Criscione Charles D, Blouin Michael S
Department of Zoology, Oregon State University, Corvallis 97331, USA.
Evolution. 2006 Mar;60(3):553-62.
Little is known about actual mating systems in natural populations of parasites or about what constitutes the limits of a parasite deme. These parameters are interesting because they affect levels of genetic diversity, opportunities for local adaptation, and other evolutionary processes. We expect that transmission dynamics and the distribution of parasites among hosts should have a large effect on mating systems and demic structure, but currently we have mostly speculation and very few data. For example, infrapopulations (all the parasites in a single host) should behave as demes if parasite offspring are transmitted as a clump from host to host over several generations. However, if offspring are well mixed, then the parasite component population (all the parasites among a host population) would function as the deme. Similarly, low mean intensities or a high proportion of worms in single infections should increase the selfing rate. For species having an asexual amplification stage, transmission between intermediate and definitive (final) hosts will control the variance in clonal reproductive success, which in turn could have a large influence on effective sizes and rates of inbreeding. We examined demic structure, selfing rates, and the variance in clonal reproductive success in natural populations of Plagioporus shawi, a hermaphroditic trematode that parasitizes salmon. Overall levels of genetic diversity were very high. An a posteriori inference of population structure overwhelmingly supports the component population as the deme, rather than individual infrapopulations. Only a single pair of 597 adult individuals was identified as clones. Thus, the variance in clonal reproductive success was almost zero. Despite being hermaphroditic, P. shawi appears to be almost entirely outcrossing. Genetic estimates of selfing (<5%) were in accordance with the proportion of parasites from single infections. Thus, it appears that individual flukes outcross whenever possible and only resort to selfing when alone. Finally, our data support the hypothesis that aquatic transmission and the use of several intermediate hosts promotes high genetic diversity and well-mixed infrapopulations.
对于寄生虫自然种群中的实际交配系统，或者关于什么构成寄生虫同类群的界限，我们知之甚少。这些参数很有趣，因为它们会影响遗传多样性水平、局部适应的机会以及其他进化过程。我们预计传播动态和寄生虫在宿主间的分布会对交配系统和同类群结构产生很大影响，但目前我们大多只有推测，数据极少。例如，如果寄生虫后代在几代中以团块形式从一个宿主传播到另一个宿主，那么虫口种群（单个宿主体内的所有寄生虫）应表现为同类群。然而，如果后代充分混合，那么寄生虫组成种群（宿主种群中的所有寄生虫）将起到同类群的作用。同样，低平均感染强度或单感染中高比例的蠕虫应会增加自交率。对于具有无性增殖阶段的物种，中间宿主和终末（最终）宿主之间的传播将控制克隆繁殖成功的方差，这反过来可能对有效种群大小和近亲繁殖率有很大影响。我们研究了寄生在鲑鱼体内的雌雄同体吸虫肖氏斜孔吸虫自然种群中的同类群结构、自交率以及克隆繁殖成功的方差。总体遗传多样性水平非常高。对种群结构的事后推断压倒性地支持组成种群作为同类群，而不是单个虫口种群。仅一对597个成年个体被鉴定为克隆体。因此，克隆繁殖成功的方差几乎为零。尽管肖氏斜孔吸虫是雌雄同体，但它似乎几乎完全进行异交。自交的遗传估计值（<5%）与单感染寄生虫的比例一致。因此，似乎单个吸虫尽可能进行异交，只有在单独存在时才 resort to自交。最后，我们的数据支持这样的假设，即水生传播和使用多种中间宿主促进了高遗传多样性和充分混合的虫口种群。
