Comparative histology of the vaginal-cloacal region in Squamata and its phylogenetic implications

Evolution and Morphology of Genitalia in Female Amniotes

Despite their evolutionary and biomedical importance, studies of the morphology and function of female genitalia have continued to lag behind those of male genitalia. While studying female genitalia can be difficult because of their soft, deformable and internal nature, recent advances in imaging, geometric analyses of shape and mechanical testing have been made, allowing for a much greater understanding of the incredible diversity of form and function of female genitalia. Here we summarize some of these methods, as well as discuss some big questions in the field that are beginning to be examined now, and will continue to benefit from further work, especially a comparative approach. Topics of further research include examination of the morphology of female genitalia in situ, in-depth anatomical work in many more species, studies of the interplay between natural and sexual selection in influencing features of vaginal morphology, how these diverse functions influence the mechanical properties of tissues, and studies of clitoris morphology and function across amniotes. Many other research topics related to female genitalia remain largely unexplored, and we hope that the papers in this issue will continue to inspire further research on female genitalia.

Divergent Genital Morphologies and Female-Male Covariation in Watersnakes

Genital evolution can be driven by diverse selective pressures. Across taxa we see evidence of covariation between males and females, as well as divergent genital morphologies between closely related species. Quantitative analyses of morphological changes in coevolving male and female genitalia have not yet been shown in vertebrates. This study uses 2D and 3D geometric morphometrics to quantitatively compare the complex shapes of vaginal pouches and hemipenes across three species of watersnakes (the sister taxa Nerodia fasciata, N. sipedon, and a close relative N. rhombifer) to address the relationship between genital morphology and divergence time in a system where sexual conflict may have driven sexually antagonistic coevolution of genital traits. Our pairwise comparisons of shape differences across species show that the sister species have male and female genitalia that are significantly different from each other, but more similar to each other than to N. rhombifer. We also determine that the main axes of shape variation are the same for males and females, with changes that relate to deeper bilobation of the vaginal pouch and hemipenes. In males, the protrusion of the region of spines at the base of the hemipene trades off with the degree of bilobation, suggesting amelioration of sexual conflict, perhaps driven by changes in the relative size of the entrance of the vaginal pouch that could have made spines less effective.

Anatomy and Physiology of the Reptile Renal System

Reptile kidneys maintain a constant extracellular environment within the body. They excrete waste products, maintain normal concentrations of salt and water, regulate acid-base balance, and produce hormones and vitamins. The kidneys contain nephrons consisting of glomeruli designed to filter the plasma, Bowman capsules that collect the filtrate, and tubules that resorb most of the filtered water and nutrients while excreting waste metabolites. A Loop of Henle is absent. Therefore, reptile kidneys cannot produce a hypertonic urine. The urinary bladder (if present) and cloaca excrete and absorb additional fluids and electrolytes. A renal portal system is present in all reptiles.

Australian lizards are outstanding models for reproductive biology research

Australian lizards are a diverse group distributed across the continent and inhabiting a wide range of environments. Together, they exhibit a remarkable diversity of reproductive morphologies, physiologies, and behaviours that is broadly representative of vertebrates in general. Many reproductive traits exhibited by Australian lizards have evolved independently in multiple lizard lineages, including sociality, complex signalling and mating systems, viviparity, and temperature-dependent sex determination. Australian lizards are thus outstanding model organisms for testing hypotheses about how reproductive traits function and evolve, and they provide an important basis of comparison with other animals that exhibit similar traits. We review how research on Australian lizard reproduction has contributed to answering broader evolutionary and ecological questions that apply to animals in general. We focus on reproductive traits, processes, and strategies that are important areas of current research, including behaviours and signalling involved in courtship; mechanisms involved in mating, egg production, and sperm competition; nesting and gestation; sex determination; and finally, birth in viviparous species. We use our review to identify important questions that emerge from an understanding of this body of research when considered holistically. Finally, we identify additional research questions within each topic that Australian lizards are well suited for reproductive biologists to address.

Are hemipenial traits under sexual selection in Tropidurus lizards? Hemipenial development, male and female genital morphology, allometry and coevolution in Tropidurus torquatus (Squamata: Tropiduridae)

Male genitalia show considerable morphological variation among animals with internal fertilization and exhibit a high level of evolvability in lizards. Studies have suggested that sexual selection may be driving hemipenial evolution against natural selection and pleiotropy. Given the direct interaction of male and female genitals, coevolution of the aforementioned is posited by several hypotheses of genital evolution. However, there are only a few studies on female genitalia morphology, resulting in a lack of coevolution description and understanding. Studies of allometric patterns have filled some gaps by answering questions about male genital evolution and could prove a powerful tool in clarifying coevolution between male and female genitals. Here, we studied the genital morphology of Tropidurus torquatus. This Tropidurus lizard species is an emerging Neotropical lizard model organism notable for having enlarged hemipenial lobes in contrast with other tropidurid species. In this study, we analyzed hemipenial development in early and late stages, describing both morphological variation and ontogenetic allometric pattern. We used quantitative traits to describe male and female genital morphology, examining their static allometric patterns and correspondence. Our study provides a quantitative discussion on the evolution of lizard genitals, suggesting that sexual selection plays an important role in genital evolution in Tropidurus lizards.

Genital morphology associated with mating strategy in the polymorphic lizard, Uta stansburiana

Sexual selection can lead to rapid evolution of sexual traits and striking morphological diversity across taxa. In populations where competition for mates is intense, males sometimes evolve distinct behavioral strategies along with morphological differences that help them secure mating opportunities. Strong postcopulatory selection and differential resource allocation across male strategy type can result in strategy-specific differences in sexual traits, such as sperm morphology, ejaculate components, and testis size. Some polymorphic species also have strategy-specific genital morphology. Thus far, among vertebrates, this has only been observed in fish. Here, we present the first morphological description of the intromittant copulatory organ, the hemipenis, of the three mating types of the side-blotched lizard, Uta stansburiana, from a population that exhibits alternative mating strategies. We found that the isometrically scaling hemipenis was shortest in the nonterritorial (yellow) morph that sneaks copulations with other males' mates. Although the hemipenes were generally the same shape across morphs, the usurping territorial (orange) morph had a significantly wider apical horn than the nonterritorial sneaker morph. Sneaker males also had smaller relative body masses than both the mate-guarding (blue) morph and the usurper morph, and shorter tibia than the usurper morph. This study using a small sample of males suggests that strong sexual selection may drive genital trait differentiation across morphs within populations of terrestrial vertebrates.

Evolution of the cloacal and genital musculature, and the genitalia morphology in liolemid lizards (Iguania: Liolaemidae) with remarks on their phylogenetic bearing

In this study, we describe the intra- and interspecific anatomical variations of cloacal and related muscles of male and female genitalia in species of five iguanian genera (three liolemid:Ctenoblepharys,Liolaemus, andPhymaturusplusDiplolaemusleopardinusandTropidurusmelanopleurusas outgroups). We found variations (seventeen characters) in topology, origin and insertion areas, tendon morphology and size of the musculature of this region. We also describe the variations of hemipeneal morphology, which is especially notable for the hemipenis ofC. adspersa,D. leopardinus, andT. melanopleurus, as this is first time they are described in the literature. Among the most significant findings are the identification of three new muscles, two of them inserted on the roof of the cloacal chamber (anterior and posterior cloacal retractor) and the third inserted superficially in the floor of the cloaca, just before anterior to the precloacal glands row (superficialis cloacalis retractor). We report sexual dimorphism in seven muscle characters. Musculature related to hemiclitoris is reduced in proportion to its size in comparison to the degree of development of male genitalia and associated musculature. The evolution of characters was traced on the known phylogenetic hypotheses of relationships among families. Characters taken from the cloacal/genital myology bring similar support to the liolaemid tree even rooting the analysis using different outgroups. In addition, a phylogenetic study using only myological characters was performed. In this case,C. adspersawas found to be more related toLiolaemusspecies instead of being basal toLiolaemusplusPhymaturus.

Spermiogenesis in the imbricate alligator lizard, Barisia imbricata (Reptilia, Squamata, Anguidae)

Although the events of spermiogenesis are commonly studied in amniotes, the amount of research available for Squamata is lacking. Many studies have described the morphological characteristics of mature spermatozoa in squamates, but few detail the ultrastructural changes that occur during spermiogenesis. This study's purpose is to gain a better understanding of the subcellular events of spermatid development within the Imbricate Alligator Lizard, Barisia imbricata. The morphological data presented here represent the first complete ultrastructural study of spermiogenesis within the family Anguidae. Samples of testes from four specimens collected on the northwest side of the Nevado de Toluca, México, were prepared using standard techniques for transmission electron microscopy. Many of the ultrastructural changes occurring during spermiogenesis within B. imbricata are similar to that of other squamates (i.e., early acrosome formation, chromatin condensation, flagella formation, annulus present, and a prominent manchette). However, there are a few unique characteristics within B. imbricata spermatids that to date have not been described during spermiogenesis in other squamates. For example, penetration of the acrosomal granule into the subacrosomal space to form the basal plate of the perforatorium during round spermatid development, the clover-shaped morphology of the developing nuclear fossa of the flagellum, and the bulbous shape to the perforatorium are all unique to the Imbricate Alligator Lizard. These anatomical character differences may be valuable nontraditional data that along with more traditional matrices (such as DNA sequences and gross morphological data) may help elucidate phylogenetic relationships, which are historically considered controversial within Squamata.

Oviductal structure and ultrastructure of the oviparous gecko, Hemidactylus mabouia (Moreau De Jonnès, 1818)

Lizards of the family Gekkonidae display a variety of reproductive patterns, as evidenced by the presence of viviparous and oviparous species. The species Hemidactylus mabouia is oviparous. We examined, in vitellogenic females, oviductal structure by light microscopy after routine histological and histochemical techniques, as well as by scanning and transmission electron microscopy. The oviduct is composed of four different regions: the infundibulum, which opens into the coelomic cavity and receives the oocyte released at the time of ovulation; the uterine tube, where sperm storage takes place; the uterus, which is responsible for the eggshell production; and the vagina, the final portion of the oviduct that leads to the cloaca. The oviductal structure of H. mabouia is similar to that of other oviparous lizard species and can be useful for morphological comparative analysis among reptile species.

Controversial snake relationships supported by reproductive anatomy

Since the advent of molecular character sets in phylogenetic systematics our understanding of the evolutionary history of snakes has changed considerably. In some cases the novel topologies reconstructed from molecular datasets have left researchers puzzled, as no morphological feature seems to support the new relationships found. This is the case for 'Amerophidia'sensu Vidal et al. (2007; Biology of the Boas and Pythons, Eagle Mountain: Eagle Mountain Publishing; Aniliidae+ Tropidophiidae), a grouping of the Red Pipesnakes and Neotropical Dwarf Boas. We contend that in some cases the apparent lack of historical morphological support for the molecular phylogenies is due to our poor understanding of the organisms as a whole, and not the complete lack of morphological support for controversial clades. For example, we found novel evidence from reproductive anatomy that demonstrates a unique association of the oviducts and cloaca in Amerophidia. Whereas in all other female squamates the oviducts communicate directly with the cloaca, the oviducts of Aniliidae and Tropidophiidae communicate with diverticuli of the cloaca. At present this is the only unambiguous synapomorphy for the Amerophidia. We feel that confirmation of controversial molecular relationships will revolve around the investigation of non-traditional morphological characters such as reproductive anatomy.