Cell size and the morphogenesis of wing hairs in Drosophila.

Almost all epidermal cells on the Drosophila wing produce a single cuticular hair. This is formed in the pupae from a microvillus-like cell projection called the prehair. Previous experiments have shown the existence of two mechanisms that ensure that only a single hair is made. One is the restriction of prehair initiation to a small subregion of the cell by the action of the frizzled tissue polarity pathway. The second is a system that ensures the integrity of the prehair. Mutations and drugs that inhibit the actin cytoskeleton lead to the splitting of a single prehair into multiple smaller hairs. We report that large polyploid cells produce multiple hairs both because they form multiple independent prehair initiation centers and because the larger than normal hairs these cells produce have a tendency to split. We show that reducing cell size by starvation partially suppresses the phenotype seen in polyploid cells and that increasing apical cell surface area by mechanical stretching also results in the formation of multiple prehair initiation centers. We also show that the frizzled tissue polarity pathway is functional in large polyploid cells even if it is unable to restrict prehair initiation to a small region of the cell. We conclude that both of these cellular systems are limited in their ability to scale to accommodate larger cell size.