Ascent of the Ascidians

Star Ascidian, Botryllus schlosseri

Phylogenetics is slowly rewriting our ideas of relationships between many organisms. Take for example the Ascidians. These tunicates (urochordates) were beliveved to once be a sister group of the vertebrates, due in part to the presence of characteristics such as segmented muscle (Lemaire et al., 2008 Current Biology 18 R620–R631). The lancelets (cephalochordates) were assumed to be the most closely related to the vertebrates as there are overall morphological similarlties and an increased complexity between these two groups when compared to the seemingly simpler tunicates. However in 2006, Delusc et al. (Nature, 23 pp. 965-968) analysed data from the sequencing of the tunicate Oikipleura dioica genome and found that tunicates represent the closest living relatives of vertebrates. Indeed, Delusc suggested that the lancelets be grouped with the echinoderms rather than than the tunicates or vertebrates.

Star Ascidian, Botryllus schlosseri

Botryllus schlosseri (Star Ascidian) is a cosmopolitan, colonial ascidian that is found encrusting on rocks, shells and large brown seaweeds (Challinor et al., 2003 A Beginner's Guide to Ireland's Seashore p. 74). It has been used for over 50 years in the laboratory as a model system for the study of blastogenesis, colony fusion and regeneration (Manni et al., 2007 Developmental Dynamics 236 pp. 335-52). It forms free living, pelagic larvae by sexual reproduction that attach to a substrate and in turn reproduce asexually to form colonies of numerous genetically identical zooids. Individuals are arranged around a common cloak siphon, thus forming the “star” of the common name.