A genetic screen for modifiers of Deformed homeotic function identifies novel genes required for head development.

Only a few genes have been identified that participate in the developmental pathways which modulate homeotic (HOX) protein specificity or mediate HOX morphogenetic function. To identify more HOX pathway genes, we screened for mutations on loci on the Drosophila second chromosome that interact with the homeotic gene Deformed (Dfd). Genetic and molecular tests on the eight genes isolated in the screen place them in three general categories. Two genes appear to encode trithorax group functions, i.e. they are general activators of Hox gene expression or function. Four genes encode abundant, widely expressed proteins that may be required to mediate Dfd morphogenetic functions in certain tissues, including two genes for collagen IV protein variants. Finally, two of the genes are required for the development of a subset of embryonic Dfd-dependent structures, while leaving many other segmental structures intact. We cloned and characterized one of these two, which we have named apontic (apt). apt is required for the elaboration of dorsal and ventral head structures. It encodes a 484-amino-acid protein with no significant similarity to known protein sequences. The apt transcript pattern is normal in Dfd and Scr mutants, and the Dfd and Scr transcript patterns are normal in apt mutants. We propose that apt acts in parallel to, or as a cofactor with, HOX proteins to regulate homeotic targets in the ventral gnathal region.