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认为，同时雌雄同体生物若具有较高的近交衰退（inbreeding depression, ID），则应采取异交策略；若近交衰退程度较低，则应选择自交。然而，诸多物种中仍存在自交与异交并存的混合繁殖模式。过往针对绦虫裂头绦虫（Schistocephalus solidus）的研究发现，即便存在近交衰退，该物种仍会对部分卵子进行自交。自交种群能否扩散，取决于自交个体的相对适合度、近交衰退的遗传基础，以及有害等位基因能否被有效清除。我们通过连续两代自交培育裂头绦虫，并在其完整生命周期内记录了多项与适合度相关的性状。经一轮自交后，近交衰退现象显著，尤其体现在早期生活史性状中；经保守估算，自交后代的终生适合度仅为异交对照组的9%。至第二代自交时，近交衰退程度仍较高，但在多个性状中出现了显著降低，这与有害隐性等位基因的清除现象相符（致死当量负载量下降了48%）。即便近交衰退可被快速清除，其严重程度仍会阻碍该寄生虫向纯粹自交策略的演化转变，不过我们亦无法排除低水平自交存在未被发现的益处这一可能性。