The Formation and Evolution of the Metameric Body Plan in Basal Annelids

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0094925MartindaleThere is considerable debate about the origin of segmental body plans in metazoan evolution. Arguments have been made that the protostome-deuterostome ancestor was already segmented, while others argue that segmentation evolved independantly in the three major clades of segmented organisms: the annelids, arthropods, and chordates. While there is alot known about the development of some arthropods and chordates, virtually nothing is known about the development of annelids. The little cellular and molecular information available comes almost exclusively from a few species of the highly derived group of freshwater clitellate annelids, the leeches. The marine polychaetes are the most diverse and speciose group of basal annelids and are clearly a critical group to study when considering the evolution of segmental development in the annelids, yet not a single intracellular fate map has been published on any polychaete embryo. This proposal describes a comprehensive and integrative cellular and molecular dissection of the origins and development of the segmental body plan in polychaetes. Rather than focusing on a single polychaete species, the investigators will examine three species of polychaetes with different life history characteristics to gain an appreciation for the diversity of mechanisms within this group of animals. Modern cell lineage analysis will be utilized in conjunction with molecular studies of two distinct patterning pathways, the formation of a segmental body plan, and the diversification of that body plan into distinct regions. Specific Aim #1: Utilize cell lineage techniques to follow the embryonic and larval origins of segmental tissues in three species of polychaetes with distinct life histories. Specific Aim #2: Examine the spatio-temporal deployment of the segmentation genes engrailed, armadillo, even-skipped, runt, and hairy in three species of polychaete embryos and larvae. Specific Aim #3: Complete the expression studies with our previously isolated Hox genes in the heteronomous species Chaetopterus and contrast the deployment of Hox genes with two species of homonomous polychaetes (Capitella and Hydroides). The proposed work will provide critical information about how the segmented body plan is generated in a basal group of annelids. This information is crucial before any kind of comparison about the homology of segmentation can be made with other metazoan phyla.