Comparing the hermaphroditic mating system of a parasitic flatworm between populations with an ancestral, 3-host life cycle and a derived, facultative-precocious life cycle.

Evolutionary changes in development and/or host number of parasite life cycles can have subsequent ecological and evolutionary consequences for parasites. One theoretical model based on the mating systems of hermaphroditic parasites assumes a life cycle with fewer hosts will result in more inbreeding, and predicts a truncated life cycle most likely evolves in the absence of inbreeding depression. Many populations of the hermaphroditic trematode Alloglossidium progeneticum maintain an ancestral obligate 3-host life cycle where obligate sexual reproduction occurs among adults in catfish third hosts. However, some populations have evolved a facultative precocious life cycle, where sexual development can occur while encysted within crayfish second hosts, likely leading to high inbreeding as individuals are forced to self-mate while encysted. Whether selfing represents a derived state remains untested. We compared selfing rates of 5 precocious populations to that of 4 populations with an ancestral obligate 3-host life cycle. We also compared demographic estimates to genetic estimates of selfing to test the prediction of no inbreeding depression in precocious populations. Results showed that while the ancestral obligate 3-host life cycle is associated with high outcrossing rates, the facultative precocious populations are highly selfing and show little evidence for inbreeding depression.