Phylogeny of some ascaridoid nematodes, inferred from comparison of 18S and 28S rRNA sequences.

Reverse transcription of cellular RNA was used to obtain sequences from regions of 18S and 26/28S ribosomal RNA for eight species of ascaridoid nematodes. Phylogenetic relationships among these species were inferred from the aligned sequences by maximum-parsimony and maximum-likelihood methods. Seventy-nine of the 168 sites that varied were phylogenetically informative in parsimony analysis. Phylogenetic inference based on maximum-likelihood analysis of all sequence sites yielded a tree of topology similar to that of the parsimony result. Monophyletic groups that were strongly supported by bootstrap resampling of these data included species constituting the Ascaridinae, as well as those representing the Toxocarinae. Alternative topologies that included a member of the Ascaridinae with the Toxocarinae were rejected statistically on the basis of analysis of the mean and variance of parsimony step differences between trees. The conformance of these sequence data to a molecular-clock model of evolution was evaluated statistically by a maximum-likelihood approach. The inferred rate of rRNA sequence change along the branch leading to Parascaris equorum is not consistent with a clocklike model of evolution.