Cellular and molecular patterning mechanisms underlying metamorphosis of the thoracic leg in Manduca sexta
Metamorphosis allows insects to possess radically different morphologies adapted for larval and adult functions. The tobacco hornworm Manduca sexta, like most other insects with complete metamorphosis, makes two versions of thoracic legs. The simple thoracic legs of the larva are formed during embryogenesis. During metamorphosis, the leg goes through extensive remodeling to give rise to the adult leg with a drastically different morphology. In addition, the adult legs on each of the three thoracic segments develop segment-specific features not present in the larval legs. In this dissertation, I explore the molecular and cellular mechanisms underlying three aspects of this morphological transformation.First I investigated the relationship between the cells in the larval and adult legs. The larval leg epidermis was found to consist of three distinct cell populations in terms of their contribution to the adult leg. The first population undergoes a rapid proliferation during the last larval instar to make up majority of the adult leg. The second population goes through no or very little proliferation and contributes to small parts of the adult leg. The last population is eliminated by programmed cell death during metamorphosis.Next, expression patterns of five leg patterning genes were examined to study their role in generating the two versions of the leg. I found that the adult leg of Manduca is produced by the conserved expression pattern of these genes that produce the adult legs of other insects. The larval leg, on the other hand, was produced by interruption of this adult patterning process. Completion of the patterning process at metamorphosis involved de novo expression of these genes in some populations of cells.In the last study, I examined the role of the Hox gene Sex combs reduced (Scr) in the segment-specific differentiation of the first thoracic (T1) legs during metamorphosis. During the early and late embryonic development, Scr protein was expressed weakly throughout the T1 leg. During the pupal leg development, its expression was up-regulated in regions of the T1 leg that develop segment-specific features consistent with its role in segment-specific development of T1 leg at this stage.
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