Ribosomal DNA and phylogeny of the Ascaridoidea (Nemata : Secernentea): Implications for morphological evolution and classification

Nematodes of the superfamily Ascaridoidea are parasites of the alimentary t ract of vertebrates and include species that are of medical and economic im portance. Existing evolutionary hypotheses for these organisms have frequen tly been based on interpretation of one or few "key" structural or life his tory features. We used nuclear-encoded small (1764 characters) and large su bunit (757 characters) ribosomal DNA sequences to estimate the phylogeny of representative taxa from this superfamily. Trees inferred by maximum parsi mony and maximum likelihood methods strongly support clades that are primar ily consistent with one recent classification of the group. In contrast, mo st previously proposed phylogenetic hypotheses were significantly worse whe n compared to the maximum likelihood tree by a statistical method. Hypothes es for the evolution of morphological and life history characters were expl ored by parsimony mapping these features on several tree topologies, includ ing optimal molecular trees and alternative topologies reflecting tradition al expectations deemed not worse in statistical tests. The results identify some consistent putative shared-derived morphological features, but also s trongly suggest that some key features emphasized by previous workers repre sent ancestral states or highly homoplastic characters. (C) 1998 Academic P ress.