The mechanism of evagination of imaginal discs ofDrosophila melanogaster : II. Studies on trypsin-accelerated evagination.

The effects of trypsin treatment on thein vitro evagination of imaginal discs under different conditions are investigated. It is found that trypsin accelerates the evagination of dises which have previously been exposed to ecdysonein vivo orin vitro. Substances which inhibit ecdysone-induced (unaccelerated) evagination, such as Cytochalasin B, Concanavalin A and Mycostatin also inhibit trypsin-accelerated evagination.On the cellular level, evagination is associated with the flattening of the disc cells. However, immature discs (i.e., those which have not been exposed to ecdysone) and discs pretreated with Cytochalasin B do not evaginate in response to trypsin even though pronounced cell flattening occurs. Cell flattening is an energy requiring process since it does not occur in response to trypsin treatment in the presence of oligomycin or nitrogen We conclude that cell flattening is an active process that takes place during evagination but which does not itself produce evagination. An alternative mechanism for evagination may involve cell rearrangement. Trypsinization could facilitate both cell flattening and cell rearrangement by reducing intercellular adhesiveness.