Autonomous control of cell and organ size by CHICO, a Drosophila homolog of vertebrate IRS1-4

The control of growth is fundamental to the developing metazoan. Here, we s how that CHICO, a Drosophila homolog of vertebrate IRS1-4, plays an essenti al role in the control of cell size and growth. Animals mutant for chico ar e less than half the size of wild-type flies, owing to fewer and smaller ce lls. In mosaic animals, chico homozygous cells grow slower than their heter ozygous siblings, show an autonomous reduction in cell size, and form organ s of reduced size. Although chico flies are smaller, they show an almost P- fold increase in lipid levels. The similarities of the growth defects cause d by mutations in chico and the insulin receptor gene in Drosophila and by perturbations of the insulin/IGF1 signaling pathway in vertebrates suggest that this pathway plays a conserved role in the regulation of overall growt h by controling cell size, cell number, and metabolism.