Date of Award
Doctor of Philosophy
The chelicerates (e.g., spiders, mites, scorpions, harvestmen and horseshoe crabs) are oneof the oldest arthropod clades, arising in the Cambrian. The chelicerates display a conserved body plan comprising of an anterior prosoma and a posterior opisthosoma. The prosoma comprises the segments bearing the chelicerae, pedipalps and the four pairs of walking legs. The opisthosoma is more variable, and either contains no appendages (e.g., in harvestmen), or extremely derived appendages (e.g., the spinnerets of spiders and the pectines of scorpions). Recent evolutionary developmental (evo devo) studies on arachnids and other arthropods have shown that a conserved suite of developmental genes pattern the chelicerate body plan, with slight modifications in their expression patterns correlating with morphological diversification and evolution of segments and the appendages that they bear. Although a few tractable evo devo chelicerate models are currently in use, there is a noted paucity of evo devo studies on mites, let alone embryological studies on mites using modern microscopic techniques. The following dissertation aimed at filling the gap in the current knowledge of mite evolution and development by studying the development of the emerging mite model mite Archegozetes longisetosus. Four main aims that focused on answering questions surrounding mite and chelicerate evolution were the subjects of the dissertation research. These aims included a.) determining the segmental composition of the mite opisthosoma, b.) determining how the segment of the suppressed fourth pair of walking legs is formed during embryogenesis, c.) determining how can the appendage-patterning genes, i.e., the "limb gap genes," aide in elucidating arthropod phylogeny and d.) determining how the evolutionarily conserved Hox genes pattern the reduced mite opisthosoma. The findings from this project include that the acariform mite opisthosoma has been reduced to two terminal segments, that the formation of the segments bearing the fourth pair of walking legs is tied to opisthosomal segmentation in A. longisetosus, that mites retain chelicerae comprised of three segments, that only the proximal elements of the fourth pair of walking legs are patterned during development in A. longisetosus, and that the mite homologues of the Hox genes Ultrabithorax and Abdominal-B are only expressed in a single segment in the opisthosoma in A. longisetosus. These findings are placed within a comparative framework to aid in the understanding of the evolution of arthropod and chelicerate body plans.
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