The developmental mechanisms that control the growth and final size of organs are poorly understood. What, for example, determines the different sizes of halteres and wings, homologous thoracic structures in Drosophila? Two papers in this issue of Development provide new insights into this process. On ,Martín and Morata show that the control of growth and final size is an autonomous feature of the Drosophila wing imaginal disc and of its anterior and posterior compartments. They used Minute mutations,which prolong the larval period of heterozygous animals (M/+) without affecting the size of the larvae or adults, in a complex genetic strategy to generate slow-growing M/+ larvae that contained fast-growing, wild-type Minute+ (M+) discs or compartments. These wild-type,M+ tissues have ∼20 hours more development time than is normal,enough to quadruple in size, yet they grow only to a normal size in M/+larvae. And although adjoining M/+ and M+ wing disc compartments in M/+ larvae initially grow at different rates, they form adult wings of the correct shape and size. The researchers conclude that an autonomous mechanism within the wing disc compartments arrests their growth once they reach the right size, probably by lengthening the cell division cycle. On , de Navas,Garaulet and Sánchez-Herrero tackle the question of what controls the size difference between halteres and wings, and show that the Ultrabithorax (Ubx) Hox gene controls haltere size by regulating Decapentaplegic (Dpp) signalling. Ubx is expressed in the haltere disc but not in the wing disc, and Ubx mutations increase haltere size, transforming them into wings. Because changes in Dpp signalling affect wing size, the authors wondered whether Ubx fixes haltere size by modifying the Dpp pathway. Their results indicate that Ubxdownregulates dpp expression and alters Dpp activity in halteres. It also reduces Dpp spread, they report, by controlling the expression of the Dpp receptor thick veins and of division abnormally delayed,which encodes a cell-surface molecule. Thus, they suggest, changes in Dpp signalling that are induced by Ubx might partly account for the different size of halteres and wings.
