Department of Evolutionary Ecology, Max Planck Institute for Evolutionary Biology, August-Thienemann-Strasse 2, 24306 Plön, Germany.
Evolution. 2014 Jun;68(6):1762-74. doi: 10.1111/evo.12388. Epub 2014 Mar 28.
Classical theory on mating system evolution suggests that simultaneous hermaphrodites should either outcross if they have high inbreeding depression (ID) or self-fertilize if they have low ID. However, a mixture of selfing and outcrossing persists in many species. Previous studies with the tapeworm Schistocephalus solidus have found worms to self-fertilize some of their eggs despite ID. The probability for selfing to spread depends on the relative fitness of selfers, as well as the genetic basis for ID and whether it can be effectively purged. We bred S. solidus through two consecutive generations of selfing and recorded several fitness correlates over the whole life cycle. After one round of selfing, ID was pronounced, particularly in early-life traits, and the conservatively estimated lifetime fitness of selfed progeny was only 9% that of the outcrossed controls. After a second generation of selfing, ID remained high but was significantly reduced in several traits, which is consistent with the purging of deleterious recessive alleles (the estimated load of lethal equivalents dropped by 48%). Severe ID, even if it can be rapidly purged, likely prevents transitions toward pure selfing in this parasite, although we also cannot exclude the possibility that low-level selfing has undetected benefits.
经典的交配系统进化理论表明，如果同时具有雌雄同体的物种存在高自交衰退（ID），它们应该进行异交；如果 ID 较低，则应该进行自交。然而，在许多物种中，自交和异交同时存在。以前对带绦虫 Schistocephalus solidus 的研究发现，尽管存在 ID，这些蠕虫仍会对部分卵子进行自交受精。自交传播的概率取决于自交者的相对适应性，以及 ID 的遗传基础以及是否可以有效清除。我们通过连续两代的自交繁殖了 S. solidus，并在整个生命周期中记录了几个与适应性相关的特征。经过一轮自交后，ID 非常明显，特别是在早期生命特征中，自交后代的保守估计终生适应性仅为异交对照组的 9%。经过第二代自交后，ID 仍然很高，但在几个特征上显著降低，这与有害隐性等位基因的清除（估计的致死等效物负荷下降了 48%）一致。尽管我们也不能排除低水平自交具有未被发现的益处的可能性，但严重的 ID 即使可以迅速清除，也可能阻止这种寄生虫向纯自交的转变。
