Aberrant chromosomal sex-determining mechanisms in mammals, with special reference to species with XY females

Abstract

Both mouse and man have the common XX/XY sex chromosome mechanism. The X chromosome is of original size (5-6% of female haploid set) and the Y is one of the smallest chromosomes of the complement. But there are species, belonging to a variety of orders, with composite sex chromosomes and multiple sex chromosome systems: XX/XY1Y2 and X1X1X2X2/X1X2Y. The original X or the Y, respectively, have been translocated on to an autosome. The sex chromosomes of these species segregate regularly at meiosis; two kinds of sperm and one kind of egg are produced and the sex ratio is the normal 1:1. Individuals with deviating sex chromosome constitutions (XXY, XYY, XO or XXX) have been found in at least 16 mammalian species other than man. The phenotypic manifestations of these deviating constitutions are briefly discussed. In the dog, pig, goat and mouse exceptional XX males and in the horse XY females attract attention. Certain rodents have complicated mechanisms for sex determination: Ellobius lutescens and Tokudaia osimensis have XO males and females. Both sexes of Microtus oregoni are gonosomic mosaics (male OY/XY, female XX/XO). The wood lemming, Myopus schisticolor, the collared lemming, Dirostonyx torquatus, and perhaps also one or two species of the genus Akodon have XX and XY females and XY males. The XX, X*X and X*Y females of Myopus and Dicrostonyx are discussed in some detail. The wood lemming has proved to be a favourable natural model for studies in sex determination, because a large variety of sex chromosome aneuploids are born relatively frequently. The dosage model for sex determination is not supported by the wood lemming data. For male development, genes on both the X and the Y chromosomes are necessary.

Sex ratio, XY females and absence of inbreeding in a population of the Wood Lemming, Myopus schisticolor Lilljeborg, 1844

“…Thus, at low density the frequency of XY females in the population was higher than expected in theory. A similar situation was reported by Fredga (1988) for Scandinavian populations of the Wood Lemming where the proportion of XY females was equal to 0.45. Fredga explained the excess of the X*Y genotype by an increased fertility of X*Y and X*XO females taking place in these populations.…”

“…In the XY females of the Varying Lemming the loss of YY zygotes is compensated for by an increased ovulation rate (Gileva et at., 1982). According to Fredga (1988), reproductive output of XY females in Scandinavian populations of the Wood Lemming is even greater than that of the XX.…”

A novel sex determination system in a close relative of the house mouse

“…It is noteworthy that similar, atypical sex determination systems have been described in three other rodent genera (see §1). The genetic bases of these modifications are all unknown, but are associated with an X-linked mutation in Myopus (deletion of Xp 21 -23 ; Liu et al 1998), and possibly so too in Dicrostonyx and Akodon (Fredga 1988;Ortiz et al 2009). Although these mutations have appeared independently in the four different lineages (even the two lemmings are not closely related; Buzan et al 2008), they are all X-linked, and may therefore involve the same gene(s).…”

Did sex chromosome turnover promote divergence of the major mammal groups?

“…Many rodent species seem now to be experimenting with novel systems 79. There are several examples of closely related rodent species with variant sex chromosomes.…”

“…Since other species in the same clade do not have SRY + females, these changes were thought to be independent; however, they may have occurred after SRY was mutationally damaged, or relocated to part of the Y subject to epigenetic silencing in an ancestral animal. Wood lemmings and a pygmy mouse species have independently evolved variant X chromosomes that suppress SRY action, again leading to XY SRY + females 79.…”