Phylogeny of Steinernema Travassos, 1927 (Cephalobina : Steinernematidae) inferred from ribosomal DNA sequences and morphological characters

Entomopathogenic nematodes in Steinernema, together with their symbiont bac teria Xenorhabdus, are obligate and lethal parasites of insects that can pr ovide effective biological control of some important lepidopteran, dipteran , and coleopteran pests of commercial crops. Phylogenetic relationships amo ng 21 Steinernema species were estimated using 28S ribosomal DNA (rDNA) seq uences and morphological characters. Sequences of the rDNA internal transcr ibed spacers were obtained to provide additional molecular characters to re solve relationships among Steinernema carpocapsae, Steinernema scapterisci, Steinernema siamkayai, and Steinernema monticolum. Four equally parsimonio us trees resulted from combined analysis of 28S sequences and 22 morphologi cal characters. Clades inferred from analyses of molecular sequences and co mbined datasets were primarily reliably supported as assessed by bootstrap resampling, whereas those inferred from morphological data alone were not. Although partially consistent with some traditional expectations and previo us phylogenetic studies, the hypotheses inferred from molecular evidence, a nd those from combined analysis of morphological and molecular data, provid e a new and comprehensive framework for evaluating character evolution of s teinernematids. Interpretation of morphological character evolution on 6 tr ees inferred from sequence data and combined evidence suggests that many st ructural features of these nematodes are highly homoplastic, and that some structures previously used to hypothesize relationships represent ancestral character states.