A SCREEN FOR MODIFIERS OF DEFORMED FUNCTION IN DROSOPHILA

Proteins produced by the homeotic genes of the Hox family assign diffe rent identities to cells on the anterior/posterior axis. Relatively li ttle is known about the signalling pathways that modulate their activi ties or the factors with which they interact to assign specific segmen tal identities. To identify genes that might encode such functions, we performed a screen for second site mutations that reduce the viabilit y of animals carrying hypomorphic mutant alleles of the Drosophila hom eotic locus, Deformed. Genes mapping to six complementation groups on the third chromosome were isolated as modifiers of Deformed function. Products of two of these genes, sallimus and moira, have been previous ly proposed as homeotic activators since they suppress the dominant ad ult phenotype of Polycomb mutants. Mutations in hedgehog, which encode s secreted signalling proteins, were also isolated as Deformed loss-of -function enhancers. Hedgehog mutant alleles also suppress the Polycom b phenotype. Mutations were also isolated in a few genes that interact with Deformed but not with Polycomb, indicating that the screen ident ified genes that are not general homeotic activators. True of these ge nes, cap 'n' collar and defaced, have defects in embryonic head develo pment that are similar to defects seen in loss of function Deformed mu tants.