Ort: Hörsaal, Museum Koenig
Zeit: 03.11.2011, 17:15h
Zeit: 03.11.2011, 17:15h
The double life of annelid worms - Phylogenomic studies unravel annelid evolution
The direction of evolution within Annelida, one of three segmented metazoan taxa, has been a long and contentious debate since the middle of the last century. Based on functional morphology two hypotheses were predominantly discussed. The first hypothesis proposed an earthworm-like ancestor and the evolution of a segmented coelom as an aid for peristaltic burrowing, whereas the second one postulated that the ancestral annelid resembled more an errant polychaete and the segmented coelom evolved as a requirement for blood transportation to the parapodia. However, approaches to assess the relationships of annelid taxa over past decades using both morphological and molecular data failed to provide a robust phylogeny of Annelida. Moreover, the traditional annelid composition comprising only Clitellata and "Polychaeta" has been changed including former phyla such as Sipuncula, Echiura and Pogonophora. Using a phylogenomic approach based on EST data I will show that a well-supported phylogeny with strong support for major splits can be recovered. Chaetopterids, myzostomids and sipunculids are placed in the basal part of the tree, although the position of Myzostomida remains uncertain owing to its long branch. The remaining taxa are in a clade named Pleistoannelida and splitted into two clades: Errantia (which includes the model annelid Platynereis), and Sedentaria (which includes Clitellata). This also reveals insights into the evolution of several morphological characters. Based on these phylogeny and the sparse fossil record the ancestral annelid is most likely a mixture of sedentary and errant annelid features indicating an evolutionary origin of Annelida as a microphagous surface deposit-feeder crawling upon and through soft-bottom habitats. Hence, the evolution of iterative parapodia as locomotive devices most likely drove the evolution of segmentation in Annelida.
