex warrants recognition as a developmental synapomorphy of the squamate clade. As in extant viviparous lizards and snakes, the YC–IYM complex presumably contributed to the placenta of extinct viviparous squamates.     

Evolutionary diversification of clades of squamate reptiles

We analysed the diversification of squamate reptiles (7488 species) based on a new molecular phylogeny, and compared the results to similar estimates for passerine birds (5712 species). The number of species in each of 36 squamate lineages showed no evidence of phylogenetic conservatism. Compared with a random speciation-extinction process with parameters estimated from the size distribution of clades, the alethinophidian snakes (2600 species) were larger than expected and 13 clades, each having fewer than 20 species, were smaller than expected, indicating rate heterogeneity. From a lineage-through-time plot, we estimated that a provisional rate of lineage extinction (0.66 per Myr) was 94% of the rate of lineage splitting (0.70 per Myr). Diversification in squamate lineages was independent of their stem age, but strongly related to the area of the region within which they occur. Tropical vs. temperate latitude exerted a marginally significant influence on species richness. In comparison with passerine birds, squamates share several clade features, including: (1) independence of species richness and age; (2) lack of phylogenetic signal with respect to clade size; (3) general absence of exceptionally large clades; (4) over-representation of small clades; (5) influence of region size on clade size; and (6) similar rates of speciation and extinction. The evidence for both groups suggests that clade size has achieved long-term equilibrium, suggesting negative feedback of species richness on the rate of diversification.     

History and the global ecology of squamate reptiles

The structure of communities may be largely a result of evolutionary changes that occurred many millions of years ago. We explore the historical ecology of squamates (lizards and snakes), identify historically derived differences among clades, and examine how this history has affected present-day squamate assemblages globally. A dietary shift occurred in the evolutionary history of squamates. Iguanian diets contain large proportions of ants, other hymenopterans, and beetles, whereas these are minor prey in scleroglossan lizards. A preponderance of termites, grasshoppers, spiders, and insect larvae in their diets suggests that scleroglossan lizards harvest higher energy prey or avoid prey containing noxious chemicals. The success of this dietary shift is suggested by dominance of scleroglossans in lizard assemblages throughout the world. One scleroglossan clade, Autarchoglossa, combined an advanced vomeronasal chemosensory system with jaw prehension and increased activity levels. We suggest these traits provided them a competitive advantage during the day in terrestrial habitats. Iguanians and gekkotans shifted to elevated microhabitats historically, and gekkotans shifted activity to nighttime. These historically derived niche differences are apparent in extant lizard assemblages and account for some observed structure. These patterns occur in a variety of habitats at both regional and local levels throughout the world.     

Convergent evolution and character correlation in burrowing reptiles: towards a resolution of squamate relationships

The affinities of three problematic groups of elongate, burrowing reptiles (amphisbaenians, dibamids and snakes) are reassessed through a phylogenetic analysis of all the major groups of squamates, including the important fossil taxa Sineoamphisbaena, mosasauroids and Pachyrhachis; 230 phylogenetically informative osteological characters were evaluated in 22 taxa. Snakes (including Pachyrhachis) are anguimorphs, being related firstly to large marine mosasauroids, and secondly to monitor lizards (varanids). Scincids and cordylids are not related to lacertiforms as previously thought, but to anguimorphs. Amphisbaenians and dibamids are closely related, and Sineoamphisbaena is the sister group to this clade. The amphisbaenian-dibamid-Sineoamphisbaena clade, in turn, is related to gekkotans and xantusiids. When the fossil taxa are ignored, snakes, amphisbaenians and dibamids form an apparently well-corroborated clade nested within anguimorphs. However, nearly all of the characters supporting this arrangement are correlated with head-first burrowing (miniaturization, cranial consolidation, body elongation, limb reduction), and invariably co-occur in other tetrapods with similar habits. These characters are potentially very misleading because of their sheer number and because they largely represent reductions or losses. It takes very drastic downweighting of these linked characters to alter tree topology: if fossils are excluded from the analysis, a (probably spurious) clade consisting of elongate, fossorial taxa almost always results. These results underscore the importance of including all relevant taxa in phylogenetic analyses. Inferring squamate phylogeny depends critically on the inclusion of certain (fossil) taxa with combinations of character states that demonstrate convergent evolution of the elongate, fossorial ecomorph in amphisbaenians and dibamids, and in snakes. In the all-taxon analysis, the position of snakes within anguimorphs is more strongly-corroborated than the association of amphisbaenians and dibamids with gekkotans. When the critical fossil taxa are deleted, snakes ‘attract’ the amphisbaenian-dibamid clade on the basis of a suite of correlated characters. While snakes remain anchored in anguimorphs, the amphisbaenian-dibamid clade moves away from gekkotans to join them. Regardless of the varying positions of the three elongate burrowing taxa, the interrelationships between the remaining limbed squamates (‘lizards’) are constant; thus, the heterodox affinities of scincids, cordylids, and xantusiids identified in this analysis appear to be robust. Finally, the position of Pachyrhachis as a basal snake rather than (as recently suggested) a derived snake is supported on both phylogenetic and evolutionary grounds.     

Correlated evolution between targets of pre‐ and postcopulatory sexual selection across squamate reptiles

Sexual selection reflects the joint contributions of precopulatory selection, which arises from variance in mating success, and postcopulatory selection, which arises from variance in fertilization success. The relative importance of each episode of selection is variable among species, and comparative evidence suggests that traits targeted by precopulatory selection often covary in expression with those targeted by postcopulatory selection when assessed across species, although the strength and direction of this association varies considerably among taxa. We tested for correlated evolution between targets of pre‐ and postcopulatory selection using data on sexual size dimorphism (SSD) and testis size from 151 species of squamate reptiles (120 lizards, 31 snakes). In squamates, male–male competition for mating opportunities often favors large body size, such that the degree of male‐biased SSD is associated with the intensity of precopulatory selection. Likewise, competition for fertilization often favors increased sperm production, such that testis size (relative to body size) is associated with the intensity of postcopulatory selection. Using both conventional and phylogenetically based analyses, we show that testis size consistently decreases as the degree of male‐biased SSD increases across lizards and snakes. This evolutionary pattern suggests that strong precopulatory selection may often constrain the opportunity for postcopulatory selection and that the relative importance of each selective episode may determine the optimal resolution of energy allocation trade‐offs between traits subject to each form of sexual selection.     

Functional and ecological correlates of ecologically-based dimorphisms in squamate reptiles

Sexual dimorphism in phenotypic traits associated with the useof resources is a widespread phenomenon throughout the animalkingdom. While ecological dimorphisms are often initially generatedby sexual selection operating on an animal's size, natural selectionis believed to maintain, or even amplify, these dimorphismsin certain ecological settings. The trophic apparatus of snakeshas proven to be a model system for testing the adaptive natureof ecological dimorphisms because head size is rarely undersexual selection and it limits the maximum ingestible size ofprey in these gape-limited predators. Significantly less attentionhas been paid to the evolution of ecological dimorphisms inlizards, however, which may be due to the fact that lizards’feeding apparatus can be under both sexual and natural selectionsimultaneously, making it difficult to formulate clear-cut hypothesesto distinguish between the influences of natural and sexualselection. In order to tease apart the respective influencesof natural selection and sexual selection on the feeding apparatusof squamates, we take an integrative approach to formulate twohypotheses for snakes and lizards, respectively: (1) For gape-limitedsnakes, we predict that natural selection will act to generatedifferences in maximum gape, which will translate into differencesin maximum ingestible prey size between the sexes. (2) For lizardswhich mechanically reduce their prey, we predict that the degreeof dimorphism in head size should be positively correlated tothe degree of dimorphism in bite force which, in turn, shouldbe correlated to dimorphism in aspects of size or hardness ofprey. Finally, we predict that functional differences in thefeeding apparatus of these animals will also be linked withdifferences in sex-based feeding behavior and with selectionof prey.     

Rampant horizontal transfer of SPIN transposons in squamate reptiles

Transposable elements (TEs) are highly abundant in the genome and capable of mobility, two properties that make them particularly prone to transfer horizontally between organisms. Although the impact of horizontal transfer (HT) of TEs is well recognized in prokaryotes, the frequency of this phenomenon and its contribution to genome evolution in eukaryotes remain poorly appreciated. Here, we provide evidence that a DNA transposon called SPIN has colonized the genome of 17 species of reptiles representing nearly every major lineage of squamates, including 14 families of lizards, snakes, and amphisbaenians. Slot blot analyses indicate that SPIN has amplified to high copy numbers in most of these species, ranging from 2,000-28,000 copies per haploid genome. In contrast, we could not detect the presence of SPIN in any of the turtles (seven species from seven families) and crocodiles (four species) examined. Genetic distances between SPIN sequences from species belonging to different squamate families are consistently very low (average = 0.1), considering the deep evolutionary divergence of the families investigated (most are >100 My diverged). Furthermore, these distances fall below interfamilial distances calculated for two genes known to have evolved under strong functional constraint in vertebrates (RAG1, average = 0.24 and C-mos, average = 0.27). These data, combined with phylogenetic analyses, indicate that the widespread distribution of SPIN among squamates is the result of at least 13 independent events of HTs. Molecular dating and paleobiogeographical data suggest that these transfers took place during the last 50 My on at least three different continents (North America, South America and, Africa). Together, these results triple the number of known SPIN transfer events among tetrapods, provide evidence for a previously hypothesized transoceanic movement of SPIN transposons during the Cenozoic, and further underscore the role of HT in the evolution of vertebrate genomes.     

Egg teeth of squamate reptiles and their phylogenetic significance

Original and published data on the structure of egg teeth in squamate reptiles and the phylogenetic significance of corresponding characters are reviewed, elaborating A.M. Sergeev’s ideas on the subject. Problems are discussed concerning the use of this character in modern phylogenetic constructions and the necessity of new embryological investigations to resolve the issue concerning the formation of an unpaired egg tooth rudiment in all Squamata except the Dibamidae and Gekkota.     

Determinate growth is predominant and likely ancestral in squamate reptiles

Body growth is typically thought to be indeterminate in ectothermic vertebrates. Indeed, until recently, this growth pattern was considered to be ubiquitous in ectotherms. Our recent observations of a complete growth plate cartilage (GPC) resorption, a reliable indicator of arrested skeletal growth, in many species of lizards clearly reject the ubiquity of indeterminate growth in reptiles and raise the question about the ancestral state of the growth pattern. Using X-ray micro-computed tomography (µCT), here we examined GPCs of long bones in three basally branching clades of squamate reptiles, namely in Gekkota, Scincoidea and Lacertoidea. A complete loss of GPC, indicating skeletal growth arrest, was the predominant finding. Using a dataset of 164 species representing all major clades of lizards and the tuataras, we traced the evolution of determinate growth on the phylogenetic tree of Lepidosauria. The reconstruction of character states suggests that determinate growth is ancestral for the squamate reptiles (Squamata) and remains common in the majority of lizard lineages, while extended (potentially indeterminate) adult growth evolved several times within squamates. Although traditionally associated with endotherms, determinate growth is coupled with ectothermy in this lineage. These findings combined with existing literature suggest that determinate growth predominates in both extant and extinct amniotes.