Abstract
The evolution of heteromorphic sex chromosomes creates a genetic condition favoring the invasion of sex-ratio meiotic drive elements, resulting in the biased transmission of one sex chromosome over the other, in violation of Mendel's first law. The molecular mechanisms of sex-ratio meiotic drive may therefore help us to understand the evolutionary forces shaping the meiotic behavior of the sex chromosomes. Here we characterize a sex-ratio distorter on the X chromosome (Dox) in Drosophila simulans by genetic and molecular means. Intriguingly, Dox has very limited coding capacity. It evolved from another X-linked gene, which also evolved de nova. Through retrotransposition, Dox also gave rise to an autosomal suppressor, not much yang (Nmy). An RNA interference mechanism seems to be involved in the suppression of the Dox distorter by the Nmy suppressor. Double mutant males of the genotype dox; nmy are normal for both sex-ratio and spermatogenesis. We postulate that recurrent bouts of sex-ratio meiotic drive and its subsequent suppression might underlie several common features observed in the heterogametic sex, including meiotic sex chromosome inactivation and achiasmy.
Highlights
Sex chromosomes are believed to evolve from a pair of autosomes [1,2,3]
A sex-ratio Distorter on the X Chromosome (Dox) We previously cloned a D. simulans gene, not much yang, in which the homozygous male mutant displays a female-biased sex ratio. This gene belongs to the Winters sex-ratio system, one of three independent sex-ratio systems found in this species [24]
We inferred that the wild-type (Nmy) function is a suppressor of sex-ratio distortion, and that there must be a corresponding Xlinked sex-ratio distorter according to Fisher’s principle of sex ratio evolution
Summary
Sex chromosomes are believed to evolve from a pair of autosomes [1,2,3]. An incipient Y chromosome, like an autosome, is largely euchromatic and free to recombine, except for a small region determining sex, as exemplified by species such as the papaya plant [4] and the medaka and stickleback fish [5,6]. A small number of genes remain active in a mature Y chromosome, such as that in humans or Drosophila. Accompanying the evolution of sex chromosomes, at least two problems of biological significance arise. One problem is the unequal gene dosage of sex-linked genes between the XY sex and the XX sex. Myriad strategies to compensate the dosage inequality have been exploited by various species, and some of these mechanisms are understood in molecular detail in model organisms of fly, worm, and mouse [11]
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