TANAKA, K; TRUMAN, JW; University of Washington: Morphogenesis of the adult legs in Manduca sexta: contribution of different larval cell populations

The tobacco hornworm Manduca sexta, like most other insects with complete metamorphosis, makes two versions of thoracic legs. The relatively simple thoracic legs of the larva are formed during embryogenesis. All the epidermal cells of these legs make leg cuticle during the subsequent larval molts. In the last larval stage, however, the adult leg primordium, a small subset of these epidermal cells occupying a dorsal longitudinal strip and the rings around the leg joints, detaches from the cuticle, and begins rapid proliferation. Eventually the proliferation spreads to the entire leg. The resulting adult leg has grown seven times in length and attained a radically different morphology. We are interested in studying the cellular events and the molecular patterning underlying this transformation. In the current study we have attempted to determine the origin of the adult leg tissue. The adult primordium in the larval leg, which proliferates rapidly in the last instar, contributes to a large part of the adult leg. In contrast, the epidermal cells outside this region do not appear to proliferate during the early stages of metamorphosis. To determine how much contribution these latter cells make to the adult leg tissue, we have investigated the adult leg morphogenesis using a cell labeling method, cell proliferation and cell death assays. Surprisingly, our results indicate that the cells outside the primordium appear to make no contribution to the adult leg. Instead, these larval cells go through programmed cell death, eliminating themselves from the newly formed adult leg. Thus, the larval leg of Manduca sexta has two distinct epidermal cell populations. One population makes both the larval and the adult leg epidermis, while the other is larval specific.