ONTOGENETIC SEQUENCES AND THE PHYLOGENETICS OF PARASITIC FLATWORM LIFE CYCLES

Abstract— Ontogenetic data can be used in phylogenetic analyses (1) as tests of character homology, (2) for polarizing transformation series, and (3) as characters themselves. The ontogenetic sequence of semaphoronts can change by addition, substitution, or deletion of stages which are either terminal or nonterminal. Outgroup comparisons can use life cycle data as well as morphological data to establish a general reference system of inferred genealogy, which can then be used to distinguish which of the six ontogenetic sequence changes has occurred in a particular phylogeny. A cladistic study of the parasitic platyhelminths suggests that the evolution of the Digenea involved a nonterminal addition of ontogenetic stages and a terminal addition of a second host to the ancestral life cycle. The evolution of the Cestoidea appears to have involved a terminal addition of ontogenetic stages and a nonterminal addition of a second host. The "complex life cycles" of the two groups are thus different in nature and etiology. Further cladistic analysis, within the Digenea, suggests that the plesiomorphic state of the intercalated ontogenetic stages was a miracidium-sporocyst-cercariae sequence. Rediae are concluded to have been intercalated between sporocysts and cercariae at a later point. The mutual exclusivity of rediae and daughter sporocysts in a life cycle is noted.