PAGE, L.R.; University of Victoria, B.C., Canada: Comparative studies of gastropod development: insights into the evolution of a novel body plan

A fundamentally important problem in Biology is to understand how new body plans originate during evolution. Insights into this central issue of evolutionary biology will come from studies on the organization of development. We study comparative patterns of gastropod development with the goal of reconstructing evolutionary events that generated the enigmatic gastropod body plan, which has been described as one of the most remarkable innovations in the history of metazoans. A longstanding proposal, popularized by Garstang as a pioneering example of evolutionary change through developmental modification, suggests that gastropods emerged as the result of a developmental macromutation. This mutation produced a 180 degree twist between the two body regions of the larva, the cephalopodium (future head and foot) and visceropallium (future viscera, mantle, and mantle cavity). However, theory on this subject is largely based on a few early studies that preceeded the advent of high resolution techniques for morphological analysis. I will present observations on developing embryos and larvae from representatives of several major gastropod clades, obtained using scanning electron microscopy, histological sections, and confocal scanning laser microscopy, that more precisely define the nature of the morphogenetic movements exhibited by these gastropod embryos. A particular focus of this work is the initial development and subsequent distortion of the visceropallial nerve connectives, which extend from the cephalopodium into the visceropallium. The trajectory of this connective, even in very early stages of neurogenesis, can be precisely characterized using fluorphore-tagged antibodies to neural markers such as serotonin and various neuropeptides. These data will be used to propose an alternative hypothesis for the evolutionary origin of the gastropod body plan.