| A common set of developmental regulators control the development of most metazoan species. Thus, alterations in the deployment of these genes during development are likely key factors that promote morphological diversity in the animal kingdom. The current paradigm for how modifications to developmental genes control morphological diversity is based largely on the investigation of the homeotic genes and implicates cis-regulatory changes as the primary factor in the evolution of morphological diversity. However, evolutionary modifications to homeotic genes may be constrained by their central role in specifying the basic body plan of most metazoans. To obtain a broader perspective on the evolutionary plasticity of key development regulators I have undertaken a comparative analysis of the ac/sc genes, a family of transcription factors that promote the initial steps of nervous system development in arthropods. In this thesis, I describe the identification and characterization of ac/sc homologs in three divergent arthropod species, propose a model for ac/sc evolution incorporating data from all four major arthropod groups, and discuss in detail the role of ac/sc genes and their developmental regulators during neurogenesis of the insect Tribolium castaneum.; These comparative studies of the ac/sc genes in Tribolium castaneum, Thermobia domestica, and Triops longicaudatis demonstrate evolutionary plasticity in ac/sc gene number, expression, and function during the evolution of the arthropods. Furthermore, these studies have revealed both conservation and variation in the development of the arthropod nervous system and implicate ac/sc gene plasticity as a key factor in their evolution. The data in this thesis show the importance of investigating evolutionarily conserved gene networks in dissecting the molecular and genetic basis of morphological diversity. |
