Bodyplan diversification in crinoid-associated myzostomes (Myzostomida, Protostomia)

摘要

When free-living organisms evolve into symbiotic organisms (parasites, commen-sals, or mutualists), their bodyplan is often dramatically modified as a consequence. Thepresent work pertains to the study of this process in a group of marine obligate symbioticworms, the Myzostomida. These are mainly ectocommensals and are only associated withechinoderms, mostly crinoids. Their usual textbook status as a class of the Annelida is gen-erally accepted, although recent molecular phylogenetic studies have raised doubts on theirrelationships with other metazoans, and the question of their status remains open. Here, wereconstruct the evolution of their bodyplans by mapping 14 external morphological charac-ters (analyzed using scanning electron microscopy) onto molecular phylogenies using max-imum parsimony (MP) and maximum likelihood (ML) optimality criteria. Rooted MP, ML,and Bayesian phylogenetic trees were obtained by analyzing the nucleotide sequences ofcytochrome oxidase subunit I, 18S rDNA, and 16S rDNA genes, separately and in combi-nation. Representatives of 34 species distributed among seven extant genera were investi-gated. Our character evolution analyses, combined with recent ontogenetic andultrastructural evidence, indicate that the organism at the base of the myzostome tree wouldhave had six body segments and five pairs of polychaete-type parapodia, and that two lin-eages emerged from it: one comprising parasites, with large females and dwarf males, whichgave rise to the extant Pulvinomyzostomum and Endomyzostoma species, and a second lineagecomprising simultaneously hermaphroditic ectocommensals, from which all other extant my-zostome taxa probably evolved.