One of the most intriguing questions in evolutionary biology at the present time is: why do so many natural populations have males and females? Stated in another way: why don't females reproduce asexually and thereby produce only clonal daughters? Assuming no concomitant reduction in fecundity, a rare asexual female would produce twice as many daughters (and four times as many granddaughters) as the average sexual female; and unchecked, the resulting clone would grow exponentially, eventually replacing the sexual females and males in the population. That sexual reproduction persists in the face of the advantage of clones suggests that there is an advantage to producing variable offspring. It is currently unknown, however, what this advantage is. To address the question, it is important to study species that have both reproduction modes, so that there is a firm basis for comparison. In one such species, a freshwater New Zealand snail, sexual reproduction is correlated with the incidence of infection by parasitic worms, which is consistent with the idea that the production of variable progeny is favored in populations where there is a high risk of parasitism. The goals present study are to: (1) determine whether there is a genetic basis to the host-parasite interaction, (2) establish whether parasites can prevent the elimination of sexual individuals by one or more clones, and (3) to assess the effect of parasites on the maintenance of genetic diversity within snail populations. In addition to addressing important questions regarding sexual versus clonal reproduction, the results of these studies have important implications for the study of diseases caused by similar worms that cycle through snail vectors (e.g. human schistosomiasis), and the role that biological enemies play in preserving genetic diversity of natural populations.
