© CATHERINE DELPHIAWhy is your right arm as long as your left? How does your heart grow just big enough to fit inside your chest, and not larger? Why is your head “smaller” in proportion to your body size now than when you were a baby? The question of how animal growth and size are regulated and coordinated has fascinated generations of biologists. However, the underlying mechanisms still remain unclear. In recent years, the power and versatility of Drosophila genetics, combined with some imaginative experiments by fruit fly researchers, have revealed new ideas about the control of animal growth. In particular, several recent reports using this model system have shown how organ-to-organ signaling can influence both the rate of Drosophila body growth and final organ size. These findings may provide clues about human growth as well.
As they develop, Drosophila progress through three main stages—embryo, larva, and pupa—before emerging as adult fruit flies. Virtually all body growth occurs in the larval stage, during which the fly’s mass increases approximately 200-fold in a coordinated fashion. This increase in mass is dependent on the availability of dietary protein. In the wild this protein comes in the form of the yeast that ferments sugars in rotting fruit; in the lab, larvae are fed yeast meal. Without access to sufficient protein, the early-stage larval fly slows its rate of growth and development, with complete starvation triggering growth arrest.
© MARTIN SHIELDS/SCIENCE SOURCEA large part of this nutrient-dependent increase in size is due to an increase in cell size of organs such as muscle, fat, epidermis, and gut. Nestled among these tissues are imaginal discs containing precursor cells that eventually contribute to adult structures, including the wings, legs, eyes, thorax, and antennae. Communication between these various larval tissues is emerging as an important regulator of overall growth rate and larval body size and ensures that the growth of each organ is proportional to the rest.
Although humans ...
