The development and evolution of arthropod appendages: Modulations of a limb patterning gene network

Abstract

A quick visual survey of the animal world around us reveals a wide array of morphological diversity, yet it is currently not known how such great diversity may have arisen in evolution. The recent emergence of molecular and developmental data from the field of developmental genetics, including genes, gene expression patterns, and most recently entire genomes, have provided a new set of characters to add in the analysis of animal evolution. One morphological character that displays a wide range of diversity is the appendage. Within the arthropods in particular, limbs vary in their size, shape and number along the body axis, providing an excellent experimental model to study how these changes have come about in evolution. The work in this dissertation uses comparative gene expression patterns to address how an arthropod limb patterning network has been modified to produce different limb morphologies. Over the past several decades, data from the field of Drosophila developmental genetics have aided our understanding of how the fly limb is patterned. Yet, Drosophila is a highly derived insect with an unbranched limb that undergoes a complete metamorphosis. I was interested in learning whether other arthropods displaying different modes of development as well as branched limbs use a similar molecular mechanism to Drosophila in the development and patterning of their limbs. The first arthropod examined was Triops, a branchiopod crustacean with a highly branched and fused limb type. Although some of the genes in the Drosophila limb patterning network appear to be conserved in this animal, the space and time in which they are expressed are different from what we observe in Drosophila. In the second species examined, the grasshopper, Schistocerca americana, again the components of the limb patterning gene network are conserved but the expression patterns are different. It appears that in the grasshopper, downstream genes involved in patterning the P/D axis are conserved in expression and perhaps function, but early in development, there is a dramatic change in the expression of one of the more upstream, dorsal components, a gene called decapentaplegic ( dpp), and it is correlated with the absence of imaginal discs. (Abstract shortened by UMI.)