How the Hox gene Ultrabithorax specifies two different segments: the significance of spatial and temporal regulation within metameres.

In Drosophila, the Hox gene Ultrabithorax (Ubx) specifies the development of two different metameres--parasegment 5, which is entirely thoracic, and parasegment 6, which includes most of the first abdominal segment. Here we investigate how a single Hox gene can specify two such different morphologies. We show that, in the early embryo, cells respond similarly to UBX protein in both parasegments. The differences between parasegments 5 and 6 can be explained by the different spatial and temporal pattern of UBX protein expression in these two metameres. We find no evidence for multiple threshold responses to different levels of UBX protein. We examine in particular the role of Ubx in limb development. We show that UBX protein will repress limb primordia before 7 hours, when Ubx is expressed in the abdomen, but not later, when UBX is first expressed in the T3 limb primordium. The regulation of one downstream target of UBX, the Distalless gene, provides a model for this transition at the molecular level.