Molecular Phylogeny of the Model Annelid Ophryotrocha

Annelids of the genus Ophryotrocha are small opportunistic worms commonly found in polluted and nutrient-rich habitats such as harbors. Within this small group of about 40 described taxa a large variety of reproductive strategies are found, ranging from gonochoristic broadcast spawners to sequential hermaphroditic brooders. Many of the species have a short generation time and are easily maintained as laboratory cultures. Thus they have become a popular system for exploring a variety of biological questions including developmental genetics, ethology, and sexual selection. Despite considerable behavioral, reproductive, and karyological studies, a phylogenetic framework is lacking because most taxa are morphologically similar. In this study we use 16S mitochondrial gene sequence data to infer the phylogeny of Ophryotrocha strains commonly used in the laboratory. The resulting mtDNA topologies are generally well resolved and support a genetic split between hermaphroditic and gonochoristic species. Although the ancestral state could not be unambiguously identified, a change in reproductive strategy (i.e., hermaphroditism and gonochorism) occurred once within Ophryotrocha. Additionally, we show that sequential hermaphroditism evolved from a simultaneous hermaphroditic ancestor, and that characters previously used in phylogenetic reconstruction (i.e., jaw morphology and shape of egg mass) are homoplasic within the group.