Sipuncula in Evolutionary Developmental Biology

Abstract

Adult sipunculans are currently placed within Annelida, mainly on the basis of molecular phylogenetic analyses. Here, we review recent advances in morphogenetic studies that have revealed numerous shared features between sipunculans and other annelids, including a metamerically formed nervous system, supporting the notion of a sipunculan/annelid clade. Similar to annelids, sipunculan myogenesis starts with the formation of four separate longitudinal muscle strands that develop from anterior to posterior, suggesting that this mechanism of myogenesis was present in the last common ancestor of both taxa. A dense arrangement of longitudinal body wall muscles in the vicinity of the retractor muscles suggests that the latter evolved from fused longitudinal body wall muscles. Although circular body wall muscles do not develop in a segmental manner during sipunculan ontogeny, traits of segmentation during neurogenesis strongly support recent molecular analyses and argue for a segmented last common ancestor of sipunculans and annelids. The establishment of a detailed morphogenetic sipunculan framework enables a careful interpretation of gene expression patterns that might shed further light on the evolution and partial loss of segmentation in Sipuncula and Annelida.