Meeting Abstract

128.1  Tuesday, Jan. 7 13:30  How the pilidium larva uses the Hox genes HIEBERT, LS*; MASLAKOVA, SA; Oregon Institute of Marine Biology, Univ. of Oregon; Oregon Institute of Marine Biology, Univ. of Oregon lhiebert@uoregon.edu

The pilidium larva is a novelty of a single group of nemertean worms called the Pilidiophora. Uniquely for this group the juvenile body develops from a series of isolated rudiments called imaginal discs. The larval and juvenile bodies have entirely different plans, though they share the same stomach. In order to understand the origins of this novel larval body plan and developmental mode, we examined patterns of expression of Hox genes in pilidial development. These conserved genes pattern major body regions and also certain evolutionarily novel structures in animals. Several nemertean Hox genes have been identified but their developmental expression has not been studied. Here we report for the first time the expression patterns of nine Hox genes during development of the pilidiophoran Micrura alaskensis, including two anterior, a PG3 gene, five central, and one posterior gene. Surprisingly, there is no Hox gene expression at any time anywhere in the pilidial larval body. And even more surprisingly, all nine are expressed in localized partially overlapping domains in the trunk imaginal disks – a pair of rudiments that gives rise to the juvenile trunk, and none in the cephalic or the cerebral organ discs, which form the juvenile head. Our findings suggest that Hox genes pattern the antero-posterior axis of the juvenile, but not the larva. Thus, a novel, apparently Hox-free mechanism must account for the patterning of the pilidium. Furthermore M. alaskensis orthologues of the more anterior genes have a broader domain of expression compared to the more posterior genes. Also, the more anterior genes begin to be expressed earlier in development than the more posterior genes, hinting at temporal and spatial colinearity, a hallmark of Hox expression in other animals.