Evolution Of Arthropod Morphological Diversity

Abstract

A fundamental problem in developmental and evolutionary biology is understanding the developmental genetic basis of morphological diversity. The current paradigm holds that a genetic and developmental program, or developmental genetic "toolkit", conserved across hundreds of millions of years patterns development in all metazoans. However, outside of a few well-characterized signal transduction pathways and developmental processes, overly broad strokes have been used to paint this "toolkit" metaphor as a hypothesis. Arthropoda, one of the largest groups of metazoans, represent the most morphologically diverse groups of metazoans, making them of particular interest for studies of morphological diversity and its evolution. Arthropoda is also home to one of the most well-understood model systems for developmental and genetic studies, the fruit fly Drosophila melanogaster. However, Drosophila is highly derived among arthropods with respect to the molecular genetic mechanisms that function during its development. As it is expected that all arthropods have access to the same development "toolkit", some changes are expected based on the observable differences in morphology, making arthropods extremely powerful tools for comparative genomic and molecular genetic studies. In this dissertation I characterize how modifications to the developmental "toolkit" contribute to the evolution of morphological diversity using emerging model arthropod systems. First, as part of a collaboration, I show that several genes expected to be conserved in all arthropods, belonging to the Hox family of transcription factors, have been lost from the genome of a phylogenetically basal arthropod, the two-spotted spider mite Tetranychus urticae. Second, I perform a genomic survey and find an overall reduction in the conservation of Drosophila orthologs from several major signal transduction pathways in the Tetranychus genome in comparison with findings from previous insect surveys. Third, I show that arthropod Hox genes, expected to be found in a tightly linked genomic cluster in most arthropod genomes, are not as tightly clustered as previously thought. Fourth, I show that changes in the genomic arrangement of Tetranychus Hox genes correspond with shifts in their expression and morphological change. Finally, I show the terminal Hox gene Abdominal-B is required for proper axial elongation and segment formation (both segment identity and number) during embryogenesis and metamorphosis in the red-flour beetle Tribolium castaneum. Taken together, these findings advance our knowledge of the evolution of morphological change, with a primary focus on Hox genes and their contribution to axial patterning during development.