RESUMO
We have investigated the molecular evolution of the nucleotide sequences of 18S ribosomal RNA genes (18S rDNA) from a set of nematodes in the family Rhabditidae (Nematoda: Secernentea). Our aim was to evaluate the usefulness of this gene for molecular systematics of this family, as well as to establish phylogenetic relationships within a group that has potential for comparative studies of the relationship between development and evolution. We determined the 18S rDNA sequences of nine species of nematodes representing six genera within this family (Caenorhabditis briggsae, C. vulgaris, C. remanei, Rhabditis blumi, Rhabditis sp. br, Rhabditella axei, Pellioditis typica. Teratorhabditis palmarum, and Pelodera strongyloides dermatitica). Using hypothetical models for secondary structure as well as nucleotide similarity, these sequences were aligned with the 18S rDNA sequence published by Ellis et al. for C. elegans and with the partial sequences published by Nadler for eight ascaridoid species. We find that 18S rDNA is likely to be a useful tool to resolve relationships at the intrafamilial level. However, 18S rDNA sequences cannot be used to resolve relationships between taxa as closely related as the Caenorhabditis species. Parsimony, minimum-evolution, and maximum-likelihood methods strongly reject Andrássy's proposed phylogenetic classification based on adult morphological characters but support that of Sudhaus as one alternative of a few possible phylogenies. Distances between genera in this family are about eight times as great as distances between tetrapod classes, suggesting rapid rates of substitution, ancient divergence, or both.