We examined phylogenetic relationships among halobatine water striders (Hemiptera, Gerridae) using molecular and morphological data. The molecular data set was 780 bp DNA sequence data from the 3' half of the mitochondrial gene encoding cytochrome oxidase subunit I from 19 species of sea skaters, Halobates, and one species from each of three related genera, Asclepios annandalei, Austrobates rivularis, and Eurymetra natalensis. The morphological data set was a slightly modified version of a previously published data set. Unweighted parsimony analyses of the molecular data set gave one tree with weak support for most branches. Maximum likelihood analysis of the same data set gave a tree with slightly different topology, but reveiled many of the clades found in parsimony analyses of the morphological data set. Parsimony analyses of the combined molecular + morphology data sets gave a better resolved and better supported tree than did analyses of any single data set. The phylogeny of Halobates presented here allows a more rigorous evaluation of several prior hypothese about evolutionary processes in marine water striders. In particular, it supports the hypothesis of at least two separate transitions from coastal to oceanic environments. (C) 2000 The Linnean Society of London.
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