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animal model; Cestoda; experimental evolution; mixed-mating; partial dominance; quantitative genetics
Abstract:
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.
