Sex chromosomes and sex determination in weird mammals

Abstract

Weird mammals are of two types. Highly divergent mammals, such as the marsupials and monotremes, have informed us of the evolutionary history of the Y chromosome and sex-determining gene, and the recently specialized rodents can help us predict its future. The Y chromosome has had a short but eventful history, and is already heading briskly for oblivion. It originated as a homologous partner of the X when it acquired a sex-determining gene (not necessarily SRY). Most of the genes on the Y, even those with a male-specific function, evolved from genes now on the X. At the mercy of a high rate of variability and the forces of drift and selection, the Y has lost genes at a rate of 3–6 genes/million years, sparing those that acquired critical male-specific functions. Even these genes have disappeared from one mammalian lineage or another as their functions were usurped by genes elsewhere in the genome. The mammalian testis-determining gene, SRY, is a typical Y-borne gene. It arose by truncation of a gene (SOX3) on the X that is expressed in brain development, and it may work by interacting with (inhibiting?) related genes, including SOX9. Variant sex-determining systems in rodents show that the action of SRY can change, as it evidently has in the mouse, and SRY can be inactivated, as in akodont rodents, or even completely superseded, as in mole voles.

Kinase CDK2 in Mammalian Meiotic Prophase I: Screening for Hetero- and Homomorphic Sex Chromosomes

“…In this case, the analysis of such a multifunctional molecule as CDK2 on other animals can be promising. Evolution has led to a diversity of sex chromosomes among the distinct phyla of animals, and rodents in particular (for example [20,21]. This study aimed to confirm the specifity of a kinase (CDK2) meiotic manifestation in the sex chromosomes of various evolutionary stories.…”

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

“…It has long been known that rodent sex chromosomes exhibit remarkable diversity. [ 1–4 ] In contrast to the relative stasis associated with sex chromosomes throughout most placental mammalian families, which largely retain the ancestral sex chromosomes and gene contents that evolved in the placental mammalian ancestor, [ 5,6 ] with occasional sex chromosome‐autosome fusions, different rodent species have undergone diverse innovations (see [ 7 ] for an excellent review). Rodent sex chromosomes have repeatedly evolved novelties ranging from non‐Mendelian inheritance, to sex chromosome‐autosome fusions, to Y chromosomal loss, to turnover of sex determination mechanisms.…”

“…Rodent sex chromosomes have repeatedly evolved novelties ranging from non-Mendelian inheritance, to sex chromosome-autosome fusions, to Y chromosomal loss, to turnover of sex determination mechanisms. [1][2][3][4]8,9] While most of these anomalies have long been known, the evolutionary origins of these systems remain obscure. It has been speculated that intragenomic conflict between sex chromosomes could drive some or all of these transitions, [10] however little empirical or modeling work has attempted to discern sources of conflict and chains of causality underlying such transitions (though see [9,[11][12] ).…”

Unusual Mammalian Sex Determination Systems: A Cabinet of Curiosities

“…However, rules are meant to be broken, and mammalian sex determination is no exception. Around a dozen therian mammals have been described with so-called "bizarre" and "weird" sex-determining systems [14,15] which deviate from the standard therian mammal XX/XY mechanism. These species, all rodents, have puzzled scientists for nearly 60 years, since Robert Matthey, who characterized a fair share of these atypical systems, analyzed karyotypes of the mole vole Ellobius lutescens, in which the Y chromosome is missing [16].…”