P3.73 Friday, Jan. 6 Comparing axial patterning in nemertean larvae - insights into the evolution of a novel larval body plan HIEBERT, L.S.*; MASLAKOVA, S.; University of Oregon; University of Oregon email@example.com
Within the nemerteans, a phylum of lophotrochozoan worms, indirect development appears to be derived. This provides a unique opportunity to explore the evolution of a novel body plan: the pilidium larva. The pilidium is a hat-shaped planktotrophic larva equipped with an apical tuft and an elaborate ciliated band. Inside the pilidial body, a juvenile worm emerges over a number of weeks from isolated rudiments that fuse around the larval gut. The juvenile body is seemingly disjointed from the larval body and the transition from larval to juvenile habitat requires a drastic metamorphosis, in which the juvenile breaks free of the larva and consumes the larval tissues. While the pilidial mode of development is derived within a single clade of nemerteans, basal nemerteans develop directly into juvenile worms, without major body plan modifications. Thus, the pilidium can be considered a novel body plan inserted into this direct-developmental pathway. During the evolution of this new body plan, morphogenesis of the larval and juvenile bodies became decoupled both spatially and temporally. It is likely that patterning mechanisms used to set up these two body plans also became developmentally decoupled. Decoupling may exist at the level of axis specification since different pilidium-bearing species harbor developing juveniles with head-tail axes positioned at wide range of angles with respect to the major axis of the pilidium. In order to understand the developmental mechanisms and evolution of the pilidial body plan, we compare body axis patterning in a species with pilidial development (Micrura alaskensis) with another possessing direct-development (Pantinonemertes californiensis). We present data on expression of axial patterning genes in both species and comparative data on axial perturbations using inhibitors of conserved developmental pathways.