Another way of looking at the enigma of sex determination in Ellobius lutescens

A number of hypotheses have been proposed over the years to account for sex determination in the vole Ellobius lutescens (Arvicolinae, Rodentia, Mammalia) (reviewed in ref. 2). The chromosomes of the males of this species are identical to those of the females in number and morphology, with the same odd number (2n = 17) being present in both sexes. The odd chromosome is the X chromosome, and it constitutes approximately 5% of the haploid genome. It therefore appears that the X chromosome is of the standard (original) type found in most mammals. Embryonic mortality is high, as one would expect when both parents are monosomic for the X chromosome and if embryos disomic for the X do not survive. The X remains unpaired at meiosis. There is no evidence of a Y chromosome, either in the soma or in the germline. The possibility of Sry (Sex determining region of the Y) being present as a cryptic translocation in another chromosome has recently been ruled out by the finding that Sry sequences are not detectable in E. lutescens. On the other hand, in Ellobius fuscocapillus, a related species with a standard XX female XY male sex-chromosome system, males are Sry-positive and females are not. Thus, there has evolved a system in which both sexes are XO, sex determination occurs without Sry, and meiosis and spermatogenesis are maintained without a Y chromosome. These cytogenetic features, unexpected in a mammal, have prompted some authors to describe sex determination in this species as enigmatic. Unravelling the complexity of this system & and that of other exceptional systems is a prerequisite for identification of the common, and therefore core, elements of mammalian sex-determining mechanisms