Meeting Abstract

10.5  Jan. 4  The evolution of Bilateria � insight from an acoel flatworm HEJNOL, A.*; MARTINDALE, M.Q.; Univ. of Hawaii, Honolulu; Univ. of Hawaii, Honolulu hejnol@hawaii.edu

Recent molecular phylogenies place the Acoelomorpha (Acoela + Nemertodermatida) as the sister to all remaining Bilateria before their split into Proto- and Deuterostomes. This phylogenetic position makes the acoel flatworms a key group to investigate for our understanding of the evolution of important features of the Bilateria, such as the mesodermal layer, an orthogonal nervous system with a brain and the position of the mouth and anus in relation to the body axis. Scenarios of the evolution of the Bilateria from over a century ago, as the Acoeloid-Planuloid-Hypothesis suggested already a common ancestor which is similar to the acoel flatworms we find today. To get more insights about the evolution of the bilaterian key features we investigated the expression patterns of genes used in proto- and deuterostomes to specifiy the axial patterning, mesoderm and orthogonal nervous system in the acoel Convolutriloba longifissura. Here we concentrate on the axial relationships and the origin of the mouth of the Acoela using the expression of anterior-posterior (HOX and paraHOX) and dorso-ventral patterning (e.g. NK2.1 and vax) genes as well as the genes goosecoid and brachyury, which play a role in the determination of the mouth and anus. The genes specifying the dorsal-ventral axis show a similar pattern as it is found in protostomes and hemichordates, and the expression of brachyury and goosecoid mark the regions which take part in fore- and hindgut formation in the remaining Bilateria. Our results and the given position of the Acoelomorpha as the sister group to the remaining Bilateria strongly supports the Acoeloid-Planuloid-scenario of bilaterian evolution.