Sculpting reproductive circuits: relationships among hormones, morphology and behavior in anole lizards

Characterizing seasonal transitions: Breeding-like morphology and behavior during the late non-breeding season in green anole lizards

Seasonally breeding animals, such as green anole lizards (Anolis carolinensis), allow for the examination of the control of reproduction during different reproductive states. During the breeding season, the gonads are large and reproductively active. Following the breeding season, gonads regress and become less active, and the lizards enter a refractory period where breeding is inhibited. After this stage, a post-refractory period occurs during which the lizards are still in a non-breeding state, but environmental changes can trigger the onset of breeding. However, it is unclear what causes these changes in reproductive state and we hypothesized that this may be due to alterations in gonadotropin-releasing hormone (GnRH) signaling. The present study aimed to identify morphological and behavioral differences in GnRH- and saline-injected refractory and post-refractory male anoles when housed under the same non-breeding environmental conditions. We found that post-refractory anoles had increased testicular weight, recrudescence, sperm presence, and reproductive behavior, with no impact of GnRH injection. Renal sex segment size and steroidogenic acute regulatory protein (StAR) mRNA levels did not differ among groups, indicating that testosterone levels likely had not increased in post-refractory lizards. Post-refractory anoles in this study were beginning to transition towards a breeding state without exposure to changing environmental conditions, and GnRH was not necessary for these changes. These data reveal a complex interaction between the activation of breeding, changing environmental conditions, and the underlying physiology regulating reproduction in seasonally breeding lizards. Future studies are needed to further elucidate the mechanisms that regulate this relationship.

Recent Secondary Contacts, Linked Selection, and Variable Recombination Rates Shape Genomic Diversity in the Model Species Anolis carolinensis

Gaining a better understanding on how selection and neutral processes affect genomic diversity is essential to gain better insights into the mechanisms driving adaptation and speciation. However, the evolutionary processes affecting variation at a genomic scale have not been investigated in most vertebrate lineages. Here, we present the first population genomics survey using whole genome resequencing in the green anole (Anolis carolinensis). Anoles have been intensively studied to understand mechanisms underlying adaptation and speciation. The green anole in particular is an important model to study genome evolution. We quantified how demography, recombination, and selection have led to the current genetic diversity of the green anole by using whole-genome resequencing of five genetic clusters covering the entire species range. The differentiation of green anole's populations is consistent with a northward expansion from South Florida followed by genetic isolation and subsequent gene flow among adjacent genetic clusters. Dispersal out-of-Florida was accompanied by a drastic population bottleneck followed by a rapid population expansion. This event was accompanied by male-biased dispersal and/or selective sweeps on the X chromosome. We show that the interaction between linked selection and recombination is the main contributor to the genomic landscape of differentiation in the anole genome.

Comparative Genomics Reveals Accelerated Evolution in Conserved Pathways during the Diversification of Anole Lizards

Squamates include all lizards and snakes, and display some of the most diverse and extreme morphological adaptations among vertebrates. However, compared with birds and mammals, relatively few resources exist for comparative genomic analyses of squamates, hampering efforts to understand the molecular bases of phenotypic diversification in such a speciose clade. In particular, the ∼400 species of anole lizard represent an extensive squamate radiation. Here, we sequence and assemble the draft genomes of three anole species-Anolis frenatus, Anolis auratus, and Anolis apletophallus-for comparison with the available reference genome of Anolis carolinensis. Comparative analyses reveal a rapid background rate of molecular evolution consistent with a model of punctuated equilibrium, and strong purifying selection on functional genomic elements in anoles. We find evidence for accelerated evolution in genes involved in behavior, sensory perception, and reproduction, as well as in genes regulating limb bud development and hindlimb specification. Morphometric analyses of anole fore and hindlimbs corroborated these findings. We detect signatures of positive selection across several genes related to the development and regulation of the forebrain, hormones, and the iguanian lizard dewlap, suggesting molecular changes underlying behavioral adaptations known to reinforce species boundaries were a key component in the diversification of anole lizards.

Evolution of Dosage Compensation in Anolis carolinensis, a Reptile with XX/XY Chromosomal Sex Determination

In species with highly heteromorphic sex chromosomes, the degradation of one of the sex chromosomes will result in unequal gene expression between the sexes (e.g. between XX females and XY males) and between the sex chromosomes and the autosomes. Dosage compensation is a process whereby genes on the sex chromosomes achieve equal gene expression. We compared genome-wide levels of transcription between males and females, and between the X chromosome and the autosomes in the green anole, Anolis carolinensis. We present evidence for dosage compensation between the sexes, and between the sex chromosomes and the autosomes. When dividing the X chromosome into regions based on linkage groups, we discovered that genes in the first reported X-linked region, anole linkage group b (LGb), exhibit complete dosage compensation, although the rest of the X-linked genes exhibit incomplete dosage compensation. Our data further suggest that the mechanism of this dosage compensation is upregulation of the X chromosome in males. We report that approximately 10% of coding genes, most of which are on the autosomes, are differentially expressed between males and females. In addition, genes on the X chromosome exhibited higher ratios of nonsynonymous to synonymous substitution than autosomal genes, consistent with the fast-X effect. Our results from the green anole add an additional observation of dosage compensation in a species with XX/XY sex determination.

The evolution of androgen receptor expression and behavior in Anolis lizard forelimb muscles

The motor systems that produce behavioral movements are among the primary targets for the action of steroid hormones, including androgens. Androgens such as testosterone bind to androgen receptors (AR) to induce physiological changes in the size, strength, and energetic capacity of skeletal muscles, which can directly influence the performance of behaviors in which those muscles are used. Because tissues differentially express AR, resulting in tissue-specific sensitivity to androgens, AR expression may be a major target of selection for the evolution of behavior. Anolis lizards (i.e., anoles) provide a robust system for the study of androgen-regulated traits, including the behavioral traits that facilitate social display and locomotion. In this study, we examined six anole species that demonstrate significant variation in the behavioral use of the forelimbs to measure the proportion of myonuclei in the bicep muscles that express AR. Using phylogenetic comparative analyses, we found that species with a greater proportion of nuclei positive for AR expression in the biceps exhibited greater frequencies of locomotor movements and pushup displays. These results suggest that AR expression in skeletal muscles may influence the evolution of androgen-regulated behaviors in this group.

Diversification in wild populations of the model organism Anolis carolinensis: A genome-wide phylogeographic investigation

The green anole (Anolis carolinensis) is a lizard widespread throughout the southeastern United States and is a model organism for the study of reproductive behavior, physiology, neural biology, and genomics. Previous phylogeographic studies of A. carolinensis using mitochondrial DNA and small numbers of nuclear loci identified conflicting and poorly supported relationships among geographically structured clades; these inconsistencies preclude confident use of A. carolinensis evolutionary history in association with morphological, physiological, or reproductive biology studies among sampling localities and necessitate increased effort to resolve evolutionary relationships among natural populations. Here, we used anchored hybrid enrichment of hundreds of genetic markers across the genome of A. carolinensis and identified five strongly supported phylogeographic groups. Using multiple analyses, we produced a fully resolved species tree, investigated relative support for each lineage across all gene trees, and identified mito‐nuclear discordance when comparing our results to previous studies. We found fixed differences in only one clade—southern Florida restricted to the Everglades region—while most polymorphisms were shared between lineages. The southern Florida group likely diverged from other populations during the Pliocene, with all other diversification during the Pleistocene. Multiple lines of support, including phylogenetic relationships, a latitudinal gradient in genetic diversity, and relatively more stable long‐term population sizes in southern phylogeographic groups, indicate that diversification in A. carolinensis occurred northward from southern Florida.

Sexually dimorphic expression of CREB binding protein in the green anole brain

Green anoles are seasonally breeding lizards in which male sexual behavior is primarily regulated by an annual increase in testosterone. This hormone activates stereotyped behaviors, as well as morphological and biochemical changes in the brain, with greater effect in the breeding season than in the non-breeding season. This study is the first description of CREB binding protein (CBP) in the reptilian brain, and investigates the possibility that changes in CBP, an androgen receptor coactivator, may facilitate differences in responsiveness to testosterone across seasons. A portion of this gene was cloned for the green anole, and in situ hybridization was performed to examine the expression of CBP in the brains of gonadally intact male and female green anoles across breeding states. Additionally, hormonal regulation of CBP was evaluated across sex and season in animals that were gonadectomized and treated with testosterone or a control. Similar to other vertebrates, CBP was expressed at relatively high levels in steroid-sensitive brain regions. In the anole ventromedial amygdala, CBP mRNA levels were nearly twice as high in gonadally intact females compared to males. In contrast, CBP expression did not differ across seasons or hormone manipulation in this brain region. No significant effects were detected in the preoptic area or ventromedial hypothalamus. This pattern suggests that CBP might influence female-biased functions controlled by the ventromedial amygdala, but is not consistent with a role in mediating seasonal differences in responsiveness to testosterone in these areas associated with reproductive function.

Relationships among sex, season and testosterone in the expression of androgen receptor mRNA and protein in the green anole forebrain

Sexual behavior in male green anole lizards is regulated by a seasonal increase in testosterone (T). However, T is much more effective at activating behavioral, morphological and biochemical changes related to reproduction in the breeding season (BS; spring) compared to nonbreeding season (NBS; fall). An increase in androgen receptor (AR) during the BS is one potential mechanism for this differential responsiveness. AR expression has not been investigated in specific brain regions across seasons in anoles. The present studies were designed to determine relative AR expression in areas important for male (preoptic area, ventromedial amygdala) and female (ventromedial hypothalamus) sexual behavior, as well as whether T upregulates AR in the anole brain. In situ hybridization and Western blot analyses were performed in unmanipulated animals across sex and season, as well as in gonadectomized animals with and without T treatment. Among hormone-manipulated animals, more cells expressing AR mRNA were detected in females than males in the amygdala. T treatment increased the volume of the ventromedial hypothalamus of gonadectomized animals in the BS, but not the NBS. AR protein in dissections of the hypothalamus and preoptic area was increased in males compared to females specifically in the BS. Additionally, among females, it was increased in the NBS compared to the BS. Collectively, these results indicate that differences in central AR expression probably do not facilitate a seasonal responsiveness to T. However, they are consistent with a role for AR in regulating some differences between sexes in the display of reproductive behaviors.

Endocrine regulation and sexual differentiation of avian copulatory sexually selected characters

Reproductive specializations in birds have provided intriguing model systems to better understand the role of endocrine mechanisms that regulate phenotype expression and the action of sexual selection. A comparative approach can elucidate how endocrine systems associated with control of sexual differentiation, sexual maturation, and reproductive physiology and behavior have diversified. Here we compare the copulatory sexually selected traits of two members of the galloanseriform superfamily: quail and ducks. Japanese quail have a non-intromittent penis, and they have evolved a unique foam gland that is known to be involved in post-copulatory sexual selection. In contrast, ducks have maintained a large intromittent penis that has evolved via copulatory male-male competition and has been elaborated in a sexually antagonistic race due to sexual conflict with females over mating. These adaptations function in concert with sex-specific and, in part, species-specific behaviors. Although the approaches to study these traits have been different, exploring the differences in neuroendocrine regulation of sexual behavior, development and seasonality of the foam gland and the penis side by side, allow us to suggest some areas where future research would be productive to better understand the evolution of novelty in sexually selected traits.

Transcriptomic analysis of tail regeneration in the lizard Anolis carolinensis reveals activation of conserved vertebrate developmental and repair mechanisms

Lizards, which are amniote vertebrates like humans, are able to lose and regenerate a functional tail. Understanding the molecular basis of this process would advance regenerative approaches in amniotes, including humans. We have carried out the first transcriptomic analysis of tail regeneration in a lizard, the green anole Anolis carolinensis, which revealed 326 differentially expressed genes activating multiple developmental and repair mechanisms. Specifically, genes involved in wound response, hormonal regulation, musculoskeletal development, and the Wnt and MAPK/FGF pathways were differentially expressed along the regenerating tail axis. Furthermore, we identified 2 microRNA precursor families, 22 unclassified non-coding RNAs, and 3 novel protein-coding genes significantly enriched in the regenerating tail. However, high levels of progenitor/stem cell markers were not observed in any region of the regenerating tail. Furthermore, we observed multiple tissue-type specific clusters of proliferating cells along the regenerating tail, not localized to the tail tip. These findings predict a different mechanism of regeneration in the lizard than the blastema model described in the salamander and the zebrafish, which are anamniote vertebrates. Thus, lizard tail regrowth involves the activation of conserved developmental and wound response pathways, which are potential targets for regenerative medical therapies.

Lizards and LINEs: selection and demography affect the fate of L1 retrotransposons in the genome of the green anole (Anolis carolinensis)

Autonomous retrotransposons lacking long terminal repeats (LTR) account for much of the variation in genome size and structure among vertebrates. Mammalian genomes contain hundreds of thousands of non-LTR retrotransposon copies, mostly resulting from the amplification of a single clade known as L1. The genomes of teleost fish and squamate reptiles contain a much more diverse array of non-LTR retrotransposon families, whereas copy number is relatively low. The majority of non-LTR retrotransposon insertions in nonmammalian vertebrates also appear to be very recent, suggesting strong purifying selection limits the accumulation of non-LTR retrotransposon copies. It is however unclear whether this turnover model, originally proposed in Drosophila, applies to nonmammalian vertebrates. Here, we studied the population dynamics of L1 in the green anole lizard (Anolis carolinensis). We found that although most L1 elements are recent in this genome, truncated insertions accumulate readily, and many are fixed at both the population and species level. In contrast, full-length L1 insertions are found at lower population frequencies, suggesting that the turnover model only applies to longer L1 elements in Anolis. We also found that full-length L1 inserts are more likely to be fixed in populations of small effective size, suggesting that the strength of purifying selection against deleterious alleles is highly dependent on host demographic history. Similar mechanisms seem to be controlling the fate of non-LTR retrotransposons in both Anolis and teleostean fish, which suggests that mammals have considerably diverged from the ancestral vertebrate in terms of how they interact with their intragenomic parasites.

Genetic variation in the green anole lizard (Anolis carolinensis) reveals island refugia and a fragmented Florida during the quaternary

The green anole lizard (Anolis carolinensis) is a model organism for behavior and genomics that is native to the southeastern United States. It is currently thought that the ancestors of modern green anoles dispersed to peninsular Florida from Cuba. However, the climatic changes and geological features responsible for the early diversification of A. carolinensis in North America have remained largely unexplored. This is because previous studies (1) differ in their estimates of the divergence times of populations, (2) are based on a single genetic locus or (3) did not test specific hypotheses regarding the geologic and topographic history of Florida. Here we provide a multi-locus study of green anole genetic diversity and find that the Florida peninsula contains a larger number of genetically distinct populations that are more diverse than those on the continental mainland. As a test of the island refugia hypothesis in Pleistocene Florida, we use a coalescent approach to estimate the divergence times of modern green anole lineages. We find that all demographic events occurred during or after the Upper Pliocene and suggest that green anole diversification was driven by population divergence on interglacial island refugia in Florida during the Lower Pleistocene, while the region was often separated from continental North America. When Florida reconnected to the mainland, two separate dispersal events led to the expansion of green anole populations across the Atlantic Seaboard and Gulf Coastal Plain.

Seasonal and sexual dimorphisms in expression of androgen receptor and its coactivators in brain and peripheral copulatory tissues of the green anole

Green anoles are seasonally breeding lizards, with an annual rise in testosterone (T) being the primary activator of male sexual behaviors. Responsiveness to T is decreased in the non-breeding season (NBS) compared to breeding season (BS) on a variety of levels, including displays of reproductive behavior and the morphology and biochemistry of associated tissues. To evaluate the possibility that seasonal changes in responsiveness to T are regulated by androgen receptors (AR) and/or two of its coactivators, CREB binding protein (CBP) and steroid receptor coactivator-1 (SRC-1), we tested whether they differ in expression across season in brains of both sexes and in peripheral copulatory tissues of males (hemipenis and retractor penis magnus muscle). AR mRNA was increased in the brains of males compared to females and in copulatory muscle in the BS compared to NBS. In the hemipenis, transcriptional activity appeared generally diminished in the NBS. T-treatment increased AR mRNA in the copulatory muscle and AR protein in the hemipenis, the latter to a greater extent in the BS than the NBS. T also decreased SRC-1 protein in hemipenis. Interpretations are complicated, in part because levels of mRNA and protein expression were not correlated and multiple sizes of the AR and CBP proteins were detected, with some tissue specificity. However, the results are consistent with the idea that differences in receptor and coactivator expression at central and peripheral levels may play roles in regulating sex and seasonal differences in the motivation or physical ability to engage in sexual behavior.