Data from: Lifetime inbreeding depression, purging, and mating system evolution in a simultaneous hermaphrodite tapeworm

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 towards pure selfing in this parasite, though we also cannot exclude the possibility that low-level selfing has undetected benefits.

交配系统演化的经典理论指出，同时雌雄同体生物若面临较高近交衰退（inbreeding depression, ID）则应进行异交，若近交衰退程度较低则可进行自体受精。然而，诸多物种中仍同时存在自体受精与异交的混合交配模式。先前以裂头绦虫（Schistocephalus solidus）为对象的研究发现，即便存在近交衰退，该绦虫仍会对部分卵进行自体受精。自体受精得以扩散的概率，既取决于自体受精个体的相对适合度，也与近交衰退的遗传基础及其能否被有效清除密切相关。本研究对裂头绦虫（Schistocephalus solidus）进行了连续两代自体受精繁育，并在其整个生命周期内记录了多项与适合度相关的性状指标。经过一轮自体受精后，近交衰退表现显著，尤其体现在早期生活史性状中；经保守估算，自体受精后代的终生适合度仅为异交对照组的9%。经过第二代自体受精后，近交衰退程度仍处于较高水平，但在多项性状中显著降低，这与有害隐性等位基因的清除效应相符（致死当量负荷的估算值下降了48%）。即便近交衰退可被快速清除，严重的近交衰退仍可能阻碍该寄生虫向专性自体受精演化；不过本研究也无法排除低水平自体受精存在未被发现的适应性优势的可能性。