Ras signaling in either prothoracic gland cells or cholinergic neurons of Drosophila melanogaster regulates fly size
Caldwell, Philip E.
Doctor of Philosophy
Body size in multicellular organisms is determined by the integartion of two factors: the rate of growth and the duration of growth. In most animals, the rate of growth is controlled cell autonomously by the insulin-stimulated Pi3 kinase (Pi3K) pathway. However, the duration of growth is controlled in a more complex manner that involves endocrine factors that act cell non-autonomously. For example, in insects such as Drosophila, the duration of each larval phase is regulated by the timing of release of the molting hormone ecdysone from the prothoracic gland (PG). The molecular mechanisms by which the rate of growth and the duration of growth are integrated remain poorly understood. To help shed light in this area, I have investigated the intracellular signaling events that regulate ecdysone release in the Drosophila PG. I have found that expressing activated Ras, or the targets of Ras signaling Raf or Pi3K, in the PG reduces fly size and accelerates larval development via precocious synthesis and release of ecdysone. In contrast, expression of dominant-negative (dn) Ras, Raf, or Pi3K increases fly size and prolongs larval development via delayed synthesis and release of ecdysone. These results indicate that Ras-Raf and Pi3K signaling act in the PG to regulate the duration of growth by altering the timing of ecdysone synthesis and release. Conversely, I have found that expressing dn-Ras or dn-Raf, but not dn-Pi3K, in cholinergic neurons increases fly size and prolongs larval development, whereas, expression of activated Ras or Raf, but not Pi3K, in cholinergic nerurons decreases fly size, but delays larval development. Inhibition of insulin signaling in flies, via chromosomal loss-of-function mutations, also decreases fly size and delays development, raising the possibility that Ras-Raf signaling in cholinergic neurons may affect fly size by controlling the rate of growth via systemic insulin signaling.