Resolving evolutionary changes in parasite life cycle complexity: Molecular phylogeny of the trematode genus Alloglossidium indicates more than one origin of precociousness.

The evolutionary causes and consequences of changes in complex life cycles are of central importance in parasitology. However, data remain limited because in part, knowledge on phylogenetic relationships among species that differ in life cycle patterns remains scarce. We present a molecular phylogeny of the trematode genus Alloglossidium, which contains several species that display precocious (a.k.a., progenetic) life cycles (i.e., maturation in what is typically regarded as an intermediate host). The molecular phylogeny contrasts with previous morphological and life-history based phylogenetic hypotheses. In particular, a precocious life cycle wherein leeches are used as final hosts evolved early in the history of the genus. Among the remaining species, which are a separate clade, a three-host life cycle using ictalurid catfishes is ancestral. Furthermore, there are at least two additional independent evolutionary events that lead to a precocious life cycle where a catfish host is lost and a crustacean is used as a final host. We conclude with a discussion on how existing hypotheses on the evolution of precociousness, and parasite life cycle complexity in general, may or may not relate to the patterns observed in genus Alloglossidium.