Sex chromosomes, sex-linked genes, and sex determination in the vertebrate class amphibia

A candidate sex determination locus in amphibians which evolved by structural variation between X- and Y-chromosomes

Most vertebrates develop distinct females and males, where sex is determined by repeatedly evolved environmental or genetic triggers. Undifferentiated sex chromosomes and large genomes have caused major knowledge gaps in amphibians. Only a single master sex-determining gene, the dmrt1-paralogue (dm-w) of female-heterogametic clawed frogs (Xenopus; ZW♀/ZZ♂), is known across >8740 species of amphibians. In this study, by combining chromosome-scale female and male genomes of a non-model amphibian, the European green toad, Bufo(tes) viridis, with ddRAD- and whole genome pool-sequencing, we reveal a candidate master locus, governing a male-heterogametic system (XX♀/XY♂). Targeted sequencing across multiple taxa uncovered structural X/Y-variation in the 5'-regulatory region of the gene bod1l, where a Y-specific non-coding RNA (ncRNA-Y), only expressed in males, suggests that this locus initiates sex-specific differentiation. Developmental transcriptomes and RNA in-situ hybridization show timely and spatially relevant sex-specific ncRNA-Y and bod1l-gene expression in primordial gonads. This coincided with differential H3K4me-methylation in pre-granulosa/pre-Sertoli cells, pointing to a specific mechanism of amphibian sex determination.

Unveiling the roles of Sertoli cells lineage differentiation in reproductive development and disorders: a review

In mammals, gonadal somatic cell lineage differentiation determines the development of the bipotential gonad into either the ovary or testis. Sertoli cells, the only somatic cells in the spermatogenic tubules, support spermatogenesis during gonadal development. During embryonic Sertoli cell lineage differentiation, relevant genes, including WT1, GATA4, SRY, SOX9, AMH, PTGDS, SF1, and DMRT1, are expressed at specific times and in specific locations to ensure the correct differentiation of the embryo toward the male phenotype. The dysregulated development of Sertoli cells leads to gonadal malformations and male fertility disorders. Nevertheless, the molecular pathways underlying the embryonic origin of Sertoli cells remain elusive. By reviewing recent advances in research on embryonic Sertoli cell genesis and its key regulators, this review provides novel insights into sex determination in male mammals as well as the molecular mechanisms underlying the genealogical differentiation of Sertoli cells in the male reproductive ridge.

Unleashing diversity through flexibility: The evolutionary journey of sex chromosomes in amphibians and reptiles

Sex determination systems have greatly diversified between amphibians and reptiles, with such as the different sex chromosome compositions within a single species and transition between temperature-dependent sex determination (TSD) and genetic sex determination (GSD). In most sex chromosome studies on amphibians and reptiles, the whole-genome sequence of Xenopous tropicalis and chicken have been used as references to compare the chromosome homology of sex chromosomes among each of these taxonomic groups, respectively. In the present study, we reviewed existing reports on sex chromosomes, including karyotypes, in amphibians and reptiles. Furthermore, we compared the identified genetic linkages of sex chromosomes in amphibians and reptiles with the chicken genome as a reference, which is believed to resemble the ancestral tetrapod karyotype. Our findings revealed that sex chromosomes in amphibians are derived from genetic linkages homologous to various chicken chromosomes, even among several frogs within single families, such as Ranidae and Pipidae. In contrast, sex chromosomes in reptiles exhibit conserved genetic linkages with chicken chromosomes, not only across most species within a single family, but also within closely related families. The diversity of sex chromosomes in amphibians and reptiles may be attributed to the flexibility of their sex determination systems, including the ease of sex reversal in these animals.

Tenuous Transcriptional Threshold of Human Sex Determination. I. SRY and Swyer Syndrome at the Edge of Ambiguity

Male sex determination in mammals is initiated by SRY, a Y-encoded transcription factor. The protein contains a high-mobility-group (HMG) box mediating sequence-specific DNA bending. Mutations causing XY gonadal dysgenesis (Swyer syndrome) cluster in the box and ordinarily arise de novo. Rare inherited variants lead to male development in one genetic background (the father) but not another (his sterile XY daughter). De novo and inherited mutations occur at an invariant Tyr adjoining the motif's basic tail (box position 72; Y127 in SRY). In SRY-responsive cell lines CH34 and LNCaP, de novo mutations Y127H and Y127C reduced SRY activity (as assessed by transcriptional activation of principal target gene Sox9) by 5- and 8-fold, respectively. Whereas Y127H impaired testis-specific enhancer assembly, Y127C caused accelerated proteasomal proteolysis; activity was in part rescued by proteasome inhibition. Inherited variant Y127F was better tolerated: its expression was unperturbed, and activity was reduced by only twofold, a threshold similar to other inherited variants. Biochemical studies of wild-type (WT) and variant HMG boxes demonstrated similar specific DNA affinities (within a twofold range), with only subtle differences in sharp DNA bending as probed by permutation gel electrophoresis and fluorescence resonance-energy transfer (FRET); thermodynamic stabilities of the free boxes were essentially identical. Such modest perturbations are within the range of species variation. Whereas our cell-based findings rationalize the de novo genotype-phenotype relationships, a molecular understanding of inherited mutation Y127F remains elusive. Our companion study uncovers cryptic biophysical perturbations suggesting that the para-OH group of Y127 anchors a novel water-mediated DNA clamp.

Sex-chromosome evolution in frogs: what role for sex-antagonistic genes?

Sex-antagonistic (SA) genes are widely considered to be crucial players in the evolution of sex chromosomes, being instrumental in the arrest of recombination and degeneration of Y chromosomes, as well as important drivers of sex-chromosome turnovers. To test such claims, one needs to focus on systems at the early stages of differentiation, ideally with a high turnover rate. Here, I review recent work on two families of amphibians, Ranidae (true frogs) and Hylidae (tree frogs), to show that results gathered so far from these groups provide no support for a significant role of SA genes in the evolutionary dynamics of their sex chromosomes. The findings support instead a central role for neutral processes and deleterious mutations. This article is part of the theme issue 'Challenging the paradigm in sex chromosome evolution: empirical and theoretical insights with a focus on vertebrates (Part I)'.

A brief review of vertebrate sex evolution with a pledge for integrative research: towards 'sexomics'

Triggers and biological processes controlling male or female gonadal differentiation vary in vertebrates, with sex determination (SD) governed by environmental factors or simple to complex genetic mechanisms that evolved repeatedly and independently in various groups. Here, we review sex evolution across major clades of vertebrates with information on SD, sexual development and reproductive modes. We offer an up-to-date review of divergence times, species diversity, genomic resources, genome size, occurrence and nature of polyploids, SD systems, sex chromosomes, SD genes, dosage compensation and sex-biased gene expression. Advances in sequencing technologies now enable us to study the evolution of SD at broader evolutionary scales, and we now hope to pursue a sexomics integrative research initiative across vertebrates. The vertebrate sexome comprises interdisciplinary and integrated information on sexual differentiation, development and reproduction at all biological levels, from genomes, transcriptomes and proteomes, to the organs involved in sexual and sex-specific processes, including gonads, secondary sex organs and those with transcriptional sex-bias. The sexome also includes ontogenetic and behavioural aspects of sexual differentiation, including malfunction and impairment of SD, sexual differentiation and fertility. Starting from data generated by high-throughput approaches, we encourage others to contribute expertise to building understanding of the sexomes of many key vertebrate species. This article is part of the theme issue 'Challenging the paradigm in sex chromosome evolution: empirical and theoretical insights with a focus on vertebrates (Part I)'.

Testis Development and Differentiation in Amphibians

Sex is determined genetically in amphibians; however, little is known about the sex chromosomes, testis-determining genes, and the genes involved in testis differentiation in this class. Certain inherent characteristics of the species of this group, like the homomorphic sex chromosomes, the high diversity of the sex-determining mechanisms, or the existence of polyploids, may hinder the design of experiments when studying how the gonads can differentiate. Even so, other features, like their external development or the possibility of inducing sex reversal by external treatments, can be helpful. This review summarizes the current knowledge on amphibian sex determination, gonadal development, and testis differentiation. The analysis of this information, compared with the information available for other vertebrate groups, allows us to identify the evolutionarily conserved and divergent pathways involved in testis differentiation. Overall, the data confirm the previous observations in other vertebrates-the morphology of the adult testis is similar across different groups; however, the male-determining signal and the genetic networks involved in testis differentiation are not evolutionarily conserved.

The genomes of a monogenic fly: views of primitive sex chromosomes

The production of male and female offspring is often determined by the presence of specific sex chromosomes which control sex-specific expression, and sex chromosomes evolve through reduced recombination and specialized gene content. Here we present the genomes of Chrysomya rufifacies, a monogenic blow fly (females produce female or male offspring, exclusively) by separately sequencing and assembling each type of female and the male. The genomes (> 25X coverage) do not appear to have any sex-linked Muller F elements (typical for many Diptera) and exhibit little differentiation between groups supporting the morphological assessments of C. rufifacies homomorphic chromosomes. Males in this species are associated with a unimodal coverage distribution while females exhibit bimodal coverage distributions, suggesting a potential difference in genomic architecture. The presence of the individual-sex draft genomes herein provides new clues regarding the origination and evolution of the diverse sex-determining mechanisms observed within Diptera. Additional genomic analysis of sex chromosomes and sex-determining genes of other blow flies will allow a refined evolutionary understanding of how flies with a typical X/Y heterogametic amphogeny (male and female offspring in similar ratios) sex determination systems evolved into one with a dominant factor that results in single sex progeny in a chromosomally monomorphic system.

Polyploidy breaks speciation barriers in Australian burrowing frogs Neobatrachus

Polyploidy has played an important role in evolution across the tree of life but it is still unclear how polyploid lineages may persist after their initial formation. While both common and well-studied in plants, polyploidy is rare in animals and generally less understood. The Australian burrowing frog genus Neobatrachus is comprised of six diploid and three polyploid species and offers a powerful animal polyploid model system. We generated exome-capture sequence data from 87 individuals representing all nine species of Neobatrachus to investigate species-level relationships, the origin and inheritance mode of polyploid species, and the population genomic effects of polyploidy on genus-wide demography. We describe rapid speciation of diploid Neobatrachus species and show that the three independently originated polyploid species have tetrasomic or mixed inheritance. We document higher genetic diversity in tetraploids, resulting from widespread gene flow between the tetraploids, asymmetric inter-ploidy gene flow directed from sympatric diploids to tetraploids, and isolation of diploid species from each other. We also constructed models of ecologically suitable areas for each species to investigate the impact of climate on differing ploidy levels. These models suggest substantial change in suitable areas compared to past climate, which correspond to population genomic estimates of demographic histories. We propose that Neobatrachus diploids may be suffering the early genomic impacts of climate-induced habitat loss, while tetraploids appear to be avoiding this fate, possibly due to widespread gene flow. Finally, we demonstrate that Neobatrachus is an attractive model to study the effects of ploidy on the evolution of adaptation in animals.

Identifying sex-linked markers in Litoria aurea: a novel approach to understanding sex chromosome evolution in an amphibian

Few taxa exhibit the variability of sex-determining modes as amphibians. However, due to the presence of homomorphic sex chromosomes in many species, this phenomenon has been difficult to study. The Australian frog, Litoria aurea, has been relatively well studied over the past 20 years due to widespread declines largely attributable to chytrid fungus. However, it has been subject to few molecular studies and its mode of sex determination remained unknown. We applied DArTseq™ to develop sex-linked single nucleotide polymorphisms (SNPs) and restriction fragment presence/absence (PA) markers in 44 phenotypically sexed L. aurea individuals from the Molonglo River in NSW, Australia. We conclusively identified a male heterogametic (XX-XY) sex determination mode in this species, identifying 11 perfectly sex-linked SNP and six strongly sex-linked PA markers. We identified a further 47 moderately sex-linked SNP loci, likely serving as evidence indicative of XY recombination. Furthermore, within these 47 loci, a group of nine males were found to have a feminised Y chromosome that significantly differed to all other males. We postulate ancestral sex-reversal as a means for the evolution of this now pseudoautosomal region on the Y chromosome. Our findings present new evidence for the ‘fountain of youth’ hypothesis for the retention of homomorphic sex chromosomes in amphibians and describe a novel approach for the study of sex chromosome evolution in amphibia.

Genomic Data Reveal Conserved Female Heterogamety in Giant Salamanders with Gigantic Nuclear Genomes

Systems of genetic sex determination and the homology of sex chromosomes in different taxa vary greatly across vertebrates. Much progress remains to be made in understanding systems of genetic sex determination in non-model organisms, especially those with homomorphic sex chromosomes and/or large genomes. We used reduced representation genome sequencing to investigate genetic sex determination systems in the salamander family Cryptobranchidae (genera Cryptobranchus and Andrias), which typifies both of these inherent difficulties. We tested hypotheses of male- or female-heterogamety by sequencing hundreds of thousands of anonymous genomic regions in a panel of known-sex cryptobranchids and characterized patterns of presence/absence, inferred zygosity, and depth of coverage to identify sex-linked regions of these 56 gigabase genomes. Our results strongly support the hypothesis that all cryptobranchid species possess homologous systems of female heterogamety, despite maintenance of homomorphic sex chromosomes over nearly 60 million years. Additionally, we report a robust, non-invasive genetic assay for sex diagnosis in Cryptobranchus and Andrias which may have great utility for conservation efforts with these endangered salamanders. Co-amplification of these W-linked markers in both cryptobranchid genera provides evidence for long-term sex chromosome stasis in one of the most divergent salamander lineages. These findings inform hypotheses about the ancestral mode of sex determination in salamanders, but suggest that comparative data from other salamander families are needed. Our results further demonstrate that massive genomes are not necessarily a barrier to effective genome-wide sequencing and that the resulting data can be highly informative about sex determination systems in taxa with homomorphic sex chromosomes.

Miniscule differences between sex chromosomes in the giant genome of a salamander

In the Mexican axolotl (Ambystoma mexicanum), sex is determined by a single Mendelian factor, yet its sex chromosomes do not exhibit morphological differentiation typical of many vertebrate taxa that possess a single sex-determining locus. As sex chromosomes are theorized to differentiate rapidly, species with undifferentiated sex chromosomes provide the opportunity to reconstruct early events in sex chromosome evolution. Whole genome sequencing of 48 salamanders, targeted chromosome sequencing and in situ hybridization were used to identify the homomorphic sex chromosome that carries an A. mexicanum sex-determining factor and sequences that are present only on the W chromosome. Altogether, these sequences cover ~300 kb of validated female-specific (W chromosome) sequence, representing ~1/100,000^th of the 32 Gb genome. Notably, a recent duplication of ATRX, a gene associated with mammalian sex-determining pathways, is one of few functional (non-repetitive) genes identified among these W-specific sequences. This duplicated gene (ATRW) was used to develop highly predictive markers for diagnosing sex and represents a strong candidate for a recently-acquired sex determining locus (or sexually antagonistic gene) in A. mexicanum.

Suppressed Recombination of Sex Chromosomes Is Not Caused by Chromosomal Reciprocal Translocation in Spiny Frog (Quasipaa boulengeri)

Chromosome rearrangements (CRs) are perceived to be related to sex chromosome evolution, but it is a matter of controversy whether CRs are the initial causative mechanism of suppressed recombination for sex differentiation. The early stages of sex chromosome evolution in amphibians may represent intermediate states of differentiation, and if so, they potentially shed light on the ultimate cause of suppressed recombination and the role of CRs in sex chromosome differentiation. In this paper, we showed that sex determination differs among 16 populations of spiny frog (Quasipaa boulengeri), in which individuals have normal and rearranged chromosomes caused by reciprocal translocation. In eastern areas, without translocation, genetic differentiation between sexes was relatively low, suggesting unrestricted recombination. In comparison, in western populations that have both normal and translocated chromosomes, a male-heterogametic system and lack of X-Y recombination were identified by male-specific alleles and heterozygote excess. However, such genetic differentiation between sexes in western populations was not directly related to karyotypes, as it was found in individuals with both normal and translocated karyotypes. In the western Sichuan Basin, male-specific and translocation-specific allelic frequency distributions suggested that recombination of sex-differentiation ceased in all populations, but recombination suppression caused by translocation did not exist in some populations. Combined with phylogenetic inference, this indicated that the establishment of sex-linkage had taken place independently of reciprocal translocation, and translocation was not the ultimate cause of sex chromosome differentiation. Furthermore, comparison of the genetic diversity of alleles on Y chromosomes, X chromosomes, and autosomes in western populations showed a reduction of effective population size on sex chromosomes, which may be caused by reciprocal translocation. It indicates that, although it is not the ultimate cause of recombination suppression, reciprocal translocation may enhance sex chromosome differentiation.

Tissue Specificity and Dynamics of Sex-Biased Gene Expression in a Common Frog Population with Differentiated, Yet Homomorphic, Sex Chromosomes

Sex-biased genes are central to the study of sexual selection, sexual antagonism, and sex chromosome evolution. We describe a comprehensive de novo assembled transcriptome in the common frog Rana temporaria based on five developmental stages and three adult tissues from both sexes, obtained from a population with karyotypically homomorphic but genetically differentiated sex chromosomes. This allows the study of sex-biased gene expression throughout development, and its effect on the rate of gene evolution while accounting for pleiotropic expression, which is known to negatively correlate with the evolutionary rate. Overall, sex-biased genes had little overlap among developmental stages and adult tissues. Late developmental stages and gonad tissues had the highest numbers of stage- or tissue-specific genes. We find that pleiotropic gene expression is a better predictor than sex bias for the evolutionary rate of genes, though it often interacts with sex bias. Although genetically differentiated, the sex chromosomes were not enriched in sex-biased genes, possibly due to a very recent arrest of XY recombination. These results extend our understanding of the developmental dynamics, tissue specificity, and genomic localization of sex-biased genes.

Origin and Evolution of the Neo-Sex Chromosomes in Pamphagidae Grasshoppers through Chromosome Fusion and Following Heteromorphization

In most phylogenetic lineages, the evolution of sex chromosomes is accompanied by their heteromorphization and degradation of one of them. The neo-sex chromosomes are useful model for studying early stages of these processes. Recently two lineages of the neo-sex chromosomes on different stages of heteromorphization was discovered in Pamphagidae family. The neo-sex chromosome heteromorphization was analyzed by generation of DNA probes derived from the neo-Xs and neo-Ys followed with chromosome painting in nineteen species of Pamphagidae family. The homologous regions of the neo-sex chromosomes were determined in closely related species with the painting procedure and image analysis with application of the Visualization of the Specific Signal in Silico software package. Results of these analyses and distribution of C-positive regions in the neo-sex chromosomes revealed details of the heteromorphization of the neo-sex chromosomes in species from both phylogenetic lineages of Pamphagidae grasshoppers. The hypothetical mechanism of the neo-Y degradation was suggested. It includes expansion of different repeats from the proximal neo-Y chromosome region by inversions, spreading them towards distal region. Amplification of these repeats leads to formation of C-positive regions and elimination of the C-negative regions located between them.

Sex Reversal in Amphibians

Amphibians have been widely used to study developmental biology due to the fact that embryo development takes place independently of the maternal organism and that observations and experimental approaches are easy. Some amphibians like Xenopus became model organisms in this field. In the first part of this article, the differentiation of the gonads in amphibians and the mechanisms governing this process are reviewed. In the second part, the state of the art about sex reversal, which can be induced by steroid hormones in general and by temperature in some species, is presented. Also information about pollutants found in the environment that could interfere with the development of the amphibian reproductive apparatus or with their reproductive physiology is given. Such compounds could play a part in the amphibian decline, since in the wild, many amphibians are endangered species.

Dmrt1 polymorphism covaries with sex-determination patterns in Rana temporaria

Patterns of sex-chromosome differentiation and gonadal development have been shown to vary among populations of Rana temporaria along a latitudinal transect in Sweden. Frogs from the northern-boreal population of Ammarnäs displayed well-differentiated X and Y haplotypes, early gonadal differentiation, and a perfect match between phenotypic and genotypic sex. In contrast, no differentiated Y haplotypes could be detected in the southern population of Tvedöra, where juveniles furthermore showed delayed gonadal differentiation. Here, we show that Dmrt1, a gene that plays a key role in sex determination and sexual development across all metazoans, displays significant sex differentiation in Tvedöra, with a Y-specific haplotype distinct from Ammarnäs. The differential segment is not only much shorter in Tvedöra than in Ammarnäs, it is also less differentiated and associates with both delayed gonadal differentiation and imperfect match between phenotypic and genotypic sex. Whereas Tvedöra juveniles with a local Y haplotype tend to ultimately develop as males, those without it may nevertheless become functional XX males, but with strongly female-biased progeny. Our findings suggest that the variance in patterns of sex determination documented in common frogs might result from a genetic polymorphism within a small genomic region that contains Dmrt1. They also substantiate the view that recurrent convergences of sex determination toward a limited set of chromosome pairs may result from the co-option of small genomic regions that harbor key genes from the sex-determination pathway.

Coexistence of Y, W, and Z sex chromosomes in Xenopus tropicalis

Homomorphic sex chromosomes and rapid turnover of sex-determining genes can complicate establishing the sex chromosome system operating in a given species. This difficulty exists in Xenopus tropicalis, an anuran quickly becoming a relevant model for genetic, genomic, biochemical, and ecotoxicological research. Despite the recent interest attracted by this species, little is known about its sex chromosome system. Direct evidence that females are the heterogametic sex, as in the related species Xenopus laevis, has yet to be presented. Furthermore, X. laevis' sex-determining gene, DM-W, does not exist in X. tropicalis, and the sex chromosomes in the two species are not homologous. Here we identify X. tropicalis' sex chromosome system by integrating data from (i) breeding sex-reversed individuals, (ii) gynogenesis, (iii) triploids, and (iv) crosses among several strains. Our results indicate that at least three different types of sex chromosomes exist: Y, W, and Z, observed in YZ, YW, and ZZ males and in ZW and WW females. Because some combinations of parental sex chromosomes produce unisex offspring and other distorted sex ratios, understanding the sex-determination systems in X. tropicalis is critical for developing this flexible animal model for genetics and ecotoxicology.

First-generation linkage map for the common frog Rana temporaria reveals sex-linkage group

The common frog (Rana temporaria) has become a model species in the fields of ecology and evolutionary biology. However, lack of genomic resources has been limiting utility of this species for detailed evolutionary genetic studies. Using a set of 107 informative microsatellite markers genotyped in a large full-sib family (800 F1 offspring), we created the first linkage map for this species. This partial map-distributed over 15 linkage groups-has a total length of 1698.8 cM. In line with the fact that males are the heterogametic sex in this species and a reduction of recombination is expected, we observed a lower recombination rate in the males (map length: 1371.5 cM) as compared with females (2089.8 cM). Furthermore, three loci previously documented to be sex-linked (that is, carrying male-specific alleles) in adults from the wild mapped to the same linkage group. The linkage map described in this study is one of the densest ones available for amphibians. The discovery of a sex linkage group in Rana temporaria, as well as other regions with strongly reduced male recombination rates, should help to uncover the genetic underpinnings of the sex-determination system in this species. As the number of linkage groups found (n=15) is quite close to the actual number of chromosomes (n=13), the map should provide a useful resource for further evolutionary, ecological and conservation genetic work in this and other closely related species.

Preparation of Xenopus tropicalis whole chromosome painting probes using laser microdissection and reconstruction of X. laevis tetraploid karyotype by Zoo-FISH

Laser microdissection was used for the preparation of whole chromosome painting probes in Silurana (Xenopus) tropicalis. Subsequent cross-species fluorescence in situ hybridization (Zoo-FISH) on its tetraploid relative Xenopus laevis revealed persistence of chromosomal quartets even after 50-65 million years of separate evolution. Their arrangement is in a partial concordance with previous experiments based on similarity of a high-resolution replication banding pattern. Further support for an allotetraploid origin of X. laevis was given by hybridization with a probe derived from the smallest X. tropicalis chromosome (Xt10). Here, pericentric areas of both arms of Xl 14 and 18 were stained, indicating intrachromosomal rearrangements. The positions of signals were not in agreement with the chromosomal quartets revealed by painting probes Xt 8 and 9 (Xl 11 + 14 and Xl 15 + 18, respectively). This suggests that both X. tropicalis chromosomes underwent non-reciprocal translocation of Xt10 separately in at least two different ancient ancestors. In addition, the observed translocation events could explain the origin of individuals with 18 chromosomes in diploid karyotypes, probably extinct after the genesis of the allotetraploid X. laevis (2n = 36).

Are homologies in vertebrate sex determination due to shared ancestry or to limited options?

The same candidate genes and the same autosomes are repeatedly used as sex chromosomes in vertebrates. Are these systems identical by descent, or are some genes or chromosomes intrinsically better at triggering the first steps of sex determination?

Patterns of reproductive isolation in toads

Understanding the general features of speciation is an important goal in evolutionary biology, and despite significant progress, several unresolved questions remain. We analyzed an extensive comparative dataset consisting of more than 1900 crosses between 92 species of toads to infer patterns of reproductive isolation. This unique dataset provides an opportunity to examine the strength of reproductive isolation, the development and sex ratios of hybrid offspring, patterns of fertility and infertility, and polyploidization in hybrids all in the context of genetic divergence between parental species. We found that the strength of intrinsic postzygotic isolation increases with genetic divergence, but relatively high levels of divergence are necessary before reproductive isolation is complete in toads. Fertilization rates were not correlated to genetic divergence, but hatching success, the number of larvae produced, and the percentage of tadpoles reaching metamorphosis were all inversely related with genetic divergence. Hybrids between species with lower levels of divergence developed to metamorphosis, while hybrids with higher levels of divergence stopped developing in gastrula and larval stages. Sex ratios of hybrid offspring were biased towards males in 70% of crosses and biased towards females in 30% of crosses. Hybrid females from crosses between closely related species were completely fertile, while approximately half (53%) of hybrid males were sterile, with sterility predicted by genetic divergence. The degree of abnormal ploidy in hybrids was positively related to genetic divergence between parental species, but surprisingly, polyploidization had no effect on patterns of asymmetrical inviability. We discuss explanations for these patterns, including the role of Haldane's rule in toads and anurans in general, and suggest mechanisms generating patterns of reproductive isolation in anurans.

Early aspects of gonadal sex differentiation in Xenopus tropicalis with reference to an antero-posterior gradient

In an effort to contribute to the development of Xenopus tropicalis as an amphibian model system, we carried out a detailed histological analysis of the process of gonadal sex differentiation and were able to find evidence that gonadal differentiation in X. tropicalis follows an antero-posterior gradient. Although the main reason for the presence of a gradient of sex differentiation is still unknown, this gradient enabled us to define the early events that signal ovarian and testicular differentiation and to identify the undifferentiated gonad structure. Given the various advantages of this emerging model, our work paves the way for experiments that should contribute to our understanding of the dynamics and mechanisms of gonadal sex differentiation in amphibians.

Unusual primitive heteromorphic ZZ/ZW sex chromosomes in Proceratophrys boiei (Anura, Cycloramphidae, Alsodinae), with description of C-Band interpopulational polymorphism

We performed cytogenetic analyses on specimens from three population samples of Proceratophrys boiei from southeastern and northeastern Brazil. We stained chromosomes of mitotic and meiotic cells with Giemsa, C-banding and Ag-NOR methods. All specimens of P. boiei presented a karyotype with a full chromosome complement of 2n=22, metacentric and submetacentric. We observed the secondary constriction within the short arm of pair 8, which was in the same position of the nucleolus organizer region (NOR). NOR heteromorphism was observed within two specimens from the municipality of Mata de São João (northeastern Bahia State). The C-banding evidenced an unusual heterochromatic pattern in the genome of P. boiei. In the southern most population samples (São Paulo State), we observed large blocks of heterochromatin in the centromeric regions of all chromosomes, whereas the northernmost samples (Bahia State) presented a small amount of constitutive heterochromatin. We suppose that this geographic variation in heterochromatin quantities could be due to heterochromatinization of some chromosome regions in the genome of the São Paulo samples. Furthermore, females from São Paulo presented, within chromosome pair 1 from C-banded karyotypes, one homologous chromosome almost heterochromatic, whereas males had heterochromatin restricted to the centromeric region. This unusual heterochromatic arrangement led us to assume that P. boiei owns a ZZ/ZW type of sexual determination system. This finding is very important, as this is the first record of ZZ/ZW sex chromosomes within Cycloramphidae. We believe that the cytogenetic differences found between southeastern and northeastern Brazilian population samples of P. boiei strongly supports the existence of a species complex under the name P. boiei, and the requirement of taxonomic and systematic reviews by morphological, bioacoustical, molecular, and cytogenetic data could define this taxonomic issue in the future.

Cytological evidence for population-specific sex chromosome heteromorphism in Palaearctic green toads (Amphibia, Anura)

A chromosome study was carried out on a number of European and Central Asiatic diploid green toad populations by means of standard and various other chromosome banding and staining methods (Ag-NOR-, Q-, CMA3-, late replicating [LR] banding pattern, C-and sequential C-banding + CMA3 + DAPI). This study revealed the remarkable karyological uniformity of specimens from all populations, with the only exception being specimens from a Moldavian population, where one chromosome pair was heteromorphic. Though similar in shape, size and with an identical heterochromatin distribution,the difference in the heteromorphic pair was due to a large inverted segment on its long arms. This heteromorphism was restricted to females, suggesting a female heterogametic sex chromosome system of ZZ/ZW type at a very early step of differentiation.

Establishment of high-resolution FISH mapping system and its application for molecular cytogenetic characterization of chromosomes in newt, Cynops pyrrhogaster (Urodela, Amphibia)

Urodele amphibians (newts and salamanders) are important animal models for understanding regeneration mechanisms and genome evolution. We constructed ideograms of BrdU/dT- and C-banded karyotypes in the Japanese fire-belly newt, Cynops pyrrhogaster, which is useful as a model animal with extremely high ability of regeneration. We also established a high-resolution FISH mapping system for newts, and localized satellite DNA sequences, 18S rDNAs, telomeric (TTAGGG)n repeats and seven functional genes, including genes associated with lens regeneration, tyrosinase and two types of gamma crystallins, to chromosomes of the newt. The 18S rDNAs were localized to three chromosomal pairs in males, whereas the chromosomal locations were highly variable in females. No hybridization signals were detected for the telomeric (TTAGGG)n sequence. All three lens regeneration-related genes were mapped on the short arm of chromosome 7, suggesting that the location of the genes in the same linkage group may be correlated with the regulation of gene expression associated with chromatin dynamics in interphase nuclei during regeneration. The chromosomal distribution and nucleotide sequences of pericentric satellite DNA sequences were well conserved between C. pyrrhogaster and European newts; in contrast, there was species specificity of nucleotide sequences for centromere-specific satellite DNAs.

Probing the W chromosome of the codling moth, Cydia pomonella, with sequences from microdissected sex chromatin

The W chromosome of the codling moth, Cydia pomonella, like that of most Lepidoptera species, is heterochromatic and forms a female-specific sex chromatin body in somatic cells. We collected chromatin samples by laser microdissection from euchromatin and W-chromatin bodies. DNA from the samples was amplified by degenerate oligonucleotide-primed polymerase chain reaction (DOP-PCR) and used to prepare painting probes and start an analysis of the W-chromosome sequence composition. With fluorescence in situ hybridization (FISH), the euchromatin probe labelled all chromosomes, whereas the W-chromatin DNA proved to be a highly specific W-chromosome painting probe. For sequence analysis, DOP-PCR-generated DNA fragments were cloned, sequenced, and tested by Southern hybridization. We recovered single-copy and low-copy W-specific sequences, a sequence that was located only in the W and the Z chromosome, multi-copy sequences that were enriched in the W chromosome but occurred also elsewhere, and ubiquitous multi-copy sequences. Three of the multi-copy sequences were recognized as derived from hitherto unknown retrotransposons. The results show that our approach is feasible and that the W-chromosome composition of C. pomonella is not principally different from that of Bombyx mori or from that of Y chromosomes of several species with an XY sex-determining mechanism. The W chromosome has attracted repetitive sequences during evolution but also contains unique sequences.

Quantifying patterns in the evolution of reproductive isolation

We present a likelihood-based statistical method for examining the pattern or rate of evolution of reproductive isolation. The method uses large empirical datasets to estimate, for a given clade, the average duration of two phases in the divergence of populations. The first phase is a lag phase and refers to the period during which lineages diverge but no detectable reproductive isolation evolves. The second is an accumulation phase, referring to the period during which the magnitude of reproductive isolation between diverging lineages increases. The pattern of evolution is inferred from the relative durations of these two phases. Results of analyses of postzygotic isolation data indicate significant differences among taxa in the pattern of evolution of postzygotic isolation that are consistent with predictions based on genetic differences among these groups. We also examine whether the evolution of postzygotic isolation is best explained by either of two models for the rate of accumulation: a linear model or a quadratic function as may be suggested by recent studies. Our analysis indicates that the appropriateness of either model varies among taxa.

Geographic variability in expression of the sex-linked AAT-1 gene of the bell-ring frog, Buergeria buergeri

Cytological observation and artificial crossing experiments were used to examine the geographic differences in the sex-determining mechanism and mode of inheritance of the sex-linked AAT-1 gene in the bell-ring frog, Buergeria buergeri. The AAT-1 phenotypes were also examined by allozyme analysis using field-caught females and males collected from 19 populations from the Honshu, Shikoku, and Kyushu islands of Japan, in order to comprehensively elucidate the geographic variability in the expression of the sex-linked AAT-1 gene of B. buergeri. The results showed that the Aomori population of B. buergeri from the northern end of Honshu was female heterogametic in sex determination, that chromosome No. VII was a sex chromosome of the ZZ/ZW type, and that the sex-linked AAT-1 gene was expressed on both the Z and W chromosomes. This mode of AAT-1 expression in the Aomori population was different from that in the Hiroshima population from western Honshu, in which the AAT-1 gene was expressed on the Z chromosome but not on the W chromosome. The results also showed that there was no differentiation among populations in the expression of the AAT-1 genes on the Z chromosome, whereas two populations, the Hiroshima and Aomori frogs, exhibited distinct modes of expression of the AAT-1 gene on the W chromosome. These two modes of expression may be widely distributed in western and eastern Japan, and coexist in the central part of Honshu.

Comparative genome analysis of the primary sex-determining locus in salmonid fishes

We compared the Y-chromosome linkage maps for four salmonid species (Arctic charr, Salvelinus alpinus; Atlantic salmon, Salmo salar; brown trout, Salmo trutta; and rainbow trout, Oncorhynchus mykiss) and a putative Y-linked marker from lake trout (Salvelinus namaycush). These species represent the three major genera within the subfamily Salmoninae of the Salmonidae. The data clearly demonstrate that different Y-chromosomes have evolved in each of the species. Arrangements of markers proximal to the sex-determining locus are preserved on homologous, but different, autosomal linkage groups across the four species studied in detail. This indicates that a small region of DNA has been involved in the rearrangement of the sex-determining region. Placement of the sex-determining region appears telomeric in brown trout, Atlantic salmon, and Arctic charr, whereas an intercalary location for SEX may exist in rainbow trout. Three hypotheses are proposed to account for the relocation: translocation of a small chromosome arm; transposition of the sex-determining gene; or differential activation of a primary sex-determining gene region among the species.