to population extinction as the population would fix for the driver (e.g. a selfish X chromosome), removing the Y chromosome (and therefore males) from the population. The idea that meiotic drivers and their suppressors may govern speciation is supported by observations that mouse Y chromosomes lacking certain genetic loci produce female-biased offspring, implying these loci encode suppressors of drive. Moreover, matings of certain mouse strains used in research results in unequal offspring ratios. One gene responsible for sex ratio distortion in mice is r2d2 (r2d2 – responder to meiotic drive 2), which predicts which strains of mice can successfully breed without offspring sex ratio distortion. Meiotic drive in flies Selfish chromosomes of stalk-eyed flies have had ecological consequences. Driving X chromosomes lead to reductions in male fecundity and mating success, leading to frequency dependent selection maintaining both the driving alleles and wild-type alleles. Multiple species of fruit fly are known to have driving X chromosomes, of which the best-characterized are found in Drosophila simulans. Three independent driving X chromosomes are known in D. simulans, called Paris, Durham, and Winters. In Paris, the driving gene encodes a DNA modelling protein ("heterochromatin protein 1 D2" or HP1D2), where the allele of the driving copy fails to prepare the male Y chromosome for meiosis. In Winters, the gene responsible ("Distorter on the X" or Dox) has been identified, though the mechanism by which it acts is still unknown. The strong selective pressure imposed by these driving X chromosomes has given rise to suppressors of drive, of which the genes are somewhat known for Winters, Durham, and Paris. These suppressors encode hairpin RNAs which match the sequence of driver genes (such as Dox), leading host RNA interference pathways to degrade Dox sequence. Autosomal suppressors of drive are known in Drosophila mediopunctata, Drosophila paramelanica, Drosophila quinaria, and Drosophila testacea, emphasizing the importance of these drive systems in natural populations. See also Fixed allele References Genetics