Characterization of the Xp21-23 region in the wood lemming, a region involved in XY sex reversal

The role of conflict in the formation and maintenance of variant sex chromosome systems in mammals

The XX/XY sex chromosome system is deeply conserved in therian mammals, as is the role of Sry in testis determination, giving the impression of stasis relative to other taxa. However, the long tradition of cytogenetic studies in mammals documents sex chromosome karyotypes that break this norm in myriad ways, ranging from fusions between sex chromosomes and autosomes to Y chromosome loss. Evolutionary conflict, in the form of sexual antagonism or meiotic drive, is the primary predicted driver of sex chromosome transformation and turnover. Yet conflict-based hypotheses are less considered in mammals, perhaps because of the perceived stability of the sex chromosome system. To address this gap, we catalog and characterize all described sex chromosome variants in mammals, test for family-specific rates of accumulation, and consider the role of conflict between the sexes or within the genome in the evolution of these systems. We identify 152 species with sex chromosomes that differ from the ancestral state and find evidence for different rates of ancestral to derived transitions among families. Sex chromosome-autosome fusions account for 79% of all variants whereas documented sex chromosome fissions are limited to three species. We propose that meiotic drive and drive suppression provide viable explanations for the evolution of many of these variant systems, particularly those involving autosomal fusions. We highlight taxa particularly worthy of further study and provide experimental predictions for testing the role of conflict and its alternatives in generating observed sex chromosome diversity.

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.

Sex determination, gonadal sex differentiation, and plasticity in vertebrate species

A diverse array of sex determination (SD) mechanisms, encompassing environmental to genetic, have been found to exist among vertebrates, covering a spectrum from fixed SD mechanisms (mammals) to functional sex change in fishes (sequential hermaphroditic fishes). A major landmark in vertebrate SD was the discovery of the SRY gene in 1990. Since that time, many attempts to clone an SRY ortholog from nonmammalian vertebrates remained unsuccessful, until 2002, when DMY/dmrt1by was discovered as the SD gene of a small fish, medaka. Surprisingly, however, DMY/dmrt1by was found in only 2 species among more than 20 species of medaka, suggesting a large diversity of SD genes among vertebrates. Considerable progress has been made over the last 3 decades, such that it is now possible to formulate reasonable paradigms of how SD and gonadal sex differentiation may work in some model vertebrate species. This review outlines our current understanding of vertebrate SD and gonadal sex differentiation, with a focus on the molecular and cellular mechanisms involved. An impressive number of genes and factors have been discovered that play important roles in testicular and ovarian differentiation. An antagonism between the male and female pathway genes exists in gonads during both sex differentiation and, surprisingly, even as adults, suggesting that, in addition to sex-changing fishes, gonochoristic vertebrates including mice maintain some degree of gonadal sexual plasticity into adulthood. Importantly, a review of various SD mechanisms among vertebrates suggests that this is the ideal biological event that can make us understand the evolutionary conundrums underlying speciation and species diversity.

Sex Reversal in Non-Human Placental Mammals

Gonads are very peculiar organs given their bipotential competence. Indeed, early differentiating genital ridges evolve into either of 2 very distinct organs: the testis or the ovary. Accumulating evidence now demonstrates that both genetic pathways must repress the other in order for the organs to differentiate properly, meaning that if this repression is disrupted or attenuated, the other pathway may completely or partially be expressed, leading to disorders of sex development. Among these disorders are the cases of XY male-to-female and XX female-to-male sex reversals as well as true hermaphrodites, in which there is a discrepancy between the chromosomal and gonadal sex. Here, we review known cases of XY and XX sex reversals described in mammals, focusing mostly on domestic animals where sex reversal pathologies occur and on wild species in which deviations from the usual XX/XY system have been documented.

Reactivation of the inactive X chromosome in development and reprogramming

In mammals, one of the two X chromosomes of female cells is inactivated for dosage compensation between the sexes. X chromosome inactivation is initiated in early embryos by the noncoding Xist RNA. Subsequent chromatin modifications on the inactive X chromosome (Xi) lead to a remarkable stability of gene repression in somatic cell lineages. In mice, reactivation of genes on the Xi accompanies the establishment of pluripotent cells of the female blastocyst and the development of primordial germ cells. Xi reactivation also occurs when pluripotency is established during the reprogramming of somatic cells to induced pluripotent stem cells. The mechanism of Xi reactivation has attracted increasing interest for studying changes in epigenetic patterns and for improving methods of cell reprogramming. Here, we review recent advances in the understanding of Xi reactivation during development and reprogramming and illustrate potential clinical applications.

Natural exceptions to normal gonad development in mammals

Gonads are the only organs with 2 possible developmental pathways, testis or ovary. A consequence of this unique feature is that mutations in genes controlling gonad development give rise not only to gonadal malformation or dysfunction but also to frequent cases of sex reversal, including XY females, XX males and intersexes. Most of our current knowledge on mammalian sex determination, the genetic process by which the gonadal primordia are committed to differentiate as either testes or ovaries, has derived mainly from the study of sex-reversed mice obtained by direct genetic manipulation. However, there are also numerous cases of natural exceptions to normal gonad development which have been described in a variety of mammals, including both domestic and wild species. Here, we review the most relevant cases of: (1) natural, non-induced sex reversal and intersexuality described in laboratory rodents, including Sxr and B6-Y(DOM) mice; (2) sex reversal in domestic animals, including freemartinism in bovids and pigs, XX sex reversal in pigs, goats and dogs, XY sex reversal in the horse, and sex chromosome chimerism and sex reversal in the cat, and (3) sex reversal in wild mammals, including the generalised true hermaphroditism described in talpid moles, XY sex reversal in Akodon, Microtus and Dicrostonyx species, males lacking a Y chromosome and SRY in Ellobius lutescens, the X* chromosome of Myopus schisticolor, and sex chromosome mosaicism and X0 females in Microtus oregoni. These studies are necessary to elucidate particular aspects of mammalian gonad development in some instances and to understand how the genetic mechanisms controlling gonad development have evolved.

Exceptional minute sex-specific region in the X0 mammal, Ryukyu spiny rat

The Ryukyu spiny rats (genus Tokudaia) inhabit only three islands in the Nansei Shoto archipelago in Japan, and have the variations of karyotype among the islands. The chromosome number of T. osimensis in Amami-Oshima Island is 2n = 25, and T. tokunoshimensis in Tokunoshima Island is 2n = 45, and the two species have X0 sex chromosome constitution with no cytogenetically visible Y chromosome in both sexes. We constructed the standard ideograms for these species at the 100 and 200 band levels. Comparing the banding patterns between these species, it was suggested that at least 10 times the number of Robertsonian fusions occurred in T. osimensis chromosomes. However, no karyotypic differences were observed between sexes in each species. To detect the sex-specific chromosomal region of these X0 species we applied the comparative genomic hybridization (CGH) method. Although the male- and female-derived gains and losses were detected in several chromosome regions, all of them were located in the heterochromatic and/or telomeric regions. This result suggested that the differences detected by CGH might be caused by the polymorphism on the copy numbers of repeated sequences in the heterochromatic and telomeric regions. Our result indicated that the sex-specific region, where the key to sex determination lies, is very minute in X0 species of Tokudaia.

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.

The sex determination in Ellobius lutescens remains bizarre

Mammalian sex determination and gonad differentiation are the result of a complex interaction of fine-tuned spatial and temporal gene expression with threshold levels of individual genes. The male pathway is initiated by SRY. Some exceptional mammals determine male sex without the SRY gene and even without a Y chromosome. Ellobius lutescens in this report is one example of this "weird" species. We provide key data on the genomic level that there are no coarse differences in the genomes of male and female animals by comparative genomic hybridization. On the gene level we studied the gene Nr5a1 for the orphan nuclear receptor, steroidogenic factor SF-1, a central constituent for gonad differentiation and adrenal gland development. The Ellobius lutescens Nr5a1 gene was mapped to the proximal short arm of chromosome 2 by fluorescence in situ hybridization. In addition, we provide evidence by linkage analysis in two E. lutescens pedigrees that Nr5a1 is not the key male sex-determining gene in Ellobius lutescens.

X-Autosome translocations, meiotic synapsis, chromosome evolution and speciation

Several theories have been proposed to explain the often-noted sterility of both reciprocal and Robertsonian X-autosome translocations in male mammals. However, there are a number of species in which all members of the species carry a Robertsonian X-autosome translocation. Meiosis in spermatocytes from these sterile vs. fertile animals is compared within the context of these theories. New technologies and insights into underlying mechanisms are summarized and suggestions presented for further studies.

Exploring the envelope. Systematic alteration in the sex-determination system of the nematode caenorhabditis elegans

The natural sexes of the nematode Caenorhabditis elegans are the self-fertilizing hermaphrodite (XX) and the male (XO). The underlying genetic pathway controlling sexual phenotype has been extensively investigated. Mutations in key regulatory genes have been used to create a series of stable populations in which sex is determined not by X chromosome dosage, but in a variety of other ways, many of which mimic the diverse sex-determination systems found in different animal species. Most of these artificial strains have male and female sexes. Each of seven autosomal genes can be made to adopt a role as the primary determinant of sex, and each of the five autosomes can carry the primary determinant, thereby becoming a sex chromosome. Strains with sex determination by fragment chromosomes, episomes, compound chromosomes, or environmental factors have also been constructed. The creation of these strains demonstrates the ease with which one sex-determination system can be transformed into another.