Cytogenetics of the endemic New Zealand frog, Leiopelma hochstetteri: extraordinary supernumerary chromosome variation and a unique sex-chromosome system

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.

The first linkage map for Australo-Papuan Treefrogs (family: Pelodryadidae) reveals the sex-determination system of the Green-eyed Treefrog (Litoria serrata)

Amphibians represent a useful taxon to study the evolution of sex determination because of their highly variable sex-determination systems. However, the sex-determination system for many amphibian families remains unknown, in part because of a lack of genomic resources. Here, using an F1 family of Green-eyed Treefrogs (Litoria serrata), we produce the first genetic linkage map for any Australo-Papuan Treefrogs (family: Pelodryadidae). The resulting linkage map contains 8662 SNPs across 13 linkage groups. Using an independent set of sexed adults, we identify a small region in linkage group 6 matching an XY sex-determination system. These results suggest Litoria serrata possesses a male heterogametic system, with a candidate sex-determination locus on linkage group 6. Furthermore, this linkage map represents the first genomic resource for Australo-Papuan Treefrogs, an ecologically diverse family of over 220 species.

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)'.

Sex chromosomes in the Vizcacheras' White-lipped frog, Leptodactylus bufonius (Anura, Leptodactylidae)

Cytogenetic analyses were performed on specimens of Leptodactylus bufonius from different localities in Argentina. Mitotic chromosomes were studied with Giemsa and differential staining techniques (Ag-NOR, C-banding, and CMA3/DAPI) and fluorescence in situ hybridization with the 18S DNAr probe. All specimens showed karyotypes with 2n = 2x = 22 and FN = 44. Secondary constrictions were present in the long arm of chromosome pair 8, coincident with Ag-NOR and hybridization signals of the 18S DNAr probe. The C-banding technique evidenced an important amount of heterochromatin with a sex-linked pericentromeric band in the short arm of chromosome pair 4. This heterochromatic band was heteromorphic in males but present in both homologues of females, and it was CMA3 positive (DAPI negative) at fluorescence staining. The occurrence of heteromorphic XY sex chromosomes in L. bufonius is the second known case in Leptodactylus and the fifth within the speciose family Leptodactylidae.

Sex Differentiation in Amphibians: Effect of Temperature and Its Influence on Sex Reversal

The role of environmental factors in sexual differentiation in amphibians is not new. The effect of hormones or hormone-like compounds is widely demonstrated. However, the effect of temperature has traditionally been regarded as something anecdotal that occurs in extreme situations and not as a factor to be considered. The data currently available reveal a different situation. Sexual differentiation in some amphibian species can be altered even by small changes in temperature. On the other hand, although not proven, it is possible that temperature is related to the appearance of sex-reversed individuals in natural populations under conditions unrelated to environmental contaminants. According to this, temperature, through sex reversal (phenotypic sex opposed to genetic sex), could play an important role in the turnover of sex-determining genes and in the maintenance of homomorphic sex chromosomes in this group. Accordingly, and given the expected increase in global temperatures, growth and sexual differentiation in amphibians could easily be affected, altering the sex ratio in natural populations and posing major conservation challenges for a group in worldwide decline. It is therefore particularly urgent to understand the mechanism by which temperature affects sexual differentiation in amphibians.

Comparative Distribution of Repetitive Sequences in the Karyotypes of Xenopus tropicalis and Xenopus laevis (Anura, Pipidae)

Xenopus laevis and its diploid relative, Xenopus tropicalis, are the most used amphibian models. Their genomes have been sequenced, and they are emerging as model organisms for research into disease mechanisms. Despite the growing knowledge on their genomes based on data obtained from massive genome sequencing, basic research on repetitive sequences in these species is lacking. This study conducted a comparative analysis of repetitive sequences in X. laevis and X. tropicalis. Genomic in situ hybridization (GISH) and fluorescence in situ hybridization (FISH) with Cot DNA of both species revealed a conserved enrichment of repetitive sequences at the ends of the chromosomes in these Xenopus species. The repeated sequences located on the short arm of chromosome 3 from X. tropicalis were not related to the sequences on the short arm of chromosomes 3L and 3S from X. laevis, although these chromosomes were homoeologous, indicating that these regions evolved independently in these species. Furthermore, all the other repetitive sequences in X. tropicalis and X. laevis may be species-specific, as they were not revealed in cross-species hybridizations. Painting experiments in X. laevis with chromosome 7 from X. tropicalis revealed shared sequences with the short arm of chromosome 3L. These regions could be related by the presence of the nucleolus organizer region (NOR) in both chromosomes, although the region revealed by chromosome painting in the short arm of chromosome 3L in X. laevis did not correspond to 18S + 28S rDNA sequences, as they did not colocalize. The identification of these repeated sequences is of interest as they provide an explanation to some problems already described in the genome assemblies of these species. Furthermore, the distribution of repetitive DNA in the genomes of X. laevis and X. tropicalis might be a valuable marker to assist us in understanding the genome evolution in a group characterized by numerous polyploidization events coupled with hybridizations.

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.

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.

Sex Chromosome Evolution: So Many Exceptions to the Rules

Genomic analysis of many nonmodel species has uncovered an incredible diversity of sex chromosome systems, making it possible to empirically test the rich body of evolutionary theory that describes each stage of sex chromosome evolution. Classic theory predicts that sex chromosomes originate from a pair of homologous autosomes and recombination between them is suppressed via inversions to resolve sexual conflict. The resulting degradation of the Y chromosome gene content creates the need for dosage compensation in the heterogametic sex. Sex chromosome theory also implies a linear process, starting from sex chromosome origin and progressing to heteromorphism. Despite many convergent genomic patterns exhibited by independently evolved sex chromosome systems, and many case studies supporting these theoretical predictions, emerging data provide numerous interesting exceptions to these long-standing theories, and suggest that the remarkable diversity of sex chromosomes is matched by a similar diversity in their evolution. For example, it is clear that sex chromosome pairs are not always derived from homologous autosomes. In addition, both the cause and the mechanism of recombination suppression between sex chromosome pairs remain unclear, and it may be that the spread of recombination suppression is a more gradual process than previously thought. It is also clear that dosage compensation can be achieved in many ways, and displays a range of efficacy in different systems. Finally, the remarkable turnover of sex chromosomes in many systems, as well as variation in the rate of sex chromosome divergence, suggest that assumptions about the inevitable linearity of sex chromosome evolution are not always empirically supported, and the drivers of the birth-death cycle of sex chromosome evolution remain to be elucidated. Here, we concentrate on how the diversity in sex chromosomes across taxa highlights an equal diversity in each stage of sex chromosome evolution.

Changing sex for selfish gain: B chromosomes of Lake Malawi cichlid fish

B chromosomes are extra, non-essential chromosomes present in addition to the normal complement of A chromosomes. Many species of cichlid fish in Lake Malawi carry a haploid, female-restricted B chromosome. Here we show that this B chromosome exhibits drive, with an average transmission rate of 70%. The offspring of B-transmitting females exhibit a strongly female-biased sex ratio. Genotyping of these offspring reveals the B chromosome carries a female sex determiner that is epistatically dominant to an XY system on linkage group 7. We suggest that this sex determiner evolved to enhance the meiotic drive of the B chromosome. This is some of the first evidence that female meiotic drive can lead to the invasion of new sex chromosomes solely to benefit the driver, and not to compensate for skewed sex ratios.

Diverse and variable sex determination mechanisms in vertebrates

Sex is prevalent in nature and sex determination is one of the most fundamental biological processes, while the way of initiating female and male development exhibits remarkable diversity and variability across vertebrates. The knowledge on why and how sex determination mechanisms evolve unusual plasticity remains limited. Here, we summarize sex determination systems, master sex-determining genes and gene-regulatory networks among vertebrates. Recent research advancements on sex determination system transition are also introduced and discussed in some non-model animals with multiple sex determination mechanisms. This review will provide insights into the origin, transition and evolutionary adaption of different sex determination strategies in vertebrates, as well as clues for future perspectives in this field.

Chromosome evolution in Cophomantini (Amphibia, Anura, Hylinae)

The hylid tribe Cophomantini is a diverse clade of Neotropical treefrogs composed of the genera Aplastodiscus, Boana, Bokermannohyla, Hyloscirtus, and Myersiohyla. The phylogenetic relationships of Cophomantini have been comprehensively reviewed in the literature, providing a suitable framework for the study of chromosome evolution. Employing different banding techniques, we studied the chromosomes of 25 species of Boana and 3 of Hyloscirtus; thus providing, for the first time, data for Hyloscirtus and for 15 species of Boana. Most species showed karyotypes with 2n = 2x = 24 chromosomes; some species of the B. albopunctata group have 2n = 2x = 22, and H. alytolylax has 2n = 2x = 20. Karyotypes are all bi-armed in most species presented, with the exception of H. larinopygion (FN = 46) and H. alytolylax (FN = 38), with karyotypes that have a single pair of small telocentric chromosomes. In most species of Boana, NORs are observed in a single pair of chromosomes, mostly in the small chromosomes, although in some species of the B. albopunctata, B. pulchella, and B. semilineata groups, this marker occurs on the larger pairs 8, 1, and 7, respectively. In Hyloscirtus, NOR position differs in the three studied species: H. alytolylax (4p), H. palmeri (4q), and H. larinopygion (1p). Heterochromatin is a variable marker that could provide valuable evidence, but it would be necesserary to understand the molecular composition of the C-bands that are observed in different species in order to test its putative homology. In H. alytolylax, a centromeric DAPI+ band was observed on one homologue of chromosome pair 2. The band was present in males but absent in females, providing evidence for an XX/XY sex determining system in this species. We review and discuss the importance of the different chromosome markers (NOR position, C-bands, and DAPI/CMA3 patterns) for their impact on the taxonomy and karyotype evolution in Cophomantini.

Amphibian and Avian Karyotype Evolution: Insights from Lampbrush Chromosome Studies

Amphibian and bird karyotypes typically have a complex organization, which makes them difficult for standard cytogenetic analysis. That is, amphibian chromosomes are generally large, enriched with repetitive elements, and characterized by the absence of informative banding patterns. The majority of avian karyotypes comprise a small number of relatively large macrochromosomes and numerous tiny morphologically undistinguishable microchromosomes. A good progress in investigation of amphibian and avian chromosome evolution became possible with the usage of giant lampbrush chromosomes typical for growing oocytes. Due to the giant size, peculiarities of organization and enrichment with cytological markers, lampbrush chromosomes can serve as an opportune model for comprehensive high-resolution cytogenetic and cytological investigations. Here, we review the main findings on chromosome evolution in amphibians and birds that were obtained using lampbrush chromosomes. In particular, we discuss the data on evolutionary chromosomal rearrangements, accumulation of polymorphisms, evolution of sex chromosomes as well as chromosomal changes during clonal reproduction of interspecies hybrids.

Biological sex identification in the endangered dusky gopher frog (Lithobates sevosa): a comparison of body size measurements, secondary sex characteristics, ultrasound imaging, and urinary hormone analysis methods

BACKGROUND

Accurate sex identification techniques are important for wildlife demographic studies and for genetic management of captive breeding colonies. Various non-invasive methods for identification of biological sex in the weakly dimorphic endangered dusky gopher frog (DGF; Lithobates sevosa) were explored to support planned recovery efforts for this species including breeding and augmentation of wild populations.

METHODS

Body size (snout-vent length and body weight) measurements, observation of nuptial pads, ultrasound imaging, and urinary hormone analysis for testosterone and estrone were performed on 27 male and 19 female DGFs. For each method, the mean and range of measurement values were determined for male and female DGFs housed in a captive breeding population. The ability of these methods to accurately predict the true biological sex of the individuals was assessed retrospectively.

RESULTS

Body size measurements were of limited use for sex identification purposes, as males and females demonstrated overlapping body lengths and weights. Observation of the presence/absence of nuptial pads in males and females, respectively, proved to be accurate and easy to perform in most cases. Ultrasound imaging was useful for predicting the sex of female frogs, particularly when females were gravid. Commercial enzyme immunoassay kits were validated to measure urinary hormones in the DGF. Mean urinary testosterone (males: 2.22 ± 0.38 ng/ml; females: 0.92 ± 0.11 ng/ml) and estrone (males: 0.08 ± 0.01 ng/ml; females: 1.50 ± 0.39 ng/ml) concentrations were significantly (p < 0.05) different between the sexes. However, there was some overlap in hormone concentrations between the sexes. When a ratio of testosterone (T) to estrone (E) concentrations was calculated for each individual, males demonstrated significantly greater T/E ratios compared to females (p < 0.05). Use of this ratio showed greater accuracy in predicting the sex of the animal compared to using testosterone or estrone concentrations alone.

CONCLUSIONS

Monitoring for presence/absence of nuptial pads and using urinary testosterone to estrone hormone ratios were the most accurate methods for identifying the biological sex of adult DGFs. Urinary hormone measurements for sex identification may be useful in other weakly dimorphic and monomorphic amphibian species in both ex situ and in situ settings.

Unraveling the Sex Chromosome Heteromorphism of the Paradoxical Frog Pseudis tocantins

The paradoxical frog Pseudis tocantins is the only species in the Hylidae family with known heteromorphic Z and W sex chromosomes. The Z chromosome is metacentric and presents an interstitial nucleolar organizer region (NOR) on the long arm that is adjacent to a pericentromeric heterochromatic band. In contrast, the submetacentric W chromosome carries a pericentromeric NOR on the long arm, which is adjacent to a clearly evident heterochromatic band that is larger than the band found on the Z chromosome and justify the size difference observed between these chromosomes. Here, we provide evidence that the non-centromeric heterochromatic bands in Zq and Wq differ not only in size and location but also in composition, based on comparative genomic hybridization (CGH) and an analysis of the anuran PcP190 satellite DNA. The finding of PcP190 sequences in P. tocantins extends the presence of this satellite DNA, which was previously detected among Leptodactylidae and Hylodidae, suggesting that this family of repetitive DNA is even older than it was formerly considered. Seven groups of PcP190 sequences were recognized in the genome of P. tocantins. PcP190 probes mapped to the heterochromatic band in Wq, and a Southern blot analysis indicated the accumulation of PcP190 in the female genome of P. tocantins, which suggests the involvement of this satellite DNA in the evolution of the sex chromosomes of this species.

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.

Possible interspecific origin of the B chromosome of Hypsiboas albopunctatus (Spix, 1824) (Anura, Hylidae), revealed by microdissection, chromosome painting, and reverse hybridisation

The B chromosome in the hylid Hypsiboas albopunctatus (2n = 22 + B) is small, almost entirely composed of C-positive heterochromatin, and does not pair with any chromosome of the A complement. B probe, obtained by microdissection and DOP-PCR amplification, was used to search for homology between the B and regular chromosomes of H. albopunctatus and of the related species H. raniceps (Cope, 1862). Reverse hybridisation was also carried out in the investigation. The B probe exclusively painted the supernumerary, not hybridising any other chromosomes in H. albopunctatus, but all H. raniceps chromosomes showed small labelling signals. This result might be an indication that differences exist between the repetitive sequences of A and B chromosomes of H. albopunctatus, and that the chromosomes of H. raniceps and the heterochromatin of the B chromosome of H. albopunctatus are enriched with the same type of repetitive DNA. In meiotic preparations, the B labelled about 30% of scored spermatids, revealing a non-mendelian inheritance, and the painted B in micronucleus suggests that the supernumerary is eliminated from germ line cells. Although our results could suggest an interespecific origin of the B at first sight, further analysis on its repetitive sequences is still necessary. Nevertheless, the accumulation of repetitive sequences, detected in another species, even though closely related, remains an intriguing question.

Evolution and biology of supernumerary B chromosomes

B chromosomes (Bs) are dispensable components of the genome exhibiting non-Mendelian inheritance and have been widely reported on over several thousand eukaryotes, but still remain an evolutionary mystery ever since their first discovery over a century ago [1]. Recent advances in genome analysis have significantly improved our knowledge on the origin and composition of Bs in the last few years. In contrast to the prevalent view that Bs do not harbor genes, recent analysis revealed that Bs of sequenced species are rich in gene-derived sequences. We summarize the latest findings on supernumerary chromosomes with a special focus on the origin, DNA composition, and the non-Mendelian accumulation mechanism of Bs.

Ribosomal DNA is active in different B chromosome variants of the grasshopper Eyprepocnemis plorans

B chromosomes are considered to be genetically inert elements. However, some of them are able to show nucleolus organizer region (NOR) activity, as detected by both cytological and molecular means. The grasshopper Eyprepocnemis plorans shows a B chromosome polymorphism characterized by the existence of many B variants. One of them, B24, shows NOR activity in about half of B-carrying males in the Torrox population. Molecular data have suggested the recent origin for B chromosomes in this species, and on this basis it would be expected that NOR activity was widespread among the different B variants. Here we test this hypothesis in four different B chromosome variants (B1, B2, B5, and B24) from 11 natural populations of the grasshopper E. plorans covering the south and east of the Iberian Peninsula plus the Balearic Islands. We used two different approaches: (1) the cytological observation of nucleoli attached to the distal region of the B chromosome (where the rDNA is located), and (2) the molecular detection of the rDNA transcripts carrying an adenine insertion characteristic of B chromosome ITS2 sequences. The results showed NOR expression not only for B24 but also for the B1 and B2 variants. However, the level of B-NOR expression in these latter variants, measured by the proportion of cells showing nucleoli attached to the B chromosomes, was much lower than that previously reported for B24. This suggests the possibility that structural or genetic background conditions are enhancing the expressivity of the rDNA in the B24 variant.

B-chromosome ribosomal DNA is functional in the grasshopper Eyprepocnemis plorans

B-chromosomes are frequently argued to be genetically inert elements, but activity for some particular genes has been reported, especially for ribosomal RNA (rRNA) genes whose expression can easily be detected at the cytological level by the visualization of their phenotypic expression, i.e., the nucleolus. The B(24) chromosome in the grasshopper Eyprepocnemis plorans frequently shows a nucleolus attached to it during meiotic prophase I. Here we show the presence of rRNA transcripts that unequivocally came from the B(24) chromosome. To detect these transcripts, we designed primers specifically anchoring at the ITS-2 region, so that the reverse primer was complementary to the B chromosome DNA sequence including a differential adenine insertion being absent in the ITS2 of A chromosomes. PCR analysis carried out on genomic DNA showed amplification in B-carrying males but not in B-lacking ones. PCR analyses performed on complementary DNA showed amplification in about half of B-carrying males. Joint cytological and molecular analysis performed on 34 B-carrying males showed a close correspondence between the presence of B-specific transcripts and of nucleoli attached to the B chromosome. In addition, the molecular analysis revealed activity of the B chromosome rDNA in 10 out of the 13 B-carrying females analysed. Our results suggest that the nucleoli attached to B chromosomes are actively formed by expression of the rDNA carried by them, and not by recruitment of nucleolar materials formed in A chromosome nucleolar organizing regions. Therefore, B-chromosome rDNA in E. plorans is functional since it is actively transcribed to form the nucleolus attached to the B chromosome. This demonstrates that some heterochromatic B chromosomes can harbour functional genes.

Transitions between sex-determining systems in reptiles and amphibians

Important technological advances in genomics are driving a new understanding of the evolution of sex determination in vertebrates. In particular, comparative chromosome mapping in reptiles has shown an intriguing distribution of homology in sex chromosomes across reptile groups. When this new understanding is combined with the widespread distribution of genetic and temperature-dependent sex-determination mechanisms among reptiles, it is apparent that transitions between modes have occurred many times, as they have for amphibians (particularly between male and female heterogamety). It is also likely that thermosensitivity in sex determination is a key factor in those transitions in reptiles, and possibly in amphibians too. New models of sex determination involving temperature thresholds are providing the framework for the investigation of transitions and making possible key predictions about the homologies and sex-determination patterns expected among taxa in these groups. Molecular cytogenetics and other genomic approaches are essential to providing the fundamental material necessary to make advances in this field.

B chromosomes have a functional effect on female sex determination in Lake Victoria cichlid fishes

The endemic cichlid fishes in Lake Victoria are a model system for speciation through adaptive radiation. Although the evolution of the sex-determination system may also play a role in speciation, little is known about the sex-determination system of Lake Victoria cichlids. To understand the evolution of the sex-determination system in these fish, we performed cytogenetic analysis in 11 cichlid species from Lake Victoria. B chromosomes, which are present in addition to standard chromosomes, were found at a high prevalence rate (85%) in these cichlids. In one species, B chromosomes were female-specific. Cross-breeding using females with and without the B chromosomes demonstrated that the presence of the B chromosomes leads to a female-biased sex ratio in this species. Although B chromosomes were believed to be selfish genetic elements with little effect on phenotype and to lack protein-coding genes, the present study provides evidence that B chromosomes have a functional effect on female sex determination. FISH analysis using a BAC clone containing B chromosome DNA suggested that the B chromosomes are derived from sex chromosomes. Determination of the nucleotide sequences of this clone (104.5 kb) revealed the presence of several protein-coding genes in the B chromosome, suggesting that B chromosomes have the potential to contain functional genes. Because some sex chromosomes in amphibians and arthropods are thought to be derived from B chromosomes, the B chromosomes in Lake Victoria cichlids may represent an evolutionary transition toward the generation of sex chromosomes.

Rye B chromosomes are weakly transcribed and might alter the transcriptional activity of A chromosome sequences

B chromosomes (Bs) are dispensable components of the genomes of numerous species. To test whether the transcriptome of a host is influenced by Bs, we looked for differences in expression in response to additional Bs. Comparative complementary DNA amplified fragment length polymorphism experiments resulted in the identification of 16 putative B-chromosome-associated transcripts. This comprises 0.7% of the total transcript number and indicates a low activity of Bs. We also provide evidence that B chromosome influences in trans the transcription of A chromosome sequences. The B-specific transcribed sequences B1334, B8149, and B2465 belong to high-copy families with similarity to mobile elements. For all analyzed B-chromosome-derived transcripts, similar A chromosome-encoded sequences were found which supports an A-derived origin of rye B chromosomes.

Population dynamics of a selfish B chromosome neutralized by the standard genome in the grasshopper Eyprepocnemis plorans

Effects of the B chromosome polymorphism of the grasshopper Eyprepocnemis plorans were analyzed in two natural populations. Postmating sexual selection, female fertility, and survival were studied. The B chromosome lacks drive and has no detectable effects on fitness. A neutral B cannot invade a population and establish a polymorphism, but the confidence limits on our estimates cannot exclude the possibility that the polymorphism is maintained by a balance between weak drive and weak selection against individuals with two and three B's. However, other lines of evidence favor the following model of the dynamics of the B in E. plorans. In a newly invaded population, the B has substantial drive, but the evolution of drive suppressor genes in the A chromosomes neutralizes the B drive so that it becomes near-neutral and begins a random walk toward extinction by stochastic loss. Because the B is common by the time drive disappears, the random walk is likely to continue for a long time. If in the course of the random walk a variant B with greater drive appears, then it will displace the original variant, and a new cycle of drive suppression and drift to extinction occurs. A simulation model of this process suggested that the mean time to extinction is proportional to the two-thirds power of the population size; it is much less affected by subpopulation size or the number of populations in a subdivided population.

Disentangling genetic vs. environmental causes of sex determination in the common frog, Rana temporaria

BACKGROUND

Understanding of sex ratio dynamics in a given species requires understanding its sex determination system, as well as access for reliable tools for sex identification at different life stages. As in the case of many other amphibians, the common frogs (Rana temporaria) do not have well differentiated sex chromosomes, and an identification of individuals' genetic sex may be complicated by sex reversals. Here, we report results of studies shedding light on the sex determination system and sex ratio variation in this species.

RESULTS

A microsatellite locus RtSB03 was found to be sex-linked in four geographically disparate populations, suggesting male heterogamy in common frogs. However, in three other populations examined, no or little evidence for sex-linkage was detected suggesting either ongoing/recent recombination events, and/or frequent sex-reversals. Comparison of inheritance patterns of alleles in RtSB03 and phenotypic sex within sibships revealed a mixed evidence for sex-linkage: all individuals with male phenotype carried a male specific allele in one population, whereas results were more mixed in another population.

CONCLUSION

These results make sense only if we assume that the RtSB03 locus is linked to male sex determination factor in some, but not in all common frog populations, and if phenotypic sex-reversals - for which there is earlier evidence from this species - are frequently occurring.

Relationships between Vertebrate ZW and XY Sex Chromosome Systems

The peculiar cytology and unique evolution of sex chromosomes raise many fundamental questions. Why and how sex chromosomes evolved has been debated over a century since H.J. Muller suggested that sex chromosome pairs evolved ultimately from a pair of autosomes. This theory was adapted to explain variations in the snake ZW chromosome pair and later the mammal XY. S. Ohno pointed out similarities between the mammal X and the bird/reptile Z chromosomes forty years ago, but his speculation that they had a common evolutionary origin, or at least evolved from similar regions of the genome, has been undermined by comparative gene mapping, and it is accepted that mammal XY and reptile ZW systems evolved independently from a common ancestor. Here we review evidence for the alternative theory, that ZW<-->XY transitions occurred during evolution, citing examples from fish and amphibians, and probably reptiles. We discuss new work from comparative genomics and cytogenetics that leads to a reconsideration of Ohno's idea and advance a new hypothesis that the mammal XY system may have arisen directly from an ancient reptile ZW system.

Differential gene expression in yellow-necked mice Apodemus flavicollis (Rodentia, Mammalia) with and without B chromosomes

Most B chromosomes are heavily heterochromatic, promoting the general idea that they are genetically inert. The B chromosomes of Apodemus flavicollis are euchromatic and show a high degree of homology with the A chromosomes. The euchromatic nature of B chromosomes in A. flavicollis suggests that they may carry active genes and have transcriptional activity. We applied the differential display reverse transcription-polymerase chain reaction (DD RT-PCR) in order to analyze and compare gene expression in animals possessing B chromosomes and animals without B chromosomes. After a second and third round of amplification, three cDNA fragments were differentially expressed in +B mice compared with 0B animals. These cDNAs were Chaperonin containing TCP-1, subunit 6b (zeta) (CCT6B), Fragile histidine triad gene (FHIT) and hypothetical gene XP transcript. The differential expression pattern was confirmed by Real Time RT-PCR. We suggest that altered expression of these important genes is due to the presence of B chromosomes. In elevating the expression of these genes, B chromosomes of A. flavicollis affect some of the crucial processes in the cell. The significance of these effects and the nature of B chromosomes of A. flavicollis are discussed in the context of the data presented.

Study on karyotype and occurrence of B chromosomes in two endemic species of the genus Pimelodus (Siluriformes, Pimelodidae) from the river Igua çu

The current work describes, for the first time, the occurrence of B chromosomes in the genus Pimelodus. Cytogenetic analysis in P. ortmanni and Pimelodus sp., from the river Iguaçu basin, revealed a diploid number of 56 chromosomes. Whereas distribution pattern of C-banding in Pimelodus sp. showed predominance of heterochromatin at the two extremities of several pairs of chromosomes, centromere markings predominated in P. ortmanni. Additional chromosomes in the two species were observed with numerical intra-individual variations between zero and four B, together with interspecies differences in their frequency. In Giemsa-stained metaphases two distinct morphological types of B chromosomes were identified: Bm (B metacentric) and Ba (B acrocentric). In P. ortmanni both types of B chromosomes were totally heterochromatic; the Bm type in Pimelodus sp. had heterochromatin throughout its entire extension; the Ba type was partially heterochromatic with telomere markings on the long arm.

The B chromosome polymorphism of the grasshopper Eyprepocnemis plorans in North Africa: III. Mutation rate of B chromosomes

B chromosome variation in nine Moroccan populations of the grasshopper Eyprepocnemis plorans was analysed for 3 consecutive years. In addition to B1, which was the predominant B chromosome in all nine populations, we found 15 other B variants, albeit at very low frequency. Eight variants were found in adults caught in the wild, four appeared in adults reared in the laboratory and seven were found in embryo progeny of controlled crosses between a 0B male and a B-carrying female. Some variants were found in more than one kind of material. At least the seven B variants that appeared in embryo progeny of females carrying a different B type arose de novo through mutation of the maternal B chromosome. The mutation rate of B chromosomes was 0.73%, on average, which explains the high variety of morphs and banding patterns found. The most frequent de novo mutations observed in these chromosomes were centromere misdivision with or without chromatid nondisjunction, which generates iso-B-chromosomes or telocentric Bs, respectively, as well as translocations with A and B chromosomes and deletions. But the whole variation observed, including that found in adult individuals, suggests that other mutations such as duplications, inversions and centric fusions do usually affect B chromosomes. Finally, B chromosome mutation rate was remarkably similar in both Moroccan and Spanish populations, which suggests that it might be dependent on B chromosome intrinsic factors.

B-chromosomes and male-biased sex ratio with paternal inheritance in the fairy shrimp Branchipus schaefferi (Crustacea, Anostraca)

This study reports on male-biased sex ratios in west Mediterranean populations of the freshwater anostracan Branchipus schaefferi (Crustacea, Anostraca, Branchipodidae), in contrast to populations elsewhere. Crossing experiments over several generations indicate a clear paternal inheritance of the trait, possibly with a dosage effect. Various mechanisms which may underlie this phenomenon are discussed, the most plausible being the presence of one or more supernumerary ('B') chromosomes--as evidenced by karyological observations--interfering with sex determination and probably having an accumulation mechanism in male individuals.

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

In this chapter the different categories of homomorphic and heteromorphic sex chromosomes, types of sex-determining mechanisms, known sex-linked genes, and data about sex-determining genes in the Amphibia have been compiled. Thorough cytogenetic analyses have shown that both XY/XX and ZW/ZZ sex chromosomes exist in the order Anura and Urodela. In some species quite unusual systems of sex determination have evolved (e.g. 0W-females/00-males or the co-existence of XY/XX and ZW/ZZ sex chromosomes within the same species). In the third order of the Amphibia, the Gymnophiona (or Apoda) there is still no information regarding any aspect of sex determination. Whereas most species of Anura and Urodela present undifferentiated, homomorphic sex chromosomes, there is also a considerable number of species in which an increasing structural complexity of the Y and W chromosomes exists. In various cases, the morphological differentiation of the sex chromosomes occurred as a result of quantitative and/or qualitative changes to the repetitive DNA sequences in the constitutive heterochromatin of the Y and W chromosomes. The greater the structural differences between the sex chromosomes, the lesser the extent of pairing in meiosis. No dosage compensation of the sex-linked genes in the somatic cells of the homogametic (XX or ZZ) individuals have been detected. The genes located to date on the amphibian sex chromosomes lead to the conclusion that there is no common ancestral or conserved sex-linkage group. In all amphibians, genetic sex determination (GSD) seems to operate, although environmental factors may influence sex determination and differentiation. Despite the accumulated evidence that GSD is operating in Anura and Urodela, there is little substantial information about how it functions. Although several DNA sequences homologous to the mammalian ZFY, SRY and SOX genes have been detected in the Anura or Urodela, none of these genes is an appropriate candidate to explain sex determination in these vertebrates.

A model of the evolution of the unusual sex chromosome system of Microtus oregoni

In the creeping vole, Microtus oregoni, females are X0 and males are XY. In the female germ line, mitotic nondisjunction ensures that the products of meiosis all carry the X chromosome. Similarly, mitotic nondisjunction in the male germ line leads to the production of 0 and Y sperm. We propose that the present situation in M. oregoni has evolved by invasion of a normal XX/XY system by a mutant X chromosome, X', with a complete transmission advantage in X'X females, and a complete transmission disadvantage in X'Y males. X' is at best initially nearly neutral, but can gain a transmission advantage if it reaches a high enough frequency. This is due to the production of X0 females in matings between XX females and X'Y males; low fertility and embryo loss of such females reduce the fitness of the X chromosome in females, relative to that of X'. Under some conditions, however, the enhanced reproductive value of males, caused by the production of inviable Y0 embryos in X0 x X'Y matings, can outweigh any advantage to X'. Inbreeding also reduces any advantage to X'.