Meeting Abstract

60.6  Sunday, Jan. 5 14:45  Annelid Segmentation Genes And The Origin Of Arthropod Parasegments BALAVOINE, G; Institut Jacques Monod, CNRS / Univ. Paris Diderot, Paris balavoine.guillaume@ijm.univ-paris-diderot.fr

Segmentation is one of the most important characteristics of body plan organization. Broadly defined, it is present in most major animal groups. There is a vigorous debate about its evolutionary origin connected to the crucial question of the origin of complexity in animal evolution. We are studying the genetic networks responsible for the formation of segments in the marine annelid Platynereis dumerilii as a representative of the most extensively metamerized animal phylum and a member of the third great branch of bilaterians, Spiralia. We have identified a number of homeobox genes that displays segment polarity-like expression patterns, including engrailed. Likewise, components of the hedgehog and Wnt/β -catenin signaling pathways are expressed in striped patterns prior to morphological segmentation. By using small molecule inhibitors, we have shown that both hedgehog and β -catenin pathways are required for establishing the axial properties of segments, similar to arthropods. The alignment of the expression patterns of segment polarity genes suggests a strong correspondence between the segmental boundary of Platynereis and the parasegmental boundary of arthropods. We interpret these data as showing that: (1) the last common ancestor of protostomes was a segmented annelid-like animal; (2) the parasegmental patterning of arthropods recapitulates the ancestral protostome metamerism. The cuticular segmentation of arthropods has evolved later on, both in evolution and embryogenesis and therefore develops through a process of resegmentation.