MASLAKOVA, S. A.*; MATZ, M. V.; University of Washington, Friday Harbor Labs; University of Florida, Whitney Laboratory: Evolution of Larval Development in Nemerteans (Phylum Nemertea)

The origin of morphological diversity is the central question of evolutionary developmental biology. Nemerteans, a poorly understood lophotrochozoan phylum, present a particularly interesting study group in this respect, because they display two drastically different developmental modes: direct, with a juvenile or a juvenile-like planktonic larva hatching out of the egg envelopes and indirect, via a unique larva, the pilidium, in which the juvenile develops via a series of ectodermal invaginations of the larval body (imaginal disks). These disks merge around the larval gut during planktonic development and a fully formed juvenile abandons larval body in a rapid metamorphosis, analogous to that of the pluteus larva of sea urchins and sand dollars. Recent data on nemertean development and phylogeny suggest that the pilidial larval body plan and drastic metamorphosis evolved de novo within one clade of nemerteans, the Pilidiophora, while direct development represents the ancestral type for the phylum. However, nothing is known about underlying molecular mechanisms involved in evolution of this unique developmental mode. Morphology suggests that juvenile body plan of indirect developing nemerteans is comparable to the larval body plan of direct developers, while the pilidial larval body has no homology in direct development. Suppressive subtractive hybridization between the cDNA libraries derived from early and late stages of pilidial development may offer insights into which genes are involved in patterning the pilidial body plan, while further comparisons with direct developing species may help to understand how a novel larval body plan evolved in this group of protostome animals. Here we present first data on development to metamorphosis of a pilidiophoran Micrura alaskensis and identify stages appropriate for subtractive hybridization experiments.