Hox gene control of segment-specific bristle patterns in Drosophila.

Hox genes specify the different morphologies of segments along the anteroposterior axis of animals. How they control complex segment morphologies is not well understood. We have studied how the Hox gene Ultrabithorax (Ubx) controls specific differences between the bristle patterns of the second and third thoracic segments (T2 and T3) of Drosophila melanogaster. We find that Ubx blocks the development of two particular bristles on T3 at different points in sensory organ development. For the apical bristle, a precursor is singled out and undergoes a first division in both the second and third legs, but in the third leg further differentiation of the second-order precursors is blocked. For the posterior sternopleural bristle, development on T3 ceases after proneural cluster initiation. Analysis of the temporal requirement for Ubx shows that in both cases Ubx function is required shortly before bristle development is blocked. We suggest that interactions between Ubx and the bristle patterning hierarchy have evolved independently on many occasions, affecting different molecular steps. The effects of Ubx on bristle development are highly dependent on the context of other patterning information. Suppression of bristle development or changes in bristle morphology in response to endogenous and ectopic Ubx expression are limited to bristles at specific locations.