DMRT gene cluster analysis in the platypus: new insights into genomic organization and regulatory regions

Medaka (Oryzias latipes) Dmrt3a Is Involved in Male Fertility

Research across various species has demonstrated that the doublesex and mab-3-related transcription factor 3 (dmrt3) plays pivotal roles in testis development. However, the precise molecular mechanisms of dmrt3 remain unclear. In this study, we investigated the role of dmrt3 (dmrt3a) in testis development using the model organism medaka (Oryzias latipes). SqRT-PCR and ISH analyses revealed that dmrt3a is predominantly expressed in the testis, especially in the spermatid and spermatozoon. Using CRISPR/Cas9, we generated two dmrt3a homozygous mutants (-8 bp and -11 bp), which exhibited significantly reduced fertilization rates and embryo production. Additionally, the number of germ cells and sperm motility were markedly decreased in the dmrt3a mutants, manifesting as the symptoms of asthenozoospermia and oligozoospermia. Interestingly, RNA-Seq analysis showed that the deficiency of dmrt3a could lead to a significant downregulation of numerous genes related to gonadal development and severe disruptions in mitochondrial function. These results suggested that dmrt3a is essential for spermatogenesis and spermatozoa energy production. This paper provides new insights and perspectives for further exploring the molecular mechanisms underlying spermatogenesis and addressing male reproductive issues.

Genome-wide identification and expression analysis of Dmrt gene family and their role in gonad development of Pacific oyster (Crassostrea gigas)

Doublesex and Mab-3-related transcription factor (Dmrt) is a type of transcription factor with a zinc-finger DM structural domain, which plays a significant role in sex determination and differentiation in animals. Although Dmrt has been reported in many vertebrates and invertebrates, it has rarely been studied in bivalves. In this study, a total of three members of the Dmrt gene family were identified and characterized in Crassostrea gigas, and all these CgDmrt genes contained a conserved DM domain. Analysis of the phylogenetic tree and gene structure revealed that Dmrt genes clustered on one branch may have similar functions in bivalves. Expression profiling of CgDmrt mRNA in different tissues and stages of gonad development indicated that CgDmrt3 exhibited sexually dimorphic expression and played an important role in the development of the male gonad in C. gigas. Furthermore, analysis of CgDmrt mRNA expression between fertile triploids and sterile triploids showed that CgDmrt3 may be involved in sperm production. Collectively, the systematic analysis of the CgDmrt genes will provide potential insights into the function of these genes in gonadal development.

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)'.

Comparative Genomics Studies on the dmrt Gene Family in Fish

Doublesex and mab-3-related transcription factor (dmrt) genes are widely distributed across various biological groups and play critical roles in sex determination and neural development. Here, we applied bioinformatics methods to exam cross-species changes in the dmrt family members and evolutionary relationships of the dmrt genes based on genomes of 17 fish species. All the examined fish species have dmrt1-5 while only five species contained dmrt6. Most fish harbored two dmrt2 paralogs (dmrt2a and dmrt2b), with dmrt2b being unique to fish. In the phylogenetic tree, 147 DMRT are categorized into eight groups (DMRT1-DMRT8) and then clustered in three main groups. Selective evolutionary pressure analysis indicated purifying selections on dmrt1-3 genes and the dmrt1-3-2(2a) gene cluster. Similar genomic conservation patterns of the dmrt1-dmrt3-dmrt2(2a) gene cluster with 20-kb upstream/downstream regions in fish with various sex-determination systems were observed except for three regions with remarkable diversity. Synteny analysis revealed that dmrt1, dmrt2a, dmrt2b, and dmrt3-5 were relatively conserved in fish during the evolutionary process. While dmrt6 was lost in most species during evolution. The high conservation of the dmrt1-dmrt3-dmrt2(2a) gene cluster in various fish genomes suggests their crucial biological functions while various dmrt family members and sequences across fish species suggest different biological roles during evolution. This study provides a molecular basis for fish dmrt functional analysis and may serve as a reference for in-depth phylogenomics.

The genomic structure of the highly-conserved dmrt1 gene in Solea senegalensis (Kaup, 1868) shows an unexpected intragenic duplication

Knowing the factors responsible for sex determination in a species has significant theoretical and practical implications; the dmrt1 gene (Doublesex and Mab-3 (DM)-related Transcription factor 1) plays this role in diverse animal species. Solea senegalensis is a commercially important flat fish in which females grow 30% faster than males. It has 2n = 42 chromosomes and an XX / XY chromosome system for sex determination, without heteromorph chromosomes but with sex proto-chromosome. In the present study, we are providing the genomic structure and nucleotide sequence of dmrt1 gene obtained from cDNA from male and female adult gonads. A cDNA of 2027 containing an open-reading frame (ORF) of 1206 bp and encoding a 402 aa protein it is described for dmrt1 gene of S. senegalensis. Multiple mRNA isoforms indicating a high variable system of alternative splicing in the expression of dmrt1 of the sole in gonads were studied. None isoforms could be related to sex of individuals. The genomic structure of the dmrt1 of S. senegalensis showed a gene of 31400 bp composed of 7 exons and 6 introns. It contains an unexpected duplication of more than 10399 bp, involving part of the exon I, exons II and III and a SINE element found in the sequence that it is proposed as responsible for the duplication. A mature miRNA of 21 bp in length was localized at 336 bp from exon V. Protein-protein interacting networks of the dmrt1 gene showed matches with dmrt1 protein from Cynoglossus semilaevis and a protein interaction network with 11 nodes (dmrt1 plus 10 other proteins). The phylogenetic relationship of the dmrt1 gene in S. senegalensis is consistent with the evolutionary position of its species. The molecular characterization of this gene will enhance its functional analysis and the understanding of sex differentiation in Solea senegalensis and other flatfish.

Behavioral Characterization of dmrt3a Mutant Zebrafish Reveals Crucial Aspects of Vertebrate Locomotion through Phenotypes Related to Acceleration

Vertebrate locomotion is orchestrated by spinal interneurons making up a central pattern generator. Proper coordination of activity, both within and between segments, is required to generate the desired locomotor output. This coordination is altered during acceleration to ensure the correct recruitment of muscles for the chosen speed. The transcription factor Dmrt3 has been proposed to shape the patterned output at different gaits in horses and mice.

Vertebrate locomotion is orchestrated by spinal interneurons making up a central pattern generator. Proper coordination of activity, both within and between segments, is required to generate the desired locomotor output. This coordination is altered during acceleration to ensure the correct recruitment of muscles for the chosen speed. The transcription factor Dmrt3 has been proposed to shape the patterned output at different gaits in horses and mice. Here, we characterized dmrt3a mutant zebrafish, which showed a strong, transient, locomotor phenotype in developing larvae. During beat-and-glide swimming, mutant larvae showed fewer and shorter movements with decreased velocity and acceleration. Developmental compensation likely occurs as the analyzed behaviors did not differ from wild-type at older larval stages. However, analysis of maximum swim speed in juveniles suggests that some defects persist within the mature locomotor network of dmrt3a mutants. Our results reveal the pivotal role Dmrt3 neurons play in shaping the patterned output during acceleration in vertebrates.

Genome-Wide identification of doublesex and Mab-3-Related transcription factor (DMRT) genes in nile tilapia (oreochromis niloticus)

Doublesex and Mab-3-related transcription factor (DMRT) gene family is extensively known for its contribution in sex determination and differentiation across phyla. Here we report the identification of five DM (doublesex and mab-3) domain genes in the Nile tilapia which includes DMRT1, DMRTa2, DMRT2a, DMRT2b and DMRT3a. The full-length sequence of DMRT genes ranges from 3526 (DMRTA2) to 1471bp (DMRT1) which encode putative proteins series from 469 to 372 amino acids. All the DMRT proteins contained at least one conserved DNA-binding DM domain. Sub-cellular localization and gene ontology revealed DMRT1 protein is maximum localized in nuclear region and gene ontology analysis showed the molecular function of 48.2%, biological process 43.6% and cellular component 25%. Chromosomal location and synteny analysis displayed that DMRT genes mostly cluster linkage group 12. Altogether, our findings provide vital genomic information for future studies of biochemical, physiological, and phylogenetic studies on DMRT genes in teleost.

Implications of monotreme and marsupial chromosome evolution on sex determination and differentiation

Studies of chromosomes from monotremes and marsupials endemic to Australasia have provided important insight into the evolution of their genomes as well as uncovering fundamental differences in their sex determination/differentiation pathways. Great advances have been made this century into solving the mystery of the complicated sex chromosome system in monotremes. Monotremes possess multiple different X and Y chromosomes and a candidate sex determining gene has been identified. Even greater advancements have been made for marsupials, with reconstruction of the ancestral karyotype enabling the evolutionary history of marsupial chromosomes to be determined. Furthermore, the study of sex chromosomes in intersex marsupials has afforded insight into differences in the sexual differentiation pathway between marsupials and eutherians, together with experiments showing the insensitivity of the mammary glands, pouch and scrotum to exogenous hormones, led to the hypothesis that there is a gene (or genes) on the X chromosome responsible for the development of either pouch or scrotum. This review highlights the major advancements made towards understanding chromosome evolution and how this has impacted on our understanding of sex determination and differentiation in these interesting mammals.

Integrated gene mapping and synteny studies give insights into the evolution of a sex proto-chromosome in Solea senegalensis

The evolution of genes related to sex and reproduction in fish shows high plasticity and, to date, the sex determination system has only been identified in a few species. Solea senegalensis has 42 chromosomes and an XX/XY chromosome system for sex determination, while related species show the ZZ/ZW system. Next-generation sequencing (NGS), multi-color fluorescence in situ hybridization (mFISH) techniques, and bioinformatics analysis have been carried out, with the objective of revealing new information about sex determination and reproduction in S. senegalensis. To that end, several bacterial artificial chromosome (BAC) clones that contain candidate genes involved in such processes (dmrt1, dmrt2, dmrt3, dmrt4, sox3, sox6, sox8, sox9, lh, cyp19a1a, amh, vasa, aqp3, and nanos3) were analyzed and compared with the same region in other related species. Synteny studies showed that the co-localization of dmrt1-dmrt2-drmt3 in the largest metacentric chromosome of S. senegalensis is coincident with that found in the Z chromosome of Cynoglossus semilaevis, which would potentially make this a sex proto-chromosome. Phylogenetic studies show the close proximity of S. senegalensis to Oryzias latipes, a species with an XX/XY system and a sex master gene. Comparative mapping provides evidence of the preferential association of these candidate genes in particular chromosome pairs. By using the NGS and mFISH techniques, it has been possible to obtain an integrated genetic map, which shows that 15 out of 21 chromosome pairs of S. senegalensis have at least one BAC clone. This result is important for distinguishing those chromosome pairs of S. senegalensis that are similar in shape and size. The mFISH analysis shows the following co-localizations in the same chromosomes: dmrt1-dmrt2-dmrt3, dmrt4-sox9-thrb, aqp3-sox8, cyp19a1a-fshb, igsf9b-sox3, and lysg-sox6.

Identification of Dmrt genes and their up-regulation during gonad transformation in the swamp eel (Monopterus albus)

The swamp eel is a teleost fish with a characteristic of natural sex reversal and an ideal model for vertebrate sexual development. However, underlying molecular mechanisms are poorly understood. We report the identification of five DM (doublesex and mab-3) domain genes in the swamp eel that include Dmrt2, Dmrt2b, Dmrt3, Dmrt4 and Dmrt5, which encode putative proteins of 527, 373, 471, 420 and 448 amino acids, respectively. Phylogenetic tree showed that these genes are clustered into corresponding branches of the DM genes in vertebrates. Southern blot analysis indicated that the Dmrt1-Dmrt3-Dmrt2 genes are tightly linked in a conserved gene cluster. Notably, these Dmrt genes are up-regulated during gonad transformation. Furthermore, mRNA in situ hybridisation showed that Dmrt2, Dmrt3, Dmrt4 and Dmrt5 are expressed in developing germ cells. These results are evidence that the DM genes are involved in sexual differentiation in the swamp eel.

Characterization of sex determination and sex differentiation genes in Latimeria

Genes involved in sex determination and differentiation have been identified in mice, humans, chickens, reptiles, amphibians and teleost fishes. However, little is known of their functional conservation, and it is unclear whether there is a common set of genes shared by all vertebrates. Coelacanths, basal Sarcopterygians and unique "living fossils", could help establish an inventory of the ancestral genes involved in these important developmental processes and provide insights into their components. In this study 33 genes from the genome of Latimeria chalumnae and from the liver and testis transcriptomes of Latimeria menadoensis, implicated in sex determination and differentiation, were identified and characterized and their expression levels measured. Interesting findings were obtained for GSDF, previously identified only in teleosts and now characterized for the first time in the sarcopterygian lineage; FGF9, which is not found in teleosts; and DMRT1, whose expression in adult gonads has recently been related to maintenance of sexual identity. The gene repertoire and testis-specific gene expression documented in coelacanths demonstrate a greater similarity to modern fishes and point to unexpected changes in the gene regulatory network governing sexual development.

The evolution of marsupial and monotreme chromosomes

Marsupial and monotreme mammals fill an important gap in vertebrate phylogeny between reptile-mammal divergence 310 million years ago (mya) and the eutherian (placental) mammal radiation 105 mya. They possess many unique features including their distinctive chromosomes, which in marsupials are typically very large and well conserved between species. In contrast, monotreme genomes are divided into several large chromosomes and many smaller chromosomes, with a complicated sex chromosome system that forms a translocation chain in male meiosis. The application of molecular cytogenetic techniques has greatly advanced our understanding of the evolution of marsupial chromosomes and allowed the reconstruction of the ancestral marsupial karyotype. Chromosome painting and gene mapping have played a vital role in piecing together the puzzle of monotreme karyotypes, particularly their complicated sex chromosome system. Here, we discuss the significant insight into karyotype evolution afforded by the combination of recently sequenced marsupial and monotreme genomes with cytogenetic analysis, which has provided a greater understanding of the events that have shaped not only marsupial and monotreme genomes, but the genomes of all mammals.

The W, X, Y and Z of sex-chromosome dosage compensation

In species with highly differentiated sex chromosomes, imbalances in gene dosage between the sexes can affect overall organismal fitness. Regulatory mechanisms were discovered in several unrelated animals, which counter gene-dose differences between females and males, and these early findings suggested that dosage-compensating mechanisms were required for sex-chromosome evolution. However, recent reports in birds and moths contradict this view because these animals locally compensate only a few genes on the sex chromosomes, leaving the majority with different expression levels in males and females. These findings warrant a re-examination of the evolutionary forces underlying dosage compensation.

Characterisation of ATRX, DMRT1, DMRT7 and WT1 in the platypus (Ornithorhynchus anatinus)

One of the most puzzling aspects of monotreme reproductive biology is how they determine sex in the absence of the SRY gene that triggers testis development in most other mammals. Although monotremes share a XX female/XY male sex chromosome system with other mammals, their sex chromosomes show homology to the chicken Z chromosome, including the DMRT1 gene, which is a dosage-dependent sex determination gene in birds. In addition, monotremes feature an extraordinary multiple sex chromosome system. However, no sex determination gene has been identified as yet on any of the five X or five Y chromosomes and there is very little knowledge about the conservation and function of other known genes in the monotreme sex determination and differentiation pathway. We have analysed the expression pattern of four evolutionarily conserved genes that are important at different stages of sexual development in therian mammals. DMRT1 is a conserved sex-determination gene that is upregulated in the male developing gonad in vertebrates, while DMRT7 is a mammal-specific spermatogenesis gene. ATRX, a chromatin remodelling protein, lies on the therian X but there is a testis-expressed Y-copy in marsupials. However, in monotremes, the ATRX orthologue is autosomal. WT1 is an evolutionarily conserved gene essential for early gonadal formation in both sexes and later in testis development. We show that these four genes in the adult platypus have the same expression pattern as in other mammals, suggesting that they have a conserved role in sexual development independent of genomic location.

Monotreme chromosomes: an introductory review

The three extant genera of the prototherian mammals, Ornithorhynchus (platypus), Tachyglossus (Australian echidna) and Zaglossus (New Guinea echidna), all have a mechanism of sex determination at odds with that seen in eutherian and metatherian mammals. Indeed, they stand apart from all vertebrates. Instead of the XX/XY, X1X2Y or ZZ/ZW systems seen in the majority of vertebrates the monotremes have a chain of nine (or ten) chromosomes present during meiosis in the male. This is believed to be the consequence of a presumed series of reciprocal translocations involving four autosomal pairs and the original X and Y chromosomes. The presence of this chain in all three genera indicates that a similar chain occurred in their common ancestor. This paper provides an overview of the search to unravel the mystery of this chain and to determine the identity of the sex chromosomes and members of the chain. The development of new techniques has hugely facilitated clarification of the findings of the earlier researchers. As a result, the chromosomes of the platypus and the echidna have now been individually described, the chain elements and/or sex chromosomes have been identified unambiguously and their order in the chain has been determined. The research reviewed here has also provided insights into the evolution of mammalian sex chromosomes and given new directions for unravelling dosage compensation and sex-determination mechanisms in mammals.

The multiple sex chromosomes of platypus and echidna are not completely identical and several share homology with the avian Z

A comparative study of the karyotype of the short-beaked echidna shows that monotremes appear to have a unique XY sex chromosome system that shares some homology with the avian Z.

Background

Sex-determining systems have evolved independently in vertebrates. Placental mammals and marsupials have an XY system, birds have a ZW system. Reptiles and amphibians have different systems, including temperature-dependent sex determination, and XY and ZW systems that differ in origin from birds and placental mammals. Monotremes diverged early in mammalian evolution, just after the mammalian clade diverged from the sauropsid clade. Our previous studies showed that male platypus has five X and five Y chromosomes, no SRY, and DMRT1 on an X chromosome. In order to investigate monotreme sex chromosome evolution, we performed a comparative study of platypus and echidna by chromosome painting and comparative gene mapping.

Results

Chromosome painting reveals a meiotic chain of nine sex chromosomes in the male echidna and establishes their order in the chain. Two of those differ from those in the platypus, three of the platypus sex chromosomes differ from those of the echidna and the order of several chromosomes is rearranged. Comparative gene mapping shows that, in addition to bird autosome regions, regions of bird Z chromosomes are homologous to regions in four platypus X chromosomes, that is, X₁, X₂, X₃, X₅, and in chromosome Y₁.

Conclusion

Monotreme sex chromosomes are easiest to explain on the hypothesis that autosomes were added sequentially to the translocation chain, with the final additions after platypus and echidna divergence. Genome sequencing and contig anchoring show no homology yet between platypus and therian Xs; thus, monotremes have a unique XY sex chromosome system that shares some homology with the avian Z.

The status of dosage compensation in the multiple X chromosomes of the platypus

Dosage compensation has been thought to be a ubiquitous property of sex chromosomes that are represented differently in males and females. The expression of most X-borne genes is equalized between XX females and XY males in therian mammals (marsupials and “placentals”) by inactivating one X chromosome in female somatic cells. However, compensation seems not to be strictly required to equalize the expression of most Z-borne genes between ZZ male and ZW female birds. Whether dosage compensation operates in the third mammal lineage, the egg-laying monotremes, is of considerable interest, since the platypus has a complex sex chromosome system in which five X and five Y chromosomes share considerable genetic homology with the chicken ZW sex chromosome pair, but not with therian XY chromosomes. The assignment of genes to four platypus X chromosomes allowed us to examine X dosage compensation in this unique species. Quantitative PCR showed a range of compensation, but SNP analysis of several X-borne genes showed that both alleles are transcribed in a heterozygous female. Transcription of 14 BACs representing 19 X-borne genes was examined by RNA-FISH in female and male fibroblasts. An autosomal control gene was expressed from both alleles in nearly all nuclei, and four pseudoautosomal BACs were usually expressed from both alleles in male as well as female nuclei, showing that their Y loci are active. However, nine X-specific BACs were usually transcribed from only one allele. This suggests that while some genes on the platypus X are not dosage compensated, other genes do show some form of compensation via stochastic transcriptional inhibition, perhaps representing an ancestral system that evolved to be more tightly controlled in placental mammals such as human and mouse.

Dosage compensation equalizes the expression of genes found on sex chromosomes so that they are equally expressed in females and males. In placental and marsupial mammals, this is accomplished by silencing one of the two X chromosomes in female cells. In birds, dosage compensation seems not to be strictly required to balance the expression of most genes on the Z chromosome between ZZ males and ZW females. Whether dosage compensation exists in the third group of mammals, the egg-laying monotremes, is of considerable interest, particularly since the platypus has five different X and five different Y chromosomes. As part of the platypus genome project, genes have now been assigned to four of the five X chromosomes. We have shown that there is some evidence for dosage compensation, but it is variable between genes. Most interesting are our results showing that there is a difference in the probability of expression for X-specific genes, with about 50% of female cells having two active copies of an X gene while the remainder have only one. This means that, although the platypus has the variable compensation characteristic of birds, it also has some level of inactivation, which is characteristic of dosage compensation in other mammals.

Genome analysis of the platypus reveals unique signatures of evolution

We present a draft genome sequence of the platypus, Ornithorhynchus anatinus. This monotreme exhibits a fascinating combination of reptilian and mammalian characters. For example, platypuses have a coat of fur adapted to an aquatic lifestyle; platypus females lactate, yet lay eggs; and males are equipped with venom similar to that of reptiles. Analysis of the first monotreme genome aligned these features with genetic innovations. We find that reptile and platypus venom proteins have been co-opted independently from the same gene families; milk protein genes are conserved despite platypuses laying eggs; and immune gene family expansions are directly related to platypus biology. Expansions of protein, non-protein-coding RNA and microRNA families, as well as repeat elements, are identified. Sequencing of this genome now provides a valuable resource for deep mammalian comparative analyses, as well as for monotreme biology and conservation.

Bird-like sex chromosomes of platypus imply recent origin of mammal sex chromosomes

In therian mammals (placentals and marsupials), sex is determined by an XX female: XY male system, in which a gene (SRY) on the Y affects male determination. There is no equivalent in other amniotes, although some taxa (notably birds and snakes) have differentiated sex chromosomes. Birds have a ZW female: ZZ male system with no homology with mammal sex chromosomes, in which dosage of a Z-borne gene (possibly DMRT1) affects male determination. As the most basal mammal group, the egg-laying monotremes are ideal for determining how the therian XY system evolved. The platypus has an extraordinary sex chromosome complex, in which five X and five Y chromosomes pair in a translocation chain of alternating X and Y chromosomes. We used physical mapping to identify genes on the pairing regions between adjacent X and Y chromosomes. Most significantly, comparative mapping shows that, contrary to earlier reports, there is no homology between the platypus and therian X chromosomes. Orthologs of genes in the conserved region of the human X (including SOX3, the gene from which SRY evolved) all map to platypus chromosome 6, which therefore represents the ancestral autosome from which the therian X and Y pair derived. Rather, the platypus X chromosomes have substantial homology with the bird Z chromosome (including DMRT1) and to segments syntenic with this region in the human genome. Thus, platypus sex chromosomes have strong homology with bird, but not to therian sex chromosomes, implying that the therian X and Y chromosomes (and the SRY gene) evolved from an autosomal pair after the divergence of monotremes only 166 million years ago. Therefore, the therian X and Y are more than 145 million years younger than previously thought.

Sex determination in platypus and echidna: autosomal location of SOX3 confirms the absence of SRY from monotremes

In eutherian ('placental') mammals, sex is determined by the presence or absence of the Y chromosome-borne gene SRY, which triggers testis determination. Marsupials also have a Y-borne SRY gene, implying that this mechanism is ancestral to therians, the SRY gene having diverged from its X-borne homologue SOX3 at least 180 million years ago. The rare exceptions have clearly lost and replaced the SRY mechanism recently. Other vertebrate classes have a variety of sex-determining mechanisms, but none shares the therian SRY-driven XX female:XY male system. In monotreme mammals (platypus and echidna), which branched from the therian lineage 210 million years ago, no orthologue of SRY has been found. In this study we show that its partner SOX3 is autosomal in platypus and echidna, mapping among human X chromosome orthologues to platypus chromosome 6, and to the homologous chromosome 16 in echidna. The autosomal localization of SOX3 in monotreme mammals, as well as non-mammal vertebrates, implies that SRY is absent in Prototheria and evolved later in the therian lineage 210-180 million years ago. Sex determination in platypus and echidna must therefore depend on another male-determining gene(s) on the Y chromosomes, or on the different dosage of a gene(s) on the X chromosomes.

Characterizing the chromosomes of the platypus (Ornithorhynchus anatinus)

Like the unique platypus itself, the platypus genome is extraordinary because of its complex sex chromosome system, and is controversial because of difficulties in identification of small autosomes and sex chromosomes. A 6-fold shotgun sequence of the platypus genome is now available and is being assembled with the help of physical mapping. It is therefore essential to characterize the chromosomes and resolve the ambiguities and inconsistencies in identifying autosomes and sex chromosomes. We have used chromosome paints and DAPI banding to identify and classify pairs of autosomes and sex chromosomes. We have established an agreed nomenclature and identified anchor BAC clones for each chromosome that will ensure unambiguous gene localizations.

The function of Dmrt genes in vertebrate development: It is not just about sex

The Dmrt genes encode a large family of transcription factors whose function in sexual development has been well studied in invertebrates and vertebrates. Their expression pattern is not restricted to the developing gonads, indicating that Dmrt genes might regulate other developmental processes. Here we review the expression pattern of several members of this family across species and summarize recent findings on the function of a subset of these genes in non-gonadal tissues.

Loss of Wnt4 and Foxl2 leads to female-to-male sex reversal extending to germ cells

The discovery that the SRY gene induces male sex in humans and other mammals led to speculation about a possible equivalent for female sex. However, only partial effects have been reported for candidate genes experimentally tested so far. Here we demonstrate that inactivation of two ovarian somatic factors, Wnt4 and Foxl2, produces testis differentiation in XX mice, resulting in the formation of testis tubules and spermatogonia. These genes are thus required to initiate or maintain all major aspects of female sex determination in mammals. The two genes are independently expressed and show complementary roles in ovary morphogenesis. In addition, forced expression of Foxl2 impairs testis tubule differentiation in XY transgenic mice, and germ cell-depleted XX mice lacking Foxl2 and harboring a Kit mutation undergo partial female-to-male sex reversal. The results are all consistent with an anti-testis role for Foxl2. The data suggest that the relative autonomy of the action of Foxl2, Wnt4 and additional ovarian factor(s) in the mouse should facilitate the dissection of their respective contributions to female sex determination.

Search for the sex-determining switch in monotremes: mapping WT1, SF1, LHX1, LHX2, FGF9, WNT4, RSPO1 and GATA4 in platypus

The duck-billed platypus has five pairs of sex chromosomes, but there is no information about the primary sex-determining switch in this species. As there is no apparent SRY orthologue in platypus, another gene must acquire the function of a key regulator of the gonadal male or female fate. SOX9 was ruled out from being this key regulator as it maps to an autosome in platypus. To check whether other genes in mammalian gonadogenesis could be the primary switch in monotremes, we have mapped a number of candidates in platypus. We report here the autosomal location of WT1, SF1, LHX1, LHX9, FGF9, WNT4 and RSPO1 in platypus, thus excluding these from being key regulators of sex determination in this species. We found that GATA4 maps to sex chromosomes Y1 and X2; however, it lies in the pairing region shown by chromosome painting to be homologous, so is unlikely to be either male-specific or differentially dosed in male and female.

A new look at the evolution of avian sex chromosomes

Birds have a ubiquitous, female heterogametic, ZW sex chromosome system. The current model suggests that the Z chromosome and its degraded partner, the W chromosome, evolved from an ancestral pair of autosomes independently from the mammalian XY male heteromorphic sex chromosomes--which are similar in size, but not gene content (Graves, 1995; Fridolfsson et al., 1998). Furthermore the degradation of the W has been proposed to be progressive, with the basal clade of birds (the ratites) possessing virtually homomorphic sex chromosomes and the more recently derived birds (the carinates) possessing highly heteromorphic sex chromosomes (Ohno, 1967; Solari, 1993). Recent findings have suggested an alternative to independent evolution of bird and mammal chromosomes, in which an XY system took over directly from an ancestral ZW system. Here we examine recent research into avian sex chromosomes and offer alternative suggestions as to their evolution.