Did Lizards Follow Unique Pathways in Sex Chromosome Evolution?

Comparative Cytogenetics of the Malagasy Ground Geckos of the Paroedura bastardi and Paroedura picta Species Groups

We present a comparative chromosome study of several taxa of the Malagasy ground geckos of the Paroedura bastardi and P. picta species groups. We employed a preliminary molecular analysis using a trait of the mitochondrial 16S rRNA gene (of about 570 bp) to assess the taxonomic status of the samples studied and a cytogenetic analysis with standard karyotyping (5% Giemsa solution), silver staining (Ag-NOR staining) and sequential C-banding (C-banding + Giemsa and + fluorochromes). Our results show that all the taxa studied of the P. bastardi group (P. ibityensis, P. rennerae and P. cf. guibeae) have a similar karyotype composed of 2n = 34 chromosomes, with two metacentric pairs (1 and 3) and all other pairs being acrocentric. Chromosome diversification in the P. bastardi group was mainly linked to the diversification of heteromorphic sex chromosome systems (ZZ/ZW) in P. ibityensis and P. rennerae, while no heteromorphic sex chromosome pair was found in P. cf. guibeae. The two taxa investigated of the P. picta species group (here named P. picta and P. cf. picta based on molecular data) showed the same chromosome number of 2n = 36, mostly acrocentric elements, but differed in the number of metacentric elements, probably as a result of an inversion at chromosome pair 2. We highlight that the genus Paroedura is characterized by the independent diversification of heterogametic sex chromosomes in different evolutionary lineages and, similarly to other phylogenetically related gecko genera, by a progressive formation of a biarmed element by means of tandem fusions and inversions of distinct pairs.

The First Identification of Homomorphic XY Sex Chromosomes by Integrating Cytogenetic and Transcriptomic Approaches in Plestiodon elegans (Scincidae)

The sex chromosomes of skinks are usually poorly differentiated and hardly distinguished by cytogenetic methods. Therefore, identifying sex chromosomes in species lacking easily recognizable heteromorphic sex chromosomes is necessary to fully understand sex chromosome diversity. In this paper, we employed cytogenetics, sex quantification of genes, and transcriptomic approaches to characterize the sex chromosomes in Plestiodon elegans. Cytogenetic examination of metaphases revealed a diploid number of 2n = 26, consisting of 12 macrochromosomes and 14 microchromosomes, with no significant heteromorphic chromosome pairs, speculating that the sex chromosomes may be homomorphic or poorly differentiated. The results of the sex quantification of genes showed that Calumenin (calu), COPI coat complex subunit γ 2 (copg2), and Smoothened (smo) were at half the dose in males as in females, suggesting that they are on the X chromosome. Transcriptomic data analysis from the gonads yielded the excess expression male-specific genes (n = 16), in which five PCR molecular markers were developed. Restricting the observed heterozygosity to males suggests the presence of homomorphic sex chromosomes in P. elegans, XX/XY. This is the first breakthrough in the study of the sex chromosomes of Plestiodon.

Karyotype Diversification and Chromosome Rearrangements in Squamate Reptiles

Karyotype diversification represents an important, yet poorly understood, driver of evolution. Squamate reptiles are characterized by a high taxonomic diversity which is reflected at the karyotype level in terms of general structure, chromosome number and morphology, and insurgence of differentiated simple or multiple-sex-chromosome systems with either male or female heterogamety. The potential of squamate reptiles as unique model organisms in evolutionary cytogenetics has been recognised in recent years in several studies, which have provided novel insights into the chromosome evolutionary dynamics of different taxonomic groups. Here, we review and summarize the resulting complex, but promising, general picture from a systematic perspective, mapping some of the main squamate karyological characteristics onto their phylogenetic relationships. We highlight how all the major categories of balanced chromosome rearrangements contributed to the karyotype evolution in different taxonomic groups. We show that distinct karyotype evolutionary trends may occur, and coexist, with different frequencies in different clades. Finally, in light of the known squamate chromosome diversity and recent research advances, we discuss traditional and novel hypotheses on karyotype evolution and propose a scenario of circular karyotype evolution.

Robertsonian fusion triggers recombination suppression on sex chromosomes in Coleonyx geckos

The classical hypothesis proposes that the lack of recombination on sex chromosomes arises due to selection for linkage between a sex-determining locus and sexually antagonistic loci, primarily facilitated by inversions. However, cessation of recombination on sex chromosomes could be attributed also to neutral processes, connected with other chromosome rearrangements or can reflect sex-specific recombination patterns existing already before sex chromosome differentiation. Three Coleonyx gecko species share a complex X1X1X2X2/X1X2Y system of sex chromosomes evolved via a fusion of the Y chromosome with an autosome. We analyzed synaptonemal complexes and sequenced flow-sorted sex chromosomes to investigate the effect of chromosomal rearrangement on recombination and differentiation of these sex chromosomes. The gecko sex chromosomes evolved from syntenic regions that were also co-opted also for sex chromosomes in other reptiles. We showed that in male geckos, recombination is less prevalent in the proximal regions of chromosomes and is even further drastically reduced around the centromere of the neo-Y chromosome. We highlight that pre-existing recombination patterns and Robertsonian fusions can be responsible for the cessation of recombination on sex chromosomes and that such processes can be largely neutral.

Sex-Linked Loci on the W Chromosome in the Multi-Ocellated Racerunner (Eremias multiocellata) Confirm Genetic Sex-Determination Stability in Lacertid Lizards

The multi-ocellated racerunner, Eremias multiocellata, was considered to have temperature-dependent sex determination (TSD), as its sex ratio can be influenced at different temperatures. However, such an observation contrasts with recent findings that suggest TSD is less common than previously thought. Here, a genotyping-by-sequencing (GBS) approach was employed to identify sex-linked markers in the E. multiocellata, for which the mechanism choice of TSD or GSD is still controversial. We preliminarily identified 119 sex-linked markers based on sex-associated sex-specific sequences, 97% of which indicated female heterogamety. After eliminating the false positives, 38 sex-linked markers were recognized, all of which showed the ZW/ZZ system. Then, eight of the novel markers were verified by PCR amplification from 15 populations of E. multiocellata, which support the GSD in E. multiocellata without geographic variation. To test the conservation of sex chromosome in Eremias, the eight markers were further cross-tested by PCR amplification in 10 individuals of the Mongolian racerunner (Eremias argus), two of which exhibited cross-utility. The novel sex-linked markers could be mapped on the W chromosome of the sand lizard (Lacerta agilis). Our finding that the sex-linked markers are shared in closely related species, along with a conserved synteny of the W chromosome, further supports the homology and conservation of sex chromosomes in the lacertid lizards.

Evolutionary and Genomic Diversity of True Polyploidy in Tetrapods

True polyploid organisms have more than two chromosome sets in their somatic and germline cells. Polyploidy is a major evolutionary force and has played a significant role in the early genomic evolution of plants, different invertebrate taxa, chordates, and teleosts. However, the contribution of polyploidy to the generation of new genomic, ecological, and species diversity in tetrapods has traditionally been underestimated. Indeed, polyploidy represents an important pathway of genomic evolution, occurring in most higher-taxa tetrapods and displaying a variety of different forms, genomic configurations, and biological implications. Herein, we report and discuss the available information on the different origins and evolutionary and ecological significance of true polyploidy in tetrapods. Among the main tetrapod lineages, modern amphibians have an unparalleled diversity of polyploids and, until recently, they were considered to be the only vertebrates with closely related diploid and polyploid bisexual species or populations. In reptiles, polyploidy was thought to be restricted to squamates and associated with parthenogenesis. In birds and mammals, true polyploidy has generally been considered absent (non-tolerated). These views are being changed due to an accumulation of new data, and the impact as well as the different evolutionary and ecological implications of polyploidy in tetrapods, deserve a broader evaluation.

A Brief Review of Meiotic Chromosomes in Early Spermatogenesis and Oogenesis and Mitotic Chromosomes in the Viviparous Lizard Zootoca vivipara (Squamata: Lacertidae) with Multiple Sex Chromosomes

This brief review is focused on the viviparous lizard Zootoca vivipara (Lichtenstein, 1823), of the family Lacertidae, which possesses female heterogamety and multiple sex chromosomes (male 2n = 36, Z₁Z₁Z₂Z₂/Z₁Z₂W, female 2n = 35, with variable W sex chromosome). Multiple sex chromosomes and their changes may influence meiosis and the female meiotic drive, and they may play a role in reproductive isolation. In two cryptic taxa of Z. vivipara with different W sex chromosomes, meiosis during early spermatogenesis and oogenesis proceeds normally, without any disturbances, with the formation of haploid spermatocytes, and in female meiosis with the formation of synaptonemal complexes (SCs) and the lampbrush chromosomes. In females, the SC number was constantly equal to 19 (according to the SC length, 16 SC autosomal bivalents plus three presumed SC sex chromosome elements). No variability in the chromosomes at the early stages of meiotic prophase I, and no significant disturbances in the chromosome segregation at the anaphase-telophase I stage, have been discovered, and haploid oocytes (n = 17) at the metaphase II stage have been revealed. There should be a factor/factors that maintain the multiple sex chromosomes, their equal transmission, and the course of meiosis in these cryptic forms of Z. vivipara.

Sex-related differences in aging rate are associated with sex chromosome system in amphibians

Sex‐related differences in mortality are widespread in the animal kingdom. Although studies have shown that sex determination systems might drive lifespan evolution, sex chromosome influence on aging rates have not been investigated so far, likely due to an apparent lack of demographic data from clades including both XY (with heterogametic males) and ZW (heterogametic females) systems. Taking advantage of a unique collection of capture–recapture datasets in amphibians, a vertebrate group where XY and ZW systems have repeatedly evolved over the past 200 million years, we examined whether sex heterogamy can predict sex differences in aging rates and lifespans. We showed that the strength and direction of sex differences in aging rates (and not lifespan) differ between XY and ZW systems. Sex‐specific variation in aging rates was moderate within each system, but aging rates tended to be consistently higher in the heterogametic sex. This led to small but detectable effects of sex chromosome system on sex differences in aging rates in our models. Although preliminary, our results suggest that exposed recessive deleterious mutations on the X/Z chromosome (the “unguarded X/Z effect”) or repeat‐rich Y/W chromosome (the “toxic Y/W effect”) could accelerate aging in the heterogametic sex in some vertebrate clades.

Noncoding Genes on Sex Chromosomes and Their Function in Sex Determination, Dosage Compensation, Male Traits, and Diseases

The mammalian Y chromosome has evolved in many species into a specialized chromosome that contributes to sex development among other male phenotypes. This function is well studied in terms of protein-coding genes. Less is known about the noncoding genome on the Y chromosome and its contribution to both sex development and other traits. Once considered junk genetic material, noncoding RNAs are now known to contribute to the regulation of gene expression and to play an important role in refining cellular functions. The prime examples are noncoding genes on the X chromosome, which mitigate the differential dosage of genes on sex chromosomes. Here, we discuss the evolution of noncoding RNAs on the Y chromosome and the emerging evidence of how micro, long, and circular noncoding RNAs transcribed from the Y chromosome contribute to sex differentiation. We briefly touch on emerging evidence that these noncoding RNAs also contribute to some other important clinical phenotypes in humans.

Microchromosomes are building blocks of bird, reptile, and mammal chromosomes

Microchromosomes, once considered unimportant shreds of the chicken genome, are gene-rich elements with a high GC content and few transposable elements. Their origin has been debated for decades. We used cytological and whole-genome sequence comparisons, and chromosome conformation capture, to trace their origin and fate in genomes of reptiles, birds, and mammals. We find that microchromosomes as well as macrochromosomes are highly conserved across birds and share synteny with single small chromosomes of the chordate amphioxus, attesting to their origin as elements of an ancient animal genome. Turtles and squamates (snakes and lizards) share different subsets of ancestral microchromosomes, having independently lost microchromosomes by fusion with other microchromosomes or macrochromosomes. Patterns of fusions were quite different in different lineages. Cytological observations show that microchromosomes in all lineages are spatially separated into a central compartment at interphase and during mitosis and meiosis. This reflects higher interaction between microchromosomes than with macrochromosomes, as observed by chromosome conformation capture, and suggests some functional coherence. In highly rearranged genomes fused microchromosomes retain most ancestral characteristics, but these may erode over evolutionary time; surprisingly, de novo microchromosomes have rapidly adopted high interaction. Some chromosomes of early-branching monotreme mammals align to several bird microchromosomes, suggesting multiple microchromosome fusions in a mammalian ancestor. Subsequently, multiple rearrangements fueled the extraordinary karyotypic diversity of therian mammals. Thus, microchromosomes, far from being aberrant genetic elements, represent fundamental building blocks of amniote chromosomes, and it is mammals, rather than reptiles and birds, that are atypical.

Lizards as Model Organisms of Sex Chromosome Evolution: What We Really Know from a Systematic Distribution of Available Data?

Lizards represent unique model organisms in the study of sex determination and sex chromosome evolution. Among tetrapods, they are characterized by an unparalleled diversity of sex determination systems, including temperature-dependent sex determination (TSD) and genetic sex determination (GSD) under either male or female heterogamety. Sex chromosome systems are also extremely variable in lizards. They include simple (XY and ZW) and multiple (X1X2Y and Z1Z2W) sex chromosome systems and encompass all the different hypothesized stages of diversification of heterogametic chromosomes, from homomorphic to heteromorphic and completely heterochromatic sex chromosomes. The co-occurrence of TSD, GSD and different sex chromosome systems also characterizes different lizard taxa, which represent ideal models to study the emergence and the evolutionary drivers of sex reversal and sex chromosome turnover. In this review, we present a synthesis of general genome and karyotype features of non-snakes squamates and discuss the main theories and evidences on the evolution and diversification of their different sex determination and sex chromosome systems. We here provide a systematic assessment of the available data on lizard sex chromosome systems and an overview of the main cytogenetic and molecular methods used for their identification, using a qualitative and quantitative approach.

Pleistocene divergence in the absence of gene flow among populations of a viviparous reptile with intraspecific variation in sex determination

Polymorphisms can lead to genetic isolation if there is differential mating success among conspecifics divergent for a trait. Polymorphism for sex-determining system may fall into this category, given strong selection for the production of viable males and females and the low success of heterogametic hybrids when sex chromosomes differ (Haldane's rule). Here we investigated whether populations exhibiting polymorphism for sex determination are genetically isolated, using the viviparous snow skink Carinascincus ocellatus. While a comparatively high elevation population has genotypic sex determination, in a lower elevation population there is an additional temperature component to sex determination. Based on 11,107 SNP markers, these populations appear genetically isolated. "Isolation with Migration" analysis also suggests these populations diverged in the absence of gene flow, across a period encompassing multiple Pleistocene glaciations and likely greater geographic proximity of populations. However, further experiments are required to establish whether genetic isolation may be a cause or consequence of differences in sex determination. Given the influence of temperature on sex in one lineage, we also discuss the implications for the persistence of this polymorphism under climate change.

Network architecture and sex chromosome turnovers: Do epistatic interactions shape patterns of sex chromosome replacement?

Recent studies have revealed an astonishing diversity of sex chromosomes in many vertebrate lineages, prompting questions about the mechanisms of sex chromosome turnover. While there is considerable population genetic theory about the evolutionary forces promoting sex chromosome replacement, this theory has not yet been integrated with our understanding of the molecular and developmental genetics of sex determination. Here, we review recent data to examine four questions about how the structure of gene networks influences the evolution of sex determination. We argue that patterns of epistasis, arising from the structure of genetic networks, may play an important role in regulating the rates and patterns of sex chromosome replacement.

Karyotype Characterisation of Two Australian Dragon Lizards (Squamata: Agamidae: Amphibolurinae) Reveals Subtle Chromosomal Rearrangements Between Related Species with Similar Karyotypes

Agamid lizards (Squamata: Agamidae) are karyotypically heterogeneous. Among the 101 species currently described from Australia, all are from the subfamily Amphibolurinae. This group is, with some exceptions, karyotypically conserved, and all species involving heterogametic sex show female heterogamety. Here, we describe the chromosomes of 2 additional Australian agamid lizards, Tympanocryptis lineata and Rankinia diemensis. These species are phylogenetically and cytogenetically sisters to the well-characterised Pogona vitticeps, but their sex chromosomes and other chromosomal characteristics are unknown. In this study, we applied advanced molecular cytogenetic techniques, such as fluorescence in situ hybridisation (FISH) and cross-species gene mapping, to characterise chromosomes and to identify sex chromosomes in these species. Our data suggest that both species have a conserved karyotype with P. vitticeps but with subtle rearrangements in the chromosomal landscapes. We could identify that T. lineata possesses a female heterogametic system (ZZ/ZW) with a pair of sex microchromosomes, while R. diemensis may have heterogametic sex chromosomes, but this requires further investigations. Our study shows the pattern of chromosomal rearrangements between closely related species, explaining the speciation within Australian agamid lizards of similar karyotypes.

Genome Complexity Reduction High-Throughput Genome Sequencing of Green Iguana (Iguana iguana) Reveal a Paradigm Shift in Understanding Sex-Chromosomal Linkages on Homomorphic X and Y Sex Chromosomes

The majority of lizards classified in the superfamily Iguanoidea have an XX/XY sex-determination system in which sex-chromosomal linkage shows homology with chicken (Gallus gallus) chromosome 15 (GGA15). However, the genomics of sex chromosomes remain largely unexplored owing to the presence of homomorphic sex chromosomes in majority of the species. Recent advances in high-throughput genome complexity reduction sequencing provide an effective approach to the identification of sex-specific loci with both single-nucleotide polymorphisms (SNPs) and restriction fragment presence/absence (PA), and a better understanding of sex chromosome dynamics in Iguanoidea. In this study, we applied Diversity Arrays Technology (DArTseq^TM) in 29 phenotypic sex assignments (14 males and 15 females) of green iguana (Iguana iguana). We confirmed a male heterogametic (XX/XY) sex determination mode in this species, identifying 29 perfectly sex-linked SNP/PA loci and 164 moderately sex-linked SNP/PA loci, providing evidence probably indicative of XY recombination. Three loci from among the perfectly sex-linked SNP/PA loci showed partial homology with several amniote sex chromosomal linkages. The results support the hypothesis of an ancestral super-sex chromosome with overlaps of partial sex-chromosomal linkages. However, only one locus among the moderately sex-linked loci showed homology with GGA15, which suggests that the specific region homologous to GGA15 was located outside the non-recombination region but in close proximity to this region of the sex chromosome in green iguana. Therefore, the location of GGA15 might be further from the putative sex-determination locus in green iguana. This is a paradigm shift in understanding linkages on homomorphic X and Y sex chromosomes. The DArTseq platform provides an easy-to-use strategy for future research on the evolution of sex chromosomes in Iguanoidea, particularly for non-model species with homomorphic or highly cryptic sex chromosomes.

Meiotic analyses show adaptations to maintenance of fertility in X1Y1X2Y2X3Y3X4Y4X5Y5 system of amazon frog Leptodactylus pentadactylus (Laurenti, 1768)

Heterozygous chromosomal rearrangements can result in failures during the meiotic cycle and the apoptosis of germline, making carrier individuals infertile. The Amazon frog Leptodactylus pentadactylus has a meiotic multivalent, composed of 12 sex chromosomes. The mechanisms by which this multi-chromosome system maintains fertility in males of this species remain undetermined. In this study we investigated the meiotic behavior of this multivalent to understand how synapse, recombination and epigenetic modifications contribute to maintaining fertility and chromosomal sexual determination in this species. Our sample had 2n = 22, with a ring formed by ten chromosomes in meiosis, indicating a new system of sex determination for this species (X1Y1X2Y2X3Y3X4Y4X5Y5). Synapsis occurs in the homologous terminal portion of the chromosomes, while part of the heterologous interstitial regions performed synaptic adjustment. The multivalent center remains asynaptic until the end of pachytene, with interlocks, gaps and rich-chromatin in histone H2A phosphorylation at serine 139 (γH2AX), suggesting transcriptional silence. In late pachytene, paired regions show repair of double strand-breaks (DSBs) with RAD51 homolog 1 (Rad51). These findings suggest that Rad51 persistence creates positive feedback at the pachytene checkpoint, allowing meiosis I to progress normally. Additionally, histone H3 trimethylation at lysine 27 in the pericentromeric heterochromatin of this anuran can suppress recombination in this region, preventing failed chromosomal segregation. Taken together, these results indicate that these meiotic adaptations are required for maintenance of fertility in L. pentadactylus.

Identification of Y chromosome markers in the eastern three-lined skink (Bassiana duperreyi) using in silico whole genome subtraction

Background

Homologous sex chromosomes can differentiate over time because recombination is suppressed in the region of the sex determining locus, leading to the accumulation of repeats, progressive loss of genes that lack differential influence on the sexes and sequence divergence on the hemizygous homolog. Divergence in the non-recombining regions leads to the accumulation of Y or W specific sequence useful for developing sex-linked markers. Here we use in silico whole-genome subtraction to identify putative sex-linked sequences in the scincid lizard Bassiana duperreyi which has heteromorphic XY sex chromosomes.

Results

We generated 96.7 × 10⁹ 150 bp paired-end genomic sequence reads from a XY male and 81.4 × 10⁹ paired-end reads from an XX female for in silico whole genome subtraction to yield Y enriched contigs. We identified 7 reliable markers which were validated as Y chromosome specific by polymerase chain reaction (PCR) against a panel of 20 males and 20 females.

Conclusions

The sex of B. duperreyi can be reversed by low temperatures (XX genotype reversed to a male phenotype). We have developed sex-specific markers to identify the underlying genotypic sex and its concordance or discordance with phenotypic sex in wild populations of B. duperreyi. Our pipeline can be applied to isolate Y or W chromosome-specific sequences of any organism and is not restricted to sequence residing within single-copy genes. This study greatly improves our knowledge of the Y chromosome in B. duperreyi and will enhance future studies of reptile sex determination and sex chromosome evolution.

Karyotypic Evolution of Sauropsid Vertebrates Illuminated by Optical and Physical Mapping of the Painted Turtle and Slider Turtle Genomes

Recent sequencing and software enhancements have advanced our understanding of the evolution of genomic structure and function, especially addressing novel evolutionary biology questions. Yet fragmentary turtle genome assemblies remain a challenge to fully decipher the genetic architecture of adaptive evolution. Here, we use optical mapping to improve the contiguity of the painted turtle (Chrysemys picta) genome assembly and use de novo fluorescent in situ hybridization (FISH) of bacterial artificial chromosome (BAC) clones, BAC-FISH, to physically map the genomes of the painted and slider turtles (Trachemys scripta elegans). Optical mapping increased C. picta's N50 by ~242% compared to the previous assembly. Physical mapping permitted anchoring ~45% of the genome assembly, spanning 5544 genes (including 20 genes related to the sex determination network of turtles and vertebrates). BAC-FISH data revealed assembly errors in C. picta and T. s. elegans assemblies, highlighting the importance of molecular cytogenetic data to complement bioinformatic approaches. We also compared C. picta's anchored scaffolds to the genomes of other chelonians, chicken, lizards, and snake. Results revealed a mostly one-to-one correspondence between chromosomes of painted and slider turtles, and high homology among large syntenic blocks shared with other turtles and sauropsids. Yet, numerous chromosomal rearrangements were also evident across chelonians, between turtles and squamates, and between avian and non-avian reptiles.

Landscape of snake' sex chromosomes evolution spanning 85 MYR reveals ancestry of sequences despite distinct evolutionary trajectories

Most of snakes exhibit a ZZ/ZW sex chromosome system, with different stages of degeneration. However, undifferentiated sex chromosomes and unique Y sex-linked markers, suggest that an XY system has also evolved in ancestral lineages. Comparative cytogenetic mappings revealed that several genes share ancestry among X, Y and Z chromosomes, implying that XY and ZW may have undergone transitions during serpent's evolution. In this study, we performed a comparative cytogenetic analysis to identify homologies of sex chromosomes across ancestral (Henophidia) and more recent (Caenophidia) snakes. Our analysis suggests that, despite ~ 85 myr of independent evolution, henophidians and caenophidians retained conserved synteny over much of their genomes. However, our findings allowed us to discover that ancestral and recent lineages of snakes do not share the same sex chromosome and followed distinct pathways for sex chromosomes evolution.

Cross-Species BAC Mapping Highlights Conservation of Chromosome Synteny across Dragon Lizards (Squamata: Agamidae)

Dragon lizards (Squamata: Agamidae) comprise about 520 species in six subfamilies distributed across Asia, Australasia and Africa. Only five species are known to have sex chromosomes. All of them possess ZZ/ZW sex chromosomes, which are microchromosomes in four species from the subfamily Amphibolurinae, but much larger in Phrynocephalus vlangalii from the subfamily Agaminae. In most previous studies of these sex chromosomes, the focus has been on Australian species from the subfamily Amphibolurinae, but only the sex chromosomes of the Australian central bearded dragon (Pogona vitticeps) are well-characterized cytogenetically. To determine the level of synteny of the sex chromosomes of P. vitticeps across agamid subfamilies, we performed cross-species two-colour FISH using two bacterial artificial chromosome (BAC) clones from the pseudo-autosomal regions of P. vitticeps. We mapped these two BACs across representative species from all six subfamilies as well as two species of chameleons, the sister group to agamids. We found that one of these BAC sequences is conserved in macrochromosomes and the other in microchromosomes across the agamid lineages. However, within the Amphibolurinae, there is evidence of multiple chromosomal rearrangements with one of the BACs mapping to the second-largest chromosome pair and to the microchromosomes in multiple species including the sex chromosomes of P. vitticeps. Intriguingly, no hybridization signal was observed in chameleons for either of these BACs, suggesting a likely agamid origin of these sequences. Our study shows lineage-specific evolution of sequences/syntenic blocks and successive rearrangements and reveals a complex history of sequences leading to their association with important biological processes such as the evolution of sex chromosomes and sex determination.

Turtle Insights into the Evolution of the Reptilian Karyotype and the Genomic Architecture of Sex Determination

Sex chromosome evolution remains an evolutionary puzzle despite its importance in understanding sexual development and genome evolution. The seemingly random distribution of sex-determining systems in reptiles offers a unique opportunity to study sex chromosome evolution not afforded by mammals or birds. These reptilian systems derive from multiple transitions in sex determination, some independent, some convergent, that lead to the birth and death of sex chromosomes in various lineages. Here we focus on turtles, an emerging model group with growing genomic resources. We review karyotypic changes that accompanied the evolution of chromosomal systems of genotypic sex determination (GSD) in chelonians from systems under the control of environmental temperature (TSD). These transitions gave rise to 31 GSD species identified thus far (out of 101 turtles with known sex determination), 27 with a characterized sex chromosome system (13 of those karyotypically). These sex chromosomes are varied in terms of the ancestral autosome they co-opted and thus in their homology, as well as in their size (some are macro-, some are micro-chromosomes), heterogamety (some are XX/XY, some ZZ/ZW), dimorphism (some are virtually homomorphic, some heteromorphic with larger-X, larger W, or smaller-Y), age (the oldest system could be ~195 My old and the youngest < 25 My old). Combined, all data indicate that turtles follow some tenets of classic theoretical models of sex chromosome evolution while countering others. Finally, although the study of dosage compensation and molecular divergence of turtle sex chromosomes has lagged behind research on other aspects of their evolution, this gap is rapidly decreasing with the acceleration of ongoing research and growing genomic resources in this group.

Reproductive phenotype predicts adult bite-force performance in sex-reversed dragons (Pogona vitticeps)

Sex-related differences in morphology and behavior are well documented, but the relative contributions of genes and environment to these traits are less well understood. Species that undergo sex reversal, such as the central bearded dragon (Pogona vitticeps), offer an opportunity to better understand sexually dimorphic traits because sexual phenotypes can exist on different chromosomal backgrounds. Reproductively female dragons with a discordant sex chromosome complement (sex reversed), at least as juveniles, exhibit traits in common with males (e.g., longer tails and greater boldness). However, the impact of sex reversal on sexually dimorphic traits in adult dragons is unknown. Here, we investigate the effect of sex reversal on bite-force performance, which may be important in resource acquisition (e.g., mates and/or food). We measured body size, head size, and bite force of the three sexual phenotypes in a colony of captive animals. Among adults, we found that males (ZZm) bite more forcefully than either chromosomally concordant females (ZWf) or sex-reversed females (ZZf), and this difference is associated with having relatively larger head dimensions. Therefore, adult sex-reversed females, despite apparently exhibiting male traits as juveniles, do not develop the larger head and enhanced bite force of adult male bearded dragons. This pattern is further illustrated in the full sample by a lack of positive allometry of bite force in sex-reversed females that is observed in males. The results reveal a close association between reproductive phenotype and bite force performance, regardless of sex chromosome complement.

Evolutionary Insights of the ZW Sex Chromosomesin Snakes: A New Chapter Added by the AmazonianPuffing Snakes of the Genus Spilotes

Amazonian puffing snakes (Spilotes; Colubridae) are snakes widely distributed in the Neotropical region. However, chromosomal data are scarce in this group and, when available, are only limited to karyotype description using conventional staining. In this paper, we focused on the process of karyotype evolution and trends for sex chromosomes in two Amazonian Puffer Snakes (S. pulllatus and S. sulphureus). We performed an extensive karyotype characterization using conventional and molecular cytogenetic approaches. The karyotype of S. sulphureus (presented here for the first time) exhibits a 2n = 36, similar to that previously described in S. pullatus. Both species have highly differentiated ZZ/ZW sex chromosomes, where the W chromosome is highly heterochromatic in S. pullatus but euchromatic in S. sulphureus. Both W chromosomes are homologous between these species as revealed by cross-species comparative genomic hybridization, even with heterogeneous distributions of several repetitive sequences across their genomes, including on the Z and on the W chromosomes. Our study provides evidence that W chromosomes in these two species have shared ancestry.