BALAVOINE, G; DE ROSA, R; PRUD’HOMME, B; ADOUTTE , A; CNRS, Gif-sur-Yvette, France; Univ. of Geneva, Switzerland; Univ. of Wisconsin, Madison; CNRS, Gif-sur-Yvette, France: Posterior addition: the annelid way

The question of the origin of metameric segmentation in metazoans has become a heated debate in recent years, with some gene expression data suggesting that it might have been present in the last common ancestor of bilaterians. We have chosen a marine annelid species, Platynereis dumerilii, to study the genes responsible for segment formation in annelids. Platynereis is an advantageous choice for two main reasons: first, as an annelid, it displays a characteristic metameric segmentation of the body that most zoologists have recognized as homologous with the arthropod metamerism and second, it develops in an anamorphic way by sequential posterior addition of new segments during most of the animal’s life span. Posterior addition, either embryonic or post-embryonic, is very common in bilaterians and likely ancestral. In this talk, we will present evidence that the expression patterns of three genes (homologues of engrailed, wingless and hedgehog) during posterior addition support the hypothesis of a segmented bilaterian ancestor and that the patterns of the homologues of caudal, even-skipped and brachyury suggest a striking conservation of the role of these genes in posterior growth and posterior-end patterning accross bilaterians. Additionally, we have tested the idea that posterior growth in Platynereis is based on the activity of teloblasts, i.e. terminal stem cells dividing assymetrically, as it does in most annelid species embryogenesis. Mitotic activity, as recorded with BrdU staining, does suggest a teloblastic-like mechanism but fails so far to specifically identify teloblasts.