Cytogenetic Analysis in the Toad Species Bufo spinosus, Bufotes viridis and Epidalea calamita (Anura, Bufonidae) from the Mediterranean Area

Taxonomy in Bufonidae witnessed notable transformations. Bufotes viridis and Epidalea calamita, previously included in genus Bufo, were relocated in other genera, while the genus Bufo was restricted to members of the earlier Bufo bufo group. On the other hand, Bufo bufo sensu lato now includes four species: Bufo bufo, Bufo spinosus, Bufo verrucosissimus and Bufo eichwaldi. In this study, we examined three species of three Bufonidae genera (B. spinosus, B. viridis and E. calamita) by conventional (C-banding and Ag-NOR staining) and molecular (in situ hybridization with probes for telomeric repeats and rDNA loci, and genomic in situ hybridization (GISH)) cytogenetic methods. C-banding patterns are reported for the first time for B. spinosus and E. calamita populations from Iberian Peninsula and for B. viridis from Greece, and reveal several differences with the reported C-banded karyotypes described for other European populations of these species. Silver staining shows size heteromorphisms of the signals at the Nucleolar Organizing Region (NOR). By contrast, FISH with ribosomal probes only reveal size heteromorphism of rDNA sequences in E. calamita, suggesting that the differences observed after silver staining in B. spinosus and B. viridis should be attributed to differences in chromosomal condensation and/or gene activity rather than to differences in the copy number for ribosomal genes. Regarding telomeric repeats, E. calamita is the only species with interstitial telomeric sequences (ITS) located on centromeric regions, probably originated by accumulation of telomeric sequences in the centromeric heterochromatin. Finally, we analyzed the composition and distribution of repetitive sequences by genome in situ hybridization. These experiments reveal the accumulation of repetitive sequences in centromeric regions of the three species, although these sequences are not conserved when species from different genera are compared.

Expanding the classical paradigm: what we have learnt from vertebrates about sex chromosome evolution

Until recently, the field of sex chromosome evolution has been dominated by the canonical unidirectional scenario, first developed by Muller in 1918. This model postulates that sex chromosomes emerge from autosomes by acquiring a sex-determining locus. Recombination reduction then expands outwards from this locus, to maintain its linkage with sexually antagonistic/advantageous alleles, resulting in Y or W degeneration and potentially culminating in their disappearance. Based mostly on empirical vertebrate research, we challenge and expand each conceptual step of this canonical model and present observations by numerous experts in two parts of a theme issue of Phil. Trans. R. Soc. B. We suggest that greater theoretical and empirical insights into the events at the origins of sex-determining genes (rewiring of the gonadal differentiation networks), and a better understanding of the evolutionary forces responsible for recombination suppression are required. Among others, crucial questions are: Why do sex chromosome differentiation rates and the evolution of gene dose regulatory mechanisms between male versus female heterogametic systems not follow earlier theory? Why do several lineages not have sex chromosomes? And: What are the consequences of the presence of (differentiated) sex chromosomes for individual fitness, evolvability, hybridization and diversification? We conclude that the classical scenario appears too reductionistic. Instead of being unidirectional, we show that sex chromosome evolution is more complex than previously anticipated and principally forms networks, interconnected to potentially endless outcomes with restarts, deletions and additions of new genomic material. 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 II)'.

Interaction between sex-determining genes from two species: clues from Xenopus hybrids

Hybrids provide an interesting model to study the evolution of sex-determining genes and sex chromosome systems as they offer the opportunity to see how independently evolving sex-determining pathways interact in vivo. In this context, the genus Xenopus represents a stimulating model, since species with non-homologous sex chromosomes and different sex-determining genes have been identified. In addition, the possibility of interspecies breeding is favoured in this group, which arose by alloploidization events, with species ploidy ranging from 2n = 2x = 20 in X. tropicalis (the only diploid representative of the genus) to 2n = 12x = 108 in X. ruwenzoriensis. To study how two sex-determining genes interact in vivo, X. laevis × X. tropicalis hybrids were produced. Gonadal differentiation in these hybrids revealed that the dm-w gene is dominant over X. tropicalis male-determining sex chromosomes (Y or Z), even though the Y chromosome is dominant in X. tropicalis (Y > W>Z). In the absence of the dm-w gene (the Z chromosome from X. laevis is present), the W chromosome from X. tropicalis is able to trigger ovarian development. Testicular differentiation will take place in the absence of W chromosomes from any of the parental species. The dominance/recessivity relationships between these sex-determining loci in the context of either parental genome remains unknown. 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 II)'.

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.

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.

Characterization of the satellite DNA Msat-160 from the species Chionomys nivalis (Rodentia, Arvicolinae)

The satellite DNA Msat-160 has been previously characterized in several species of the genus Microtus. Here we present the characterization of Msat-160 from Chionomys nivalis, a species with a very primitive karyotype. As in other Microtus species analyzed, C. nivalis Msat-160 is AT rich, has a monomer length of 160 bp, is undermethylated and is mainly located in all the pericentromeric heterochromatin of all autosomes and the X chromosome, but is completely absent from the Y chromosome. Hence, our results support the hypothesis that Msat-160 was initially distributed in the pericentromeric heterochromatin of all autosomes and the X chromosome. The taxonomic status of the genus Chionomys in relation to the genus Microtus is a very interesting issue, so we constructed phylogenetic dendrograms using Msat-160 sequences from several Microtus species. Although the results were not informative about this issue, the presence of Msat-160 in C. nivalis and Microtus species suggested that both genera are closely related and that this satellite DNA was present in the common ancestor. Studies of Msat-160 in different arvicoline species could help to determine the origin of this satellite and, perhaps, to establish the phylogenetic relationships of some arvicoline groups.

Distribution of L1-retroposons on the giant sex chromosomes of Microtus cabrerae (Arvicolidae, Rodentia): functional and evolutionary implications

Long interspersed nuclear elements (L1 or LINE-1) are the most abundant and active retroposons in the mammalian genome. Traditionally, the bulk of L1 sequences have been explained by the 'selfish DNA' hypothesis; however, recently it has been also argued that L1s could play an important role in genome and gene organizations. The non-random chromosomal distribution of these retroelements is a striking feature considered to reflect this functionality. In the present study we have cloned and analyzed three different L1 fragments from the genome of the rodent Microtus cabrerae. In addition, we have examined the chromosomal distribution of this L1 in several species of Microtus, a very interesting group owing to the presence in some species of enlarged ('giant') sex chromosomes. Interestingly, in all species analyzed, L1-retroposons have preferentially accumulated on both the giant- and the normal-sized sex chromosomes compared with the autosomes. Also we have demonstrated that L1-retroposons are not similarly distributed among the heterochromatic blocks of the giant sex chromosomes in M. cabrerae and M. agrestis, which suggest that L1 retroposition and amplification over the sex heterochromatin have been different and independent processes in each species. Finally, we proposed that the main factors responsible for the L1 distribution on the mammalian sex chromosomes are the heterochromatic nature of the Y chromosome and the possible role of L1 sequences during the X-inactivation process.

X chromosome painting in Microtus: origin and evolution of the giant sex chromosomes

Sex chromosomes in species of the genus Microtus present some characteristic features that make them a very interesting group to study sex chromosome composition and evolution. M. cabrerae and M. agrestis have enlarged sex chromosomes (known as 'giant sex chromosomes') due to the presence of large heterochromatic blocks. By chromosome microdissection, we have generated probes from the X chromosome of both species and hybridized on chromosomes from six Microtus and one Arvicola species. Our results demonstrated that euchromatic regions of X chromosomes in Microtus are highly conserved, as occurs in other mammalian groups. The sex chromosomes heterochromatic blocks are probably originated by fast amplification of different sequences, each with an independent origin and evolution in each species. For this reason, the sex heterochromatin in Microtus species is highly heterogeneous within species (with different composition for the Y and X heterochromatic regions in M. cabrerae) and between species (as the composition of M. agrestis and M. cabrerae sex heterochromatin is different). In addition, the X chromosome painting results on autosomes of several species suggest that, during karyotypic evolution of the genus Microtus, some rearrangements have probably occurred between sex chromosomes and autosomes.

A repeat DNA sequence from the Y chromosome in species of the genus Microtus

In most mammals, the Y chromosome is composed of a large amount of constitutive heterochromatin. In some Microtus species, this feature is also extended to the X chromosome, resulting in enlarged (giant) sex chromosomes. Several repeated DNA sequences have been described in the gonosomal heterochromatin of these species, indicating that it has heterogeneous and species-specific composition and distribution. We have cloned an AT-rich, 851-bp long, repeated DNA sequence specific for M. cabrerae Y chromosome heterochromatin. The analysis of other species of the genus Microtus indicated that this sequence is also located on the Y chromosome (male-specific) in three species (M. agrestis, M. oeconomus and M. nivalis), present on both Y and X chromosomes and on some autosomes in M. arvalis and absent in the genome of M. guentheri. Our data also suggest that the mechanism of heterochromatin amplification operating on the sex chromosomes could have been different in each species since the repeated sequences of the gonosomal heterochromatic blocks in M. cabrerae and M. agrestis are different. The absence of this sequence in the mouse genome indicates that its evolutionary origin could be recent. Future analysis of the species distribution, localization and sequence of this repeat DNA family in arvicolid rodent species could help to establish the unsolved phylogenetic relationships in this rodent group.

Characterization of an EcoRI family of satellite DNA from two species of the genus Eptesicus (Vespertilionidae; Chiroptera)

We have cloned and sequenced a 321 bp band of repetitive DNA from Eptesicus fuscus and E. serotinus observed after gel electrophoresis of EcoRI digested genomic DNA in both species. Southern blot analysis of genomic DNA (from both species) digested with the same enzyme showed the existence of a ladder pattern indicating that the repetitive DNA is arrayed in tandem. The repetitive sequences have a monomer unit of 321 bp which is composed of two subunits of 160 bp, suggested by the existence of a 160 bp band in the ladder of E. fuscus and by the presence of some direct repeats found in the analysis of the consensus sequence. Analysis of the methylation status demonstrated that cytosines in CCGG sequences in this satellite DNA are methylated in E. fuscus but not in the E. serotinus. Alignment of the sequenced clones showed that several nucleotide positions are diagnostic species-specific and consequently the phylogenetic analysis grouped the monomer units from both species in two clearly separated groups.

Sex chromosomes pairing in two Arvicolidae species: Microtus nivalis and Arvicola sapidus

Arvicolid rodents present both synaptic and asynaptic sex chromosomes. We analyzed the pairing behaviour of sex chromosomes in two species belonging to this rodent group (Microtus nivalis and Arvicola sapidus). At pachynema, the sex chromosomes of both species paired in a small region while the rest remain unsynapsed. Consequently at metaphase I, sex chromosomes present end-to-end association. Thus, the pairing behaviour of sex chromosomes in these species is very similar to that previously described for other arvicolid rodents and for most mammals. According to this, we propose that synaptic sex chromosomes were the ancestral condition in the family Arvicolidae, including the genus Microtus. The phylogenetic origin of the asynaptic sex chromosomes in the genus Microtus would have arisen once in the lineage that originated the species M. arvalis/agrestis and related species, while the lineage that originated the species M. oeconomous and related species conserved synaptic chromosomes. Furthermore, the phylogenetic relationships between the genus Microtus, Chionomys and Pitymys are discussed in relation to the synaptic behaviour of sex chromosomes.

Sex chromosomes, sex determination, and sex-linked sequences in Microtidae

The Arvicolidae is a widely distributed rodent group with several interesting characteristics in their sex chromosomes. Here, we summarize the actual knowledge of some of these characteristics. This mammalian group has species with abnormal sex determination systems. In fact, some species present the same karyotype in both males and females, with total absence of a Y chromosome, and hence of SRY and ZFY genes. Other species present fertile, sex-reversed XY females, generally due to mutations affecting X chromosomes. Furthermore, in Microtus oregoni males and females are gonosomic mosaic (the females are XO in the soma and XX in the germ cells, while the males are XY in the soma and OY in the germ cells). Regarding sex chromosomes, some species present enlarged (giant) sex chromosomes because of the presence of large blocks of constitutive heterochromatin, which have been demonstrated to be highly heterogeneous. Furthermore, we also consider the alterations affecting composition and localization of sex-linked genes or repeated sequences. Finally, this rodent group includes species with synaptic and asynaptic sex chromosomes. In fact, several species with asynaptic sex chromosomes have been described. It is interesting to note that within the genus Microtus both types of sex chromosomes are present.

New C-band protocol by heat denaturation in the presence of formamide

C-banding techniques detect the presence of constitutive heterochromatin, which is usually located in centromeric regions of chromosomes in the majority of analysed species. The common method for C-banding used over the last 30 years involves treatment with a mild alkali barium hydroxide 5% Ba(OH)2 at 50 degrees C for 5-15 min and subsequent incubation in salt solution (2 x SSC at 60 degrees C for 1 h). We here present a new, easy and reliable technique for C-banding, which basically involves heat denaturation of chromosomal DNA in the presence of formamide and incubation in 2 x SSC at room temperature.

Pericentric satellite DNA sequences in Pipistrellus pipistrellus (Vespertilionidae; Chiroptera)

This paper reports the molecular and cytogenetic characterization of a HindIII family of satellite DNA in the bat species Pipistrellus pipistrellus. This satellite is organized in tandem repeats of 418 bp monomer units, and represents approximately 3% of the whole genome. The consensus sequence from five cloned monomer units has an A-T content of 62.20%. We have found differences in the ladder pattern of bands between two populations of the same species. These differences are probably because of the absence of the target sites for the HindIII enzyme in most monomer units of one population, but not in the other. Fluorescent in situ hybridization (FISH) localized the satellite DNA in the pericentromeric regions of all autosomes and the X chromosome, but it was absent from the Y chromosome. Digestion of genomic DNAs with HpaII and its isoschizomer MspI demonstrated that these repetitive DNA sequences are not methylated. Other bat species were tested for the presence of this repetitive DNA. It was absent in five Vespertilionidae and one Rhinolophidae species, indicating that it could be a species/genus specific, repetitive DNA family.

Repeated DNA sequences in the microbat species Miniopterus schreibersi (Vespertilionidae; Chiroptera)

Repetitive DNA sequences represent a substantial component of eukaryotic genomes. These sequences have been described and characterized in many mammalian species. However, little information about repetitive DNA sequences is available in bat species. Here we describe an EcoRI family of repetitive DNA sequences present in the species Miniopterus schreibersi. These repetitive sequences are 57.85%, A-T rich, organized in tandem, and with a monomer unit length of 904 bp. Methylation analysis using the isoesquizomer pair MspI and HpaII indicates that the cytosines present in the sequences CCGG are partially methylated. Furthermore, Southern blot analysis demonstrated that these DNA sequences are absent in the genomes of four related microbat species and suggest that it could be specific to the M. schreibersi genome.

Mapping the SRY gene in Microtus cabrerae: a vole species with multiple SRY copies in males and females

The SRY gene is a single-copy, male-specific gene, located on the Y chromosome in most mammals. However, recently we have described the presence of multiple polymorphic copies of this gene in both males and females of the vole species Microtus cabrerae. Here, we present the chromosomal localization of SRY gene copies in this species by fluorescent in situ hybridization (FISH). This technique localized these gene copies in the short arm, and hence in the euchromatic region, of the Y chromosome. Furthermore, several copies of the SRY gene are located on the X chromosome. These copies are spread along the entire heterochromatic region of the X chromosome, occupying the whole short arm, the centromeric region, and the pericentromeric region of the long arm.

Highly repeated DNA sequences in three species of the genus Pteropus (Megachiroptera, Mammalia)

Bat genomes are characterised by an A-T richness and by a small C-value compared with other mammalian groups. It has been suggested that the small C-value is mainly due to the lack of repetitive DNA sequences. However, little information about repetitive DNA sequences in this mammalian group is available at the molecular level. Here we describe a PstI family of repetitive DNA sequences present in three species of the genus Pteropus. These repetitive sequences are 54.97% G-C rich, organised in tandem and with a unit length of 744 bp. Methylation analysis indicates that some of the CCGG target sites present in these repetitive DNA sequences have methylated cytosines and that there are small differences in the methylation pattern between species. Several features of this family of repetitive sequences suggest that they evolved by concerted evolution.

HMG-box sequences from microbats homologous to the human SOX30 HMG-box

SOX genes are a family of genes that encode for proteins which are characterised by the presence of a HMG-domain related to that of the mammalian sex-determining gene (SRY). By definition, the DNA binding domain of SOX genes is at least 50% identical to the 79 amino acid HMG domain of the SRY gene. We report here two HMG-box sequences from two microbat species (R. ferrumequinum and P. Pipistrellus) which were PCR amplified using a primer pair specific to the mouse Sry HMG-box. The high percentage of identity of this sequences with the human and mouse SOX30 HMG-box suggests that they are the SOX30 HMG-box for these two bat species.

Molecular and cytogenetic characterization of highly repeated DNA sequences in the vole Microtus cabrerae

The genus Microtus presents several species with extremely large sex chromosomes that contain large blocks of constitutive heterochromatin. Several cytogenetic and molecular studies of the repetitive sequences in species of the genus Microtus have demonstrated that the heterochromatin is highly heterogeneous. We have cloned and characterized a family of repetitive DNA sequences from M. cabrerae, a species with large heterochromatic blocks on the giant sex chromosomes. These repetitive sequences are 65.84% A-T rich, organized in tandem, with a 161-bp unit and are located on the centromeric region of autosomes and the X chromosome. In addition, this repetitive DNA is located throughout the entire heterochromatic block of the X chromosome and on three interstitial bands in the heterochromatic block of the Y chromosome. Comparative analysis of this family of repetitive sequences from three Microtus species revealed that the development of these sequences has occurred by concerted evolution. Our results support the hypothesis that the heterochromatic blocks from the sex chromosomes of different species are evolving independently and they probably have the genetic capacity to amplify and retain different satellite DNAs. For a topic related to the location of these repetitive DNA sequences on the Y chromosome of M. cabrerae, we propose a model to explain the origin of a length polymorphism previously described for this chromosome.

Regulation of male sexual development by Sry and Sox9

Sry, a gene from the Y chromosome, is known to initiate testis formation and subsequent male differentiation in mammals. A related gene, Sox9, also plays a critical role in testis determination, possibly in all vertebrates. A number of models have been presented regarding the molecular modes of action of these two genes. However, details regarding their regulation, regulatory target genes, and interacting protein factors and co-factors have not been established with any certainty. In this review, we examine new evidence and re-examine existing evidence bearing on these issues, in an effort to build up an integrative model of the network of gene activity centred around Sry and Sox9. J. Exp. Zool. 290:463-474, 2001.

Searching for missing pieces of the sex-determination puzzle

Little is known of the mechanisms whereby the mammalian indifferent gonad develops into a testis or ovary. In XY individuals, Sry, the mammalian testis-determining gene, is expressed in the pre-Sertoli cells, which then differentiate into Sertoli cells. Other cell types, which include the germ cells, the steroidogenic cells and the connective tissue cells, must then be instructed to develop in a male-specific manner. Although some genes involved in sex-determination and differentiation processes have been identified, we know little of how they interact and cooperate to orchestrate the development of a testis or ovary. We have initiated an expression-screening program designed to identify additional genes, known or novel, which play a role in these processes. This approach is based on our belief that many of the genes we seek will be expressed in a sex-specific manner during the period of sex-determination and differentiation. Most of the genes identified previously are transcription factors and so we aim, in particular, to find genes involved in cell-to-cell communication, signal transduction, and transcriptional regulation, downstream of the differentiation of Sertoli cells. We have used a suppression subtractive-hybridization method to generate male- and female-enriched probes and libraries. Clones are validated as being sex-specific in their expression patterns by array screening and in situ hybridization. Here we report on our progress to date and the general applicability of the approach for studies in other systems. J. Exp. Zool. 290:517-522, 2001.

Spatially dynamic expression of Sry in mouse genital ridges

We have studied the spatial dynamics of Sry transcription in the genital ridges of mouse embryos. We find that Sry is expressed in a dynamic wave that emanates from the central and/or anterior regions, extends subsequently to both poles, and ends in the caudal pole. This dynamism may explain the relative positioning of ovarian and testicular tissue seen in ovotestes in mice. Since direct regulatory targets of SRY ought to be expressed in a corresponding or complimentary wave, our observations pave the way for identification of target genes. Sry is expressed in internal cells but not in coelomic surface epithelial cells, indicating that its effect on proliferation of surface cells is achieved non-cell-autonomously. The cellular dynamism of Sry expression revealed in this study thus provides important insights into both the cellular and molecular mode of action of SRY, and how perturbations in Sry expression can lead to anomalies of sexual development.

A subtractive gene expression screen suggests a role for vanin-1 in testis development in mice

The molecular pathways leading from indifferent mammalian gonad to either testis or ovary are not well understood. A number of genes, including the Y-linked sex determining gene SRY, have been shown to play roles in sex determination or differentiation, but there are clearly many missing elements to be found. We used suppression-subtractive hybridization to construct normalized cDNA libraries enriched for male-specific or female-specific transcripts in mouse fetal gonads. We describe the strategy used to efficiently screen these libraries for candidate sex-determination and gonadogenesis genes. One gene arising from these screens is vanin-1, which encodes a protein implicated in the induction of cell migration into the thymus. We find that vanin-1 is expressed male-specifically in Sertoli cells of the developing testis and may be involved in inducing cell migration from the adjacent mesonephros, a process known to be critical for testis development. This screening approach is likely to be applicable to the isolation and study of genes involved in a variety of developmental systems.

The SRY gene HMG-box in micro- and megabats

Sex determination in mammals is controlled by the Y-linked SRY gene, which encodes a transcription factor with a DNA-binding motif of the HMG type. The only conserved region in this gene is the HMG-box, whose nucleotide sequence is currently available in a number of mammalian taxa. However, nothing is known about this gene in bats. Here, we report partial sequences of the SRY HMG-box from four microbat and four megabat species. We used the SRY HMG- box sequences from micro- and megabats to test the phylogenetic relationships between microbats, megabats, and primates. In maximum parsimony and maximum-likelihood trees, mega- and microbat branches start in the same internal node, which is consistent with a monophyletic origin of this mammalian group.

Cloning and characterisation of the Sry-related transcription factor gene Sox8

SOX proteins form a large family of transcription factors related by a DNA-binding domain known as the HMG box. Some 30 Sox genes have been identified in mammals and orthologues have been found in a wide range of other metazoans. Sox genes are highly conserved and are known to play important roles in embryonic development, including roles in gonadal, central nervous system, neural crest and skeletal development. Several SOX genes have been implicated in human congenital diseases. We report here the isolation of Sox8 and its characterisation in mice and humans. This gene has a remarkably similar primary structure and genomic organisation to the campomelic dysplasia gene SOX9 and the Waardenburg-Shah syndrome gene SOX10. SOX8 protein is able to bind to canonical SOX target DNA sequences and activate transcription in vitro through two separate trans -activation regions. Further, Sox8 is expressed in the central nervous system, limbs, kidneys, gonads and craniofacial structures during mouse embryo development. Sox8 maps to the t complex on mouse chromosome 17 and to human chromosome 16p13.3, a region associated with the microphthalmia-cataract syndrome CATM and the alpha-thalassemia/mental retardation syndrome ATR-16.

Multiple mono- and polymorphic Y-linked copies of the SRY HMG-box in microtidae

Sex determination in mammals is controlled by SRY (sex-determining region of the Y chromosome), a single-copy gene located on the Y-specific region. Several exceptions to this rule have been described: some rodent species present Y-specific multiple copies (either mono- or polymorphic) of this gene, and two Ellobius species and one Tokudaia species determine sex without a Y chromosome or the SRY gene. Recently, we have described multiple polymorphic copies of the SRY gene in both males and females of the vole species Microtus cabrerae. The female location and the presence of stop codons in some copies from males and females also suggest that they are nonfunctional copies of this gene (pseudogenes). We have investigated the SRY HMG-box in nine species of the family Microtidae; we report here the presence, in eight of these species, of multiple mono- or polymorphic copies of the SRY gene located on the Y chromosome.

Multiple, polymorphic copies of SRY in both males and females of the vole Microtus cabrerae

In mammals, sex determination is controlled by the Y-linked gene SRY. Although SRY is male-specific in most eutherian and marsupial species, with a single copy on the Y chromosome, several rodent species have multiple Y-linked copies of SRY, and two mole-vole species of the genus Ellobius determine sex without the Y chromosome or the SRY gene. We searched for homologs of SRY in three vole species of the genus Microtus and concluded that this gene is not male-specific in M. cabrerae, as it is present in multiple, polymorphic copies in both males and females. In contrast, SRY is male-specific in the related species M. agrestis and M. nivalis. Up to 15 different partial sequences of the SRY gene were found in M. cabrerae. Southern blots suggest that most of the extra copies of SRY are X-linked. One of the copies observed only in males has a sequence identical to that of the SRY gene in M. agrestis and may represent a functional copy of the gene in this species. The rest are probably nonfunctional pseudogenes.

Females of four mole species of genus Talpa (insectivora, mammalia) are true hermaphrodites with ovotestes

We studied the anatomical, histological, and genetic features of the sexual tract in four European mole species of the genus Talpa (Insectivora, mammalia): T. occidentalis, T. europaea, T. romana, and T. stankovici. All XY individuals had a normal male phenotype, whereas all XX individuals in all four species had features that identified them as intersexes. These individuals were nonetheless presumed to be functionally fertile females. Intersexuality was manifested mainly as gonadal hermaphroditism, with all females possessing bilateral ovotestes. The gonads were composed of a small portion of histologically normal ovarian tissue and a variably sized, generally large mass of disgenetic testicular tissue, accompanied by a small, rudimentary epididymis. The rest of the sexual tract was typically female, including oviducts, uterus, and vagina of normal appearance. Polymerase chain reaction (PCR) and Southern blotting analyses showed that the mammalian testis-determining gene SRY is present in males but not in females. Part of the conserved sequence of the mole SRY gene was cloned and sequenced after PCR amplification in two of the four mole species (T. occidentalis from Spain and T. romana from Italy). Sequences were identical in these two species and were very similar to those of the human and mouse SRY gene. Our findings constitute the first evidence of the existence of a genus-specific case of true hermaphroditism, probably due to a very ancient mutation that fixed in populations of the ancestral species from which contemporary moles evolved. The possible nature of this mutation is discussed with regard to the cytologic, histologic, and genetic features of the gonads in Talpa females.