9615065 Jaenike Most animal species have a genetic system that keeps the population half female and half male. Generally, males are XY, and produce half X sperm, which result in daughters, and half Y sperm, which result in sons. In many species, some X chromosomes destroy Y sperm, which results in more females among the progeny of those individuals and in the population. If unchecked by natural selection, the alleles causing the sex-ratio bias (termed SR) will increase so that only females are produced, causing the extinction of populations or even entire species. That certain insect species harbor SR alleles at moderate frequencies indicates that the advantage in the proportion of sperm must be opposed by some other form of natural selection. The aim of this proposal is to test a specific mechanism that can potentially halt the spread of SR alleles, in the insect Drosophila neotestacea. An increase in the frequency of SR causes the ratio of females to males to increase, which leads to an increase in the potential rate of male mating. Because SR males produce only half as many sperm as do normal males, the increased rate of male mating may cause the fertility of SR males to decline relative to that of normal males and eventually halt the spread of SR. This mechanism will be tested in a series of laboratory and field experiments. An increasing number of insects have been found to harbor X-chromosome SR alleles that could drive a species extinct. Because the mechanism to be tested involves a general side-effect of SR genes, the conclusions drawn from these studies will be applicable to a wide variety of insects. This study will show how extinction risk is related to the frequency of the SR alleles and to the density of the species, which is useful for pest control and for conservation biology. More generally, the results will indicate the balance between natural selection acting at the levels of genes, individuals, and populations.
