| The earth is populated by a large diversity of animal species with many different forms. How has this diversity evolved? The integration of the three previously disparate fields of genetics, embryology and evolution was spurred by the discovery that genes critically involved in fruit fly development, Hox genes, are present in the genomes of widely divergent animal species. Comparative studies of the genetics of animal development have demonstrated that all animals have many developmental genes in common—a genetic “toolkit” of development. The diversificaton of animal body plans has therefore not largely involved the evolution of new genes, but rather changes in the number and biochemical function of toolkit genes and the evolution of new contexts of expression patterns and interactions among genes in developmental regulatory networks.; My thesis focuses on identifying and understanding the genes and the changes within them that underlie the diversity of animal morphology. Here, I present evidence that strongly suggests that both changes in cis -regulatory DNA and the coding regions of regulatory genes have played pivotal roles in the evolution of animal morphology. I have shown that changes in the expression pattern of a major developmental regulatory gene are correlated with the evolution of a novel morphological trait in butterflies. The evolution of this trait likely involved both the evolution of a new context of expression for a transcription factor and changes in its downstream targets. My work also shows that the evolution of the Dipteran hindwing, or haltere, may have evolved through the gradual accumulation of changes in the cis-regulatory DNA of a regulatory gene target that led to a qualitative change in its expression and contributed to a difference between the morphology of the haltere and that of its serial homolog, the forewing. Finally, my work provides the first example of the functional evolution of a Hox transcription factor and demonstrates that important morphological transitions can also involve changes in regulatory gene functions and are not limited to changes in the target genes of transcription factors. Thus, the underpinnings of morphological evolution comprise a diverse set of genetic changes. |
