Genomes of Ellobius species provide insight into the evolutionary dynamics of mammalian sex chromosomes

Compensation of gene dosage on the mammalian X

Changes in gene dosage can have tremendous evolutionary potential (e.g. whole-genome duplications), but without compensatory mechanisms, they can also lead to gene dysregulation and pathologies. Sex chromosomes are a paradigmatic example of naturally occurring gene dosage differences and their compensation. In species with chromosome-based sex determination, individuals within the same population necessarily show 'natural' differences in gene dosage for the sex chromosomes. In this Review, we focus on the mammalian X chromosome and discuss recent new insights into the dosage-compensation mechanisms that evolved along with the emergence of sex chromosomes, namely X-inactivation and X-upregulation. We also discuss the evolution of the genetic loci and molecular players involved, as well as the regulatory diversity and potentially different requirements for dosage compensation across mammalian species.

Specificity of Key Sex Determination Genes in a Mammal with Ovotestes: The European Mole Talpa europaea

Here, for the first time, the structure of genes involved in sex determination in mammals (full Sry and partial Rspo1, Eif2s3x, and Eif2s3y) was analyzed for the European mole Talpa europaea with ovotestes in females. We confirmed male-specificity for Eif2s3y and Sry. Five exons were revealed for Rspo1 and the deep similarity with the structure of this gene in T. occidentalis was proved. The most intriguing result was obtained for the Sry gene, which, in placental mammals, initiates male development. We described two exons for this canonically single-exon gene: the first (initial) exon is only 15 bp while the second exon includes 450 bp. The exons are divided by an extended intron of about 1894 bp, including the fragment of the LINE retroposon. Moreover, in chromatogram fragments, which correspond to intron and DNA areas, flanking both exons, we revealed double peaks, similar to heterozygous nucleotide sites of autosomal genes. This may indicate the existence of two or more copies of the Sry gene. Proof of copies requires an additional in-depth study. We hypothesize that unusual structure and possible supernumerary copies of Sry may be involved in ovotestes formation.

Loss of One X and the Y Chromosome Changes the Configuration of the X Inactivation Center in the Genus Tokudaia

INTRODUCTION

X chromosome inactivation (XCI) is an essential mechanism for dosage compensation between females and males in mammals. In females, XCI is controlled by a complex, conserved locus termed the X inactivation center (Xic), in which the lncRNA Xist is the key regulator. However, little is known about the Xic in species with unusual sex chromosomes. The genus Tokudaia includes three rodent species endemic to Japan. Tokudaia osimensis and Tokudaia tokunoshimensis lost the Y chromosome (XO/XO), while Tokudaia muenninki (TMU) acquired a neo-X region by fusion of the X chromosome and an autosome (XX/XY). We compared the gene location and structure in the Xic among Tokudaia species.

METHODS

Gene structure of nine genes in Xic was predicted, and the gene location and genome sequences of Xic were compared between mouse and Tokudaia species. The expression level of the gene was confirmed by transcripts per million calculation using RNA-seq data.

RESULTS

Compared to mouse, the Xic gene order and location were conserved in Tokudaia species. However, remarkable structure changes were observed in lncRNA genes, Xist and Tsix, in the XO/XO species. In Xist, important functional repeats, B-, C-, D-, and E-repeats, were partially or completely lost due to deletions in these species. RNA-seq data showed that female-specific expression patterns of Xist and Tsix were confirmed in TMU, however, not in the XO/XO species. Additionally, three deletions and one inversion were confirmed in the intergenic region between Jpx and Ftx in the XO/XO species.

CONCLUSION

Our findings indicate that even if the Xist and Tsix lncRNAs are expressed, they are incapable of producing a successful and lasting XCI in the XO/XO species. We hypothesized that the significant structure change in the intergenic region of Jpx-Ftx resulted in the inability to perform the XCI, and, as a result, a lack of Xist expression. Our results collectively suggest that structural changes in the Xic occurred in the ancestral lineage of XO/XO species, likely due to the loss of one X chromosome and the Y chromosome as a consequence of the degradation of the XCI system.

Where Are the Formerly Y-linked Genes in the Ryukyu Spiny Rat that has Lost its Y Chromosome?

It has been predicted that the highly degenerate mammalian Y chromosome will be lost eventually. Indeed, Y was lost in the Ryukyu spiny rat Tokudaia osimensis, but the fate of the formerly Y-linked genes is not completely known. We looked for all 12 ancestrally Y-linked genes in a draft T. osimensis genome sequence. Zfy1, Zfy2, Kdm5d, Eif2s3y, Usp9y, Uty, and Ddx3y are putatively functional and are now located on the X chromosome, whereas Rbmy, Uba1y, Ssty1, Ssty2, and Sry are missing or pseudogenized. Tissue expressions of the mouse orthologs of the retained genes are significantly broader/higher than those of the lost genes, suggesting that the destinies of the formerly Y-linked genes are related to their original expressions. Interestingly, patterns of gene retention/loss are significantly more similar than by chance across four rodent lineages where Y has been independently lost, indicating a level of certainty in the fate of Y-linked genes even when the chromosome is gone.

Did the creeping vole sex chromosomes evolve through a cascade of adaptive responses to a selfish x chromosome?

The creeping vole Microtus oregoni exhibits remarkably transformed sex chromosome biology, with complete chromosome drive/drag, X-Y fusions, sex reversed X complements, biased X inactivation, and X chromosome degradation. Beginning with a selfish X chromosome, I propose a series of adaptations leading to this system, each compensating for deleterious consequences of the preceding adaptation: (1) YY embryonic inviability favored evolution of a selfish feminizing X chromosome; (2) the consequent Y chromosome transmission disadvantage favored X-Y fusion ("XP "); (3) Xist-based silencing of Y-derived XP genes favored a second X-Y fusion ("XM "); (4) X chromosome dosage-related costs in XP XM males favored the evolution of XM loss during spermatogenesis; (5) X chromosomal dosage-related costs in XM 0 females favored the evolution of XM drive during oogenesis; and (6) degradation of the non-recombining XP favored the evolution of biased X chromosome inactivation. I discuss recurrent rodent sex chromosome transformation, and selfish genes as a constructive force in evolution.

Return of the forgotten hero: the role of Y chromosome-encoded Zfy in male reproduction

The Y-linked zinc finger gene ZFY is conserved across eutherians and is known to be a critical fertility factor in some species. The initial studies of the mouse homologues, Zfy1 and Zfy2, were performed using mice with spontaneous Y chromosome mutations and Zfy transgenes. These studies revealed that Zfy is involved in multiple processes during spermatogenesis, including removal of germ cells with unpaired chromosomes and control of meiotic sex chromosome inactivation during meiosis I, facilitating the progress of meiosis II, promoting spermiogenesis, and improving assisted reproduction outcomes. Zfy was also identified as a key gene in Y chromosome evolution, protecting this chromosome from extinction by serving as the executioner responsible for meiosis surveillance. Studies with targeted Zfy knock-outs revealed that mice lacking both homologues have severe spermatogenic defects and are infertile. Based on protein structure and in vitro assays, Zfy is expected to drive spermatogenesis as a transcriptional regulator. The combined evidence documents that the presence of at least one Zfy homologue is required for male fertility and that Zfy2 plays a more prominent role. This knowledge reinforces the importance of these factors for mouse spermatogenesis and informs our understanding of the human ZFY variants, which are homologous to the mouse Zfy1 and Zfy2.

Reproductive biology research down under: highlights from the Australian and New Zealand Annual Meeting of the Society for Reproductive Biology, 2021

Against the backdrop of a global pandemic, the Society for Reproductive Biology (SRB) 2021 meeting reunited the Australian and New Zealand reproductive research community for the first time since 2019 and was the first virtual SRB meeting. Despite the recent global research disruptions, the conference revealed significant advancements in reproductive research, the importance of which span human health, agriculture, and conservation. A core theme was novel technologies, including the use of medical microrobots for therapeutic and sperm delivery, diagnostic hyperspectral imaging, and hydrogel condoms with potential beyond contraception. The importance of challenging the contraceptive status quo was further highlighted with innovations in gene therapies, non-hormonal female contraceptives, epigenetic semen analysis, and in applying evolutionary theory to suppress pest population reproduction. How best to support pregnancies, particularly in the context of global trends of increasing maternal age, was also discussed, with several promising therapies for improved outcomes in assisted reproductive technology, pre-eclampsia, and pre-term birth prevention. The unique insights gained via non-model species was another key focus and presented research emphasised the importance of studying diverse systems to understand fundamental aspects of reproductive biology and evolution. Finally, the meeting highlighted how to effectively translate reproductive research into policy and industry practice.

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

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

Unusual Mammalian Sex Determination Systems: A Cabinet of Curiosities

Therian mammals have among the oldest and most conserved sex-determining systems known to date. Any deviation from the standard XX/XY mammalian sex chromosome constitution usually leads to sterility or poor fertility, due to the high differentiation and specialization of the X and Y chromosomes. Nevertheless, a handful of rodents harbor so-called unusual sex-determining systems. While in some species, fertile XY females are found, some others have completely lost their Y chromosome. These atypical species have fascinated researchers for over 60 years, and constitute unique natural models for the study of fundamental processes involved in sex determination in mammals and vertebrates. In this article, we review current knowledge of these species, discuss their similarities and differences, and attempt to expose how the study of their exceptional sex-determining systems can further our understanding of general processes involved in sex chromosome and sex determination evolution.

The role of Eif2s3y in mouse spermatogenesis

Eukaryotic translation initiation factor 2 subunit 3 and structural gene Y-linked (Eif2s3y) gene, the gene encoding eIF2γ protein, is located on the mouse Y chromosome short arm. The Eif2s3y gene is globally expressed in all tissues and plays an important role in regulating global and gene-specific mRNA translation initiation. During the process of protein translation initiation, Eif2s3x(its homolog) and Eif2s3y encoded eIF2γ perform similar functions. However, it has been noticed that Eif2s3y plays a crucial role in spermatogenesis, including spermatogonia mitosis, meiosis, and spermiogenesis of spermatids, which may account for infertility. In the period of spermatogenesis, the role of Eif2s3x and Eif2s3y are not equivalent. Importance of Eif2s3y has been observed in ESC and implicated in several aspects, including the pluripotency state and the proliferation rate. Here, we discuss the functional significance of Eif2s3y in mouse spermatogenesis and self-renewal of ESCs.

How did the guppy Y chromosome evolve?

The sex chromosome pairs of many species do not undergo genetic recombination, unlike the autosomes. It has been proposed that the suppressed recombination results from natural selection favouring close linkage between sex-determining genes and mutations on this chromosome with advantages in one sex, but disadvantages in the other (these are called sexually antagonistic mutations). No example of such selection leading to suppressed recombination has been described, but populations of the guppy display sexually antagonistic mutations (affecting male coloration), and would be expected to evolve suppressed recombination. In extant close relatives of the guppy, the Y chromosomes have suppressed recombination, and have lost all the genes present on the X (this is called genetic degeneration). However, the guppy Y occasionally recombines with its X, despite carrying sexually antagonistic mutations. We describe evidence that a new Y evolved recently in the guppy, from an X chromosome like that in these relatives, replacing the old, degenerated Y, and explaining why the guppy pair still recombine. The male coloration factors probably arose after the new Y evolved, and have already evolved expression that is confined to males, a different way to avoid the conflict between the sexes.

We report new findings concerning the long-studied the guppy XY pair, which has remained somewhat mystifying. We show that it can be understood as a case of a recent sex chromosome turnover event in which an older, highly degenerated Y chromosome was lost, and creation of a new sex chromosome from the ancestral X. This chromosome acquired a male-determining factor, possibly by a mutation in (or a duplication of) a previously X-linked gene, or (less likely) by movement of an ancestral Y-linked maleness factor onto the X. We relate the findings to theoretical models of such events, and argue that the proposed change was free from factors thought to impede such turnovers. The change resulted in the intriguing situation where the X chromosome is old and the Y is much younger, and we discuss some other species where a similar change seems likely to have occurred.

Evolution: Shape-shifting vole sex determination and sex chromosomes

It has long been known that some mouse and vole species have unusual sex chromosomes. A recent genome sequencing study advances understanding of a particularly puzzling vole system.

On the Form and Origins of the Bizarre Sex Chromosomal System of the Mandarin Vole

Recent work has illuminated the bizarre sex chromosomal system of the mandarin vole, Lasiopodomys mandarinus. The ancestral sex chromosomes have been replaced by 4 neo-sex chromosomes. These sex chromosomes show non-Mendelian inheritance and epistatic sex determination, as well as unaccounted-for karyotype frequencies. I suggest a model to account for the complex observed inheritance patterns. The proposed model combines putative adaptations previously observed in rodents, including feminizing X chromosomes and Y-biased spermatogenesis, with a novel proposed mechanism of genomic imprinting of X-linked genes during oogenesis in XY females. Alternative possibilities are also discussed. The proposed scenario provides a relatively simple and testable model for the function and origins of a remarkably complex mammalian sex chromosomal system.

Sex chromosome transformation and the origin of a male-specific X chromosome in the creeping vole

The mammalian sex chromosome system (XX female/XY male) is ancient and highly conserved. The sex chromosome karyotype of the creeping vole (Microtus oregoni) represents a long-standing anomaly, with an X chromosome that is unpaired in females (X0) and exclusively maternally transmitted. We produced a highly contiguous male genome assembly, together with short-read genomes and transcriptomes for both sexes. We show that M. oregoni has lost an independently segregating Y chromosome and that the male-specific sex chromosome is a second X chromosome that is largely homologous to the maternally transmitted X. Both maternally inherited and male-specific sex chromosomes carry fragments of the ancestral Y chromosome. Consequences of this recently transformed sex chromosome system include Y-like degeneration and gene amplification on the male-specific X, expression of ancestral Y-linked genes in females, and X inactivation of the male-specific chromosome in male somatic cells. The genome of M. oregoni elucidates the processes that shape the gene content and dosage of mammalian sex chromosomes and exemplifies a rare case of plasticity in an ancient sex chromosome system.

Myoglobin primary structure reveals multiple convergent transitions to semi-aquatic life in the world's smallest mammalian divers

The speciose mammalian order Eulipotyphla (moles, shrews, hedgehogs, solenodons) combines an unusual diversity of semi-aquatic, semi-fossorial, and fossorial forms that arose from terrestrial forbearers. However, our understanding of the ecomorphological pathways leading to these lifestyles has been confounded by a fragmentary fossil record, unresolved phylogenetic relationships, and potential morphological convergence, calling for novel approaches. The net surface charge of the oxygen-storing muscle protein myoglobin (ZMb), which can be readily determined from its primary structure, provides an objective target to address this question due to mechanistic linkages with myoglobin concentration. Here, we generate a comprehensive 71 species molecular phylogeny that resolves previously intractable intra-family relationships and then ancestrally reconstruct ZMb evolution to identify ancient lifestyle transitions based on protein sequence alone. Our phylogenetically informed analyses confidently resolve fossorial habits having evolved twice in talpid moles and reveal five independent secondary aquatic transitions in the order housing the world's smallest endothermic divers.

Compromised Function of the Pancreatic Transcription Factor PDX1 in a Lineage of Desert Rodents

Gerbils are a subfamily of rodents living in arid regions of Asia and Africa. Recent studies have shown that several gerbil species have unusual amino acid changes in the PDX1 protein, a homeodomain transcription factor essential for pancreatic development and β-cell function. These changes were linked to strong GC-bias in the genome that may be caused by GC-biased gene conversion, and it has been hypothesized that this caused accumulation of deleterious changes. Here we use two approaches to examine if the unusual changes are adaptive or deleterious. First, we compare PDX1 protein sequences between 38 rodents to test for association with habitat. We show the PDX1 homeodomain is almost totally conserved in rodents, apart from gerbils, regardless of habitat. Second, we use ectopic gene overexpression and gene editing in cell culture to compare functional properties of PDX1 proteins. We show that the divergent gerbil PDX1 protein inefficiently binds an insulin gene promoter and ineffectively regulates insulin expression in response to high glucose in rat cells. The protein has, however, retained the ability to regulate some other β-cell genes. We suggest that during the evolution of gerbils, the selection-blind process of biased gene conversion pushed fixation of mutations adversely affecting function of a normally conserved homeodomain protein. We argue these changes were not entirely adaptive and may be associated with metabolic disorders in gerbil species on high carbohydrate diets. This unusual pattern of molecular evolution could have had a constraining effect on habitat and diet choice in the gerbil lineage.

Sex differences in the meiotic behavior of an XX sex chromosome pair in males and females of the mole vole Ellobius tancrei: turning an X into a Y chromosome?

Sex determination in mammals is usually provided by a pair of chromosomes, XX in females and XY in males. Mole voles of the genus Ellobius are exceptions to this rule. In Ellobius tancrei, both males and females have a pair of XX chromosomes that are indistinguishable from each other in somatic cells. Nevertheless, several studies on Ellobius have reported that the two X chromosomes may have a differential organization and behavior during male meiosis. It has not yet been demonstrated if these differences also appear in female meiosis. To test this hypothesis, we have performed a comparative study of chromosome synapsis, recombination, and histone modifications during male and female meiosis in E. tancrei. We observed that synapsis between the two X chromosomes is limited to the short distal (telomeric) regions of the chromosomes in males, leaving the central region completely unsynapsed. This uneven behavior of sex chromosomes during male meiosis is accompanied by structural modifications of one of the X chromosomes, whose axial element tends to appear fragmented, accumulates the heterochromatin mark H3K9me3, and is associated with a specific nuclear body that accumulates epigenetic marks and proteins such as SUMO-1 and centromeric proteins but excludes others such as H3K4me, ubiH2A, and γH2AX. Unexpectedly, sex chromosome synapsis is delayed in female meiosis, leaving the central region unsynapsed during early pachytene. This region accumulates γH2AX up to the stage in which synapsis is completed. However, there are no structural or epigenetic differences similar to those found in males in either of the two X chromosomes. Finally, we observed that recombination in the sex chromosomes is restricted in both sexes. In males, crossover-associated MLH1 foci are located exclusively in the distal regions, indicating incipient differentiation of one of the sex chromosomes into a neo-Y. Notably, in female meiosis, the central region of the X chromosome is also devoid of MLH1 foci, revealing a lack of recombination, possibly due to insufficient homology. Overall, these results reveal new clues about the origin and evolution of sex chromosomes.

Expression Evolution of Ancestral XY Gametologs across All Major Groups of Placental Mammals

Placental mammals present 180 million-year-old Y chromosomes that have retained a handful of dosage-sensitive genes. However, the expression evolution of Y-linked genes across placental groups has remained largely unexplored. Here, we expanded the number of Y gametolog sequences by analyzing ten additional species from previously unexplored groups. We detected seven remarkably conserved genes across 25 placental species with known Y repertoires. We then used RNA-seq data from 17 placental mammals to unveil the expression evolution of XY gametologs. We found that Y gametologs followed, on average, a 3-fold expression loss and that X gametologs also experienced some expression reduction, particularly in primates. Y gametologs gained testis specificity through an accelerated expression decay in somatic tissues. Moreover, despite the substantial expression decay of Y genes, the combined expression of XY gametologs in males is higher than that of both X gametologs in females. Finally, our work describes several features of the Y chromosome in the last common mammalian ancestor.

Meiotic Executioner Genes Protect the Y from Extinction

The Y has been described as a wimpy degraded relic of the X, with imminent demise should it lose sex-determining function. Why then has it persisted in almost all mammals? Here we present a novel mechanistic explanation for its evolutionary perseverance: the persistent Y hypothesis. The Y chromosome bears genes that act as their own judge, jury, and executioner in the tightly regulated meiotic surveillance pathways. These executioners are crucial for successful meiosis, yet need to be silenced during the meiotic sex chromosome inactivation window, otherwise germ cells die. Only rare transposition events to the X, where they remain subject to obligate meiotic silencing, are heritable, posing strong evolutionary constraint for the Y chromosome to persist.

Rapid chromosomal evolution in enigmatic mammal with XX in both sexes, the Alay mole vole Ellobiusalaicus Vorontsov et al., 1969 (Mammalia, Rodentia)

Evolutionary history and taxonomic position for cryptic species may be clarified by using molecular and cytogenetic methods. The subterranean rodent, the Alay mole vole Ellobiusalaicus Vorontsov et al., 1969 is one of three sibling species constituting the subgenus Ellobius Fischer, 1814, all of which lost the Y chromosome and obtained isomorphic XX sex chromosomes in both males and females. E.alaicus is evaluated by IUCN as a data deficient species because their distribution, biology, and genetics are almost unknown. We revealed specific karyotypic variability (2n = 52-48) in E.alaicus due to different Robertsonian translocations (Rbs). Two variants of hybrids (2n = 53, different Rbs) with E.tancrei Blasius, 1884 were found at the Northern slopes of the Alay Ridge and in the Naryn district, Kyrgyzstan. We described the sudden change in chromosome numbers from 2n = 50 to 48 and specific karyotype structure for mole voles, which inhabit the entrance to the Alay Valley (Tajikistan), and revealed their affiliation as E.alaicus by cytochrome b and fragments of nuclear XIST and Rspo1 genes sequencing. To date, it is possible to expand the range of E.alaicus from the Alay Valley (South Kyrgyzstan) up to the Ferghana Ridge and the Naryn Basin, Tien Shan at the north-east and to the Pamir-Alay Mountains (Tajikistan) at the west. The closeness of E.tancrei and E.alaicus is supported, whereas specific chromosome and molecular changes, as well as geographic distribution, verified the species status for E.alaicus. The case of Ellobius species accented an unevenness in rates of chromosome and nucleotide changes along with morphological similarity, which is emblematic for cryptic species.

Evolution and meiotic organization of heteromorphic sex chromosomes

The evolution of heteromorphic sex chromosomes has occurred independently many times in different lineages. The differentiation of sex chromosomes leads to dramatic changes in sequence composition and function and guides the evolutionary trajectory and utilization of genes in pivotal sex determination and reproduction roles. In addition, meiotic recombination and pairing mechanisms are key in orchestrating the resultant impact, retention and maintenance of heteromorphic sex chromosomes, as the resulting exposure of unpaired DNA at meiosis triggers ancient repair and checkpoint pathways. In this review, we summarize the different ways in which sex chromosome systems are organized at meiosis, how pairing is affected, and differences in unpaired DNA responses. We hypothesize that lineage specific differences in meiotic organization is not only a consequence of sex chromosome evolution, but that the establishment of epigenetic changes on sex chromosomes contributes toward their evolutionary conservation.

Sex chromosome repeats tip the balance towards speciation

Because sex chromosomes, by definition, carry genes that determine sex, mutations that alter their structural and functional stability can have immediate consequences for the individual by reducing fertility, but also for a species by altering the sex ratio. Moreover, the sex-specific segregation patterns of heteromorphic sex chromosomes make them havens for selfish genetic elements that not only create suboptimal sex ratios but can also foster sexual antagonism. Compensatory mutations to mitigate antagonism or return sex ratios to a Fisherian optimum can create hybrid incompatibility and establish reproductive barriers leading to species divergence. The destabilizing influence of these selfish elements is often manifest within populations as copy number variants (CNVs) in satellite repeats and transposable elements (TE) or as CNVs involving sex-determining genes, or genes essential to fertility and sex chromosome dosage compensation. This review catalogs several examples of well-studied sex chromosome CNVs in Drosophilids and mammals that underlie instances of meiotic drive, hybrid incompatibility and disruptions to sex differentiation and sex chromosome dosage compensation. While it is difficult to pinpoint a direct cause/effect relationship between these sex chromosome CNVs and speciation, it is easy to see how their effects in creating imbalances between the sexes, and the compensatory mutations to restore balance, can lead to lineage splitting and species formation.

Recombinant IFN-γ from the bank vole Myodes glareolus: a novel tool for research on rodent reservoirs of zoonotic pathogens

Rodent species like Myodes glareolus and Microtus spp. are natural reservoirs for many zoonotic pathogens causing human diseases and are gaining increasing interest in the field of eco-immunology as candidate animal models. Despite their importance the lack of immunological reagents has hampered research in these animal species. Here we report the recombinant production and functional characterization of IFN-γ, a central mediator of host’s innate and adaptive immune responses, from the bank vole M. glareolus. Soluble dimeric recMgIFN-γ was purified in high yield from Escherichia coli. Its activity on M. glareolus and Microtus arvalis kidney cell lines was assessed by immunofluorescent detection of nuclear translocation and phosphorylation of the transcription factor STAT1. RecMgIFN-γ also induced expression of an IFN-γ-regulated innate immunity gene. Inhibition of vesicular stomatitis virus replication in vole cells upon recMgIFN-γ treatment provided further evidence of its biological activity. Finally, we established a recMgIFN-γ-responsive bank vole reporter cell line that allows the sensitive titration of the cytokine activity via a bioluminescence reporter assay. Taken together, we report valuable tools for future investigations on the immune response against zoonotic pathogens in their natural animal hosts, which might foster the development of novel animal models.

X inactivation in a mammal species with three sex chromosomes

X inactivation is a fundamental mechanism in eutherian mammals to restore a balance of X-linked gene products between XY males and XX females. However, it has never been extensively studied in a eutherian species with a sex determination system that deviates from the ubiquitous XX/XY. In this study, we explore the X inactivation process in the African pygmy mouse Mus minutoides, that harbours a polygenic sex determination with three sex chromosomes: Y, X, and a feminizing mutant X, named X*; females can thus be XX, XX*, or X*Y, and all males are XY. Using immunofluorescence, we investigated histone modification patterns between the two X chromosome types. We found that the X and X* chromosomes are randomly inactivated in XX* females, while no histone modifications were detected in X*Y females. Furthermore, in M. minutoides, X and X* chromosomes are fused to different autosomes, and we were able to show that the X inactivation never spreads into the autosomal segments. Evaluation of X inactivation by immunofluorescence is an excellent quantitative procedure, but it is only applicable when there is a structural difference between the two chromosomes that allows them to be distinguished.

Brain Sexual Differentiation and Requirement of SRY: Why or Why Not?

Brain sexual differentiation is orchestrated by precise coordination of sex steroid hormones. In some species, programming of select male brain regions is dependent upon aromatization of testosterone to estrogen. In mammals, these hormones surge during the organizational and activational periods that occur during perinatal development and adulthood, respectively. In various fish and reptiles, incubation temperature during a critical embryonic period results in male or female sexual differentiation, but this can be overridden in males by early exposure to estrogenic chemicals. Testes development in mammals requires a Y chromosome and testis determining gene SRY (in humans)/Sry (all other therian mammals), although there are notable exceptions. Two species of spiny rats: Amami spiny rat (Tokudaia osimensis) and Tokunoshima spiny rat (Tokudaia tokunoshimensis) and two species of mole voles (Ellobius lutescens and Ellobius tancrei), lack a Y chromosome/Sry and possess an XO chromosome system in both sexes. Such rodent species, prototherians (monotremes, who also lack Sry), and fish and reptile species that demonstrate temperature sex determination (TSD) seemingly call into question the requirement of Sry for brain sexual differentiation. This review will consider brain regions expressing SRY/Sry in humans and rodents, respectively, and potential roles of SRY/Sry in the brain will be discussed. The evidence from various taxa disputing the requirement of Sry for brain sexual differentiation in mammals (therians and prototherians) and certain fish and reptilian species will be examined. A comparative approach to address this question may elucidate other genes, pathways, and epigenetic modifications stimulating brain sexual differentiation in vertebrate species, including humans.

Chromosomal Evolution in Mole Voles Ellobius (Cricetidae, Rodentia): Bizarre Sex Chromosomes, Variable Autosomes and Meiosis

This study reports on extensive experimental material covering more than 30 years of studying the genetics of mole voles. Sex chromosomes of Ellobius demonstrate an extraordinary case of mammalian sex chromosomes evolution. Five species of mole voles own three types of sex chromosomes; typical for placentals: XY♂/XX♀; and atypical X0♂/X0♀; or XX♂/XX♀. Mechanisms of sex determination in all Ellobius species remain enigmatic. It was supposed that the Y chromosome was lost twice and independently in subgenera Bramus and Ellobius. Previous to the Y being lost, the X chromosome in distinct species obtained some parts of the Y chromosome, with or without Sry, and accumulated one or several copies of the Eif2s3y gene. Along with enormous variations of sex chromosomes, genes of sex determination pathway and autosomes, and five mole vole species demonstrate ability to establish different meiotic mechanisms, which stabilize their genetic systems and make it possible to overcome the evolutionary deadlocks.

Flexible adaptation of male germ cells from female iPSCs of endangered Tokudaia osimensis

In mammals, the Y chromosome strictly influences the maintenance of male germ cells. Almost all mammalian species require genetic contributors to generate testes. An endangered species, Tokudaia osimensis, has a unique sex chromosome composition XO/XO, and genetic differences between males and females have not been confirmed. Although a distinctive sex-determining mechanism may exist in T. osimensis, it has been difficult to examine thoroughly in this rare animal species. To elucidate the discriminative sex-determining mechanism in T. osimensis and to find a strategy to prevent its possible extinction, we have established induced pluripotent stem cells (iPSCs) and derived interspecific chimeras using mice as the hosts and recipients. Generated iPSCs are considered to be in the so-called "true naïve" state, and T. osimensis iPSCs may contribute as interspecific chimeras to several different tissues and cells in live animals. Surprisingly, female T. osimensis iPSCs not only contributed to the female germ line in the interspecific mouse ovary but also differentiated into spermatocytes and spermatids that survived in the adult interspecific mouse testes. Thus, T. osimensis cells have high sexual plasticity through which female somatic cells can be converted to male germline cells. These findings suggest flexibility in T. osimensis cells, which can adapt their germ cell sex to the gonadal niche. The probable reduction of the extinction risk of an endangered species through the use of iPSCs is indicated by this study.

Genetic Mechanisms Leading to Sex Differences Across Common Diseases and Anthropometric Traits

Common diseases often show sex differences in prevalence, onset, symptomology, treatment, or prognosis. Although studies have been performed to evaluate sex differences at specific SNP associations, this work aims to comprehensively survey a number of complex heritable diseases and anthropometric traits. Potential genetically encoded sex differences we investigated include differential genetic liability thresholds or distributions, gene-sex interaction at autosomal loci, major contribution of the X-chromosome, or gene-environment interactions reflected in genes responsive to androgens or estrogens. Finally, we tested the overlap between sex-differential association with anthropometric traits and disease risk. We utilized complementary approaches of assessing GWAS association enrichment and SNP-based heritability estimation to explore explicit sex differences, as well as enrichment in sex-implicated functional categories. We do not find consistent increased genetic load in the lower-prevalence sex, or a disproportionate role for the X-chromosome in disease risk, despite sex-heterogeneity on the X for several traits. We find that all anthropometric traits show less than complete correlation between the genetic contribution to males and females, and find a convincing example of autosome-wide genome-sex interaction in multiple sclerosis (P = 1 × 10^-9). We also find some evidence for hormone-responsive gene enrichment, and striking evidence of the contribution of sex-differential anthropometric associations to common disease risk, implying that general mechanisms of sexual dimorphism determining secondary sex characteristics have shared effects on disease risk.