Evolutionary History and host Ecology Determine Acanthocephalan Egg Shape

Egg morphology varies among and within groups of parasites. Morphological variation is tightly coupled with transmission, and prior work suggests this variation is associated with host type and host habitat in some cases. Here, we used phylogenetic comparative methods combined with evolutionary model fitting to determine whether egg shape in acanthocephalans is constrained by evolutionary history and if this variation is a response to differences in habitat use among the intermediate host, definitive host, or a combination of the two. A model combining a phylogenetic reconstruction of 59 acanthocephalan species and life cycle data shows that egg shape is both constrained by evolutionary history and has evolved in response to the habitat (microhabitat) of the intermediate host. These results support the hypothesis that parasite egg morphology has adapted in response to host habitat and facilitates transmission.