Meeting Abstract

87.2  Monday, Jan. 6 10:30  A convergent dorsal nervous system condensation in the Acoelomorpha driven by the BMP-synexpression group BøRVE, A.; RYAN, J.F.; HEJNOL, A.*; Sars International Centre for Marine Molecular Biology, Univ. of Bergen andreas.hejnol@sars.uib.no

Nervous system condensations such as the ventral or dorsal nerve cord or anterior condensations such as the brain are an important character of animals. How often such condensations have been evolved is still a hotly debated topic in zoology. Careful investigations of the developmental processes underlying such condensations can contribute to the understanding of the evolutionary origins of such condensation-events. We have focused our investigations of the group Acoelomorpha that comprises Acoela, Nemertodermatida and Xenoturbellida. All species share a basiepidermal nerve-net-like nervous system but Acoela can have additional 3-4 pairs of nerve cords along the body and an anterior brain. Nemertodermatids in contrast only possess two cords composed out of axon-tracts on the dorsal side. We investigated the molecular patterning of the nervous system in two species the nemertodermatid Meara stichopi and the acoel Isodiametra pulchra. Our results show that the anterior-posterior and dorso-ventral patterning systems in nemertodermatids are acting similar to what is found in other bilaterians. Surprisingly, the two thickenings in the dorsal side however, are located next to the expression of the BMP-synexpression group. In addition, the nerve cords of acoels show an co-opotion of neural genes that are expressed in the region where the cords are located, e.g. NK2.1 in the ventral cord. Our results indicate an independent condensation of nerve cords in the Acoela for which epidermal patterning genes have been co-opted. Our results illustrate that homologizations of nerve cords that are based on similar arrangement of gene expression have to be performed on the background of a phylogenetic framework.