Gemmill Alan W, Viney Mark E, Read Andrew F
Institute of Cell, Animal and Population Biology, Division of Biological Sciences, University of Edinburgh, Edinburgh, EH9 3JT, United Kingdom.
Evolution. 1997 Apr;51(2):393-401. doi: 10.1111/j.1558-5646.1997.tb02426.x.
We examine the hypothesis that sexual reproduction by parasites is an adaptation to counter the somatic evolution of vertebrate immune responses. This is analogous to the idea that antagonistic coevolution between hosts and their parasites maintains sexual reproduction in host populations. Strongyloides ratti is a parasitic nematode of rats. It can have a direct life cycle, with clonal larvae of the wholly parthenogenetic parasites becoming infective, or an indirect life cycle, with clonal larvae developing into free-living dioecious adults. These free-living adults produce infective larvae by conventional meiosis and syngamy. The occurrence of the sexual cycle is determined by both environmental and genetic factors. By experimentally manipulating host immune status using hypothymic mutants, corticosteroids, whole-body γ-irradiation and previous exposure to S. ratti, we show that larvae from hosts that have acquired immune protection are more likely to develop into sexual adults. This effect is independent of the method of manipulation, larval density, and the number of days postinfection. This immune-determined sexuality is consistent with the idea that sexual reproduction by parasites is adaptive in the face of specific immunity, an idea which, if true, has clinical and epidemiological consequences.
我们检验了这样一种假说，即寄生虫的有性生殖是一种适应机制，用以对抗脊椎动物免疫反应的体细胞进化。这类似于宿主与其寄生虫之间的拮抗协同进化维持宿主种群中有性生殖的观点。鼠类圆线虫是大鼠的一种寄生线虫。它可以有直接生命周期，完全孤雌生殖的寄生虫的克隆幼虫直接具有感染性；或者有间接生命周期，克隆幼虫发育成自由生活的雌雄异体成虫。这些自由生活的成虫通过传统的减数分裂和配子融合产生感染性幼虫。有性生殖周期的出现由环境和遗传因素共同决定。通过使用胸腺发育不全突变体、皮质类固醇、全身γ射线照射以及先前感染鼠类圆线虫等方法对宿主免疫状态进行实验性操控，我们发现，来自已获得免疫保护的宿主的幼虫更有可能发育成性成熟成虫。这种效应与操控方法、幼虫密度以及感染后天数无关。这种由免疫决定的有性生殖与寄生虫的有性生殖在面对特异性免疫时具有适应性这一观点相符，该观点若成立，则会产生临床和流行病学方面的影响。
