A comparative study of lower jaw form, function, ecology and development in the damselfishes (Family pomacentridae)

Rachel Marie Wirgau

We examined the developmental underpinnings of the damselfish adaptive radiation. Because the embryos of damselfish species with diverse skull shapes and feeding ecologies are easily obtained, the damselfishes present a unique opportunity to understand the developmentalgenetic underpinnings of a successful adaptive radiation. We have several hundred specimens that represent complete developmental series (fertilization-hatchling) of 12 species of damselfishes that occupy a range of adult feeding niches. Similar work with embryos of Darwin's Finches has examined no more than 6 species. Our objective was to understand how early developmental patterning determines larval and adult jaw shape, and to determine how larval jaw shape relates to adult jaw shape. We used "whole-mount in situ hybridization" (WISH) labeling of the gene dlx2a, which is expressed in the cranial neural crest cells that form the pharyngeal arches, to visualize the formation of the first pharyngeal arch (i.e., visualize initial jaw patterning). We combined this developmental work with a comparative analysis of larval jaw shape and biomechanics using hatchlings of the vi same species. It is at the hatchling stage that first feeding (first jaw use) occurs. We used a similar approach to examine skull form and function in adults. Finally, we used phylogenetic comparative methods to test hypotheses of damselfish skull evolution using our phenotypic data from adults, larvae and embryos. The research examined the developmental factors that shape adaptive radiation using a larger number of species than has been previously examined in this context. There is no connection between early patterning of the jaws and adult lower jaw shape, and no correspondence between larval jaw shape and adult jaw shape. Our data indicate that the evolutionary of damselfish jaw morphology has involved important changes to jaw formation during late development. We examined the role of fgf20a in directing jaw morphogenesis in late development. We used geometric morphometrics, expression labeling of fgf20a and assays for osteoblast and osteoclast activity to describe the anatomical effects of an fgf20a null mutation on skull morphogenesis. The results indicate that silencing of fgf20a signalling disrupts bone deposition and bone resorption in the mid-facial region in late development.

A comparative study of lower jaw form, function, ecology and development in the damselfishes (Family pomacentridae)

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