Origin and evolution of alternative developmental strategies in amphibioussarcopterygian parasites (Platyhelminthes, Monogenea, Polystomatidae)

摘要

Most integrative studies involving phylogenetic, developmental and .ecological trends showed that the diversityof developmental modifications among the Platyhelminthes was linked to transmission opportunity pressures. Forparasitic flatworms with complex life cycles it was suggested that the evolutionary forces that constrained or enhanceddevelopmental strategies implied heterochronic patterns. Similar patterns were also reported from the Monogenea withdirect life cycles, especi ally for Polystomatidae, which infest amphibious Sarcopterygians. Polystoma, whose membersare recovered almost exclusively from anuran hosts of the Neobatrachia, is capable of following two alternativedevelopmental strategies depending on the physiological stage of its host. Processes by which parasites reach maturityare strikingly different, and lead to discrete adult phenotypes within the same parasite species. In the present study,we investigate the origin and evolution of developmental patterns of polystomatids in a phylogenetic framework, usingan integrative approach of heterochrony and evolutionary ecology. The results suggest that both phenotypes havecoexisted during the early stages of polystome evolution, and that neither of them can be considered as the ancestralone. The two developmental pathways, each associated with one life cycle, may have arisen independently prior topolystome diversification, when strictly aquatic sarcopterygians attempted colonization of temporary freshwaterenvironments. The occurrence of these two patterns within species of the genus Polystoma is suggested to reflect theancestral condition, and to have allowed both developmental strategies to be successful depending on shifts intransmission opportunities. Thus, host evolutionary ecology may be the main factor in shaping developmentalstrategies within polystomatids.