Parasitism and the Maintenance of Genetic Variation in a Partial Parthenogen

目前进化生物学中最耐人寻味的问题之一是：为什么有这么多自然种群有雄性和雌性？换一种方式说：为什么雌性不无性繁殖，因此只产生克隆的女儿？假设繁殖力没有随之降低，一只罕见的无性雌性会生出两倍于普通有性雌性的女儿(四倍于普通有性雌性的孙女)；如果不加以控制，由此产生的克隆将呈指数级增长，最终取代种群中的有性雌性和雄性。面对克隆的优势，有性繁殖仍然存在，这表明产生可变后代是有优势的。然而，目前还不清楚这一优势是什么。为了解决这个问题，重要的是研究具有这两种繁殖模式的物种，以便有坚实的基础进行比较。在这样的物种中，新西兰淡水蜗牛的有性繁殖与寄生虫感染的发生率相关，这与寄生风险高的种群倾向于产生可变后代的观点是一致的。本研究的目的是：(1)确定宿主-寄生虫相互作用是否存在遗传基础；(2)确定寄生虫是否可以阻止一个或多个克隆消除有性个体；(3)评估寄生虫对维持钉螺种群遗传多样性的影响。除了解决有性繁殖与克隆繁殖有关的重要问题外，这些研究的结果还对研究由类似的蠕虫引起的疾病(例如人类血吸虫病)以及生物敌人在保护自然种群遗传多样性方面所起的作用具有重要意义。

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.