SEGMENTAL POLARITY IN DROSOPHILA-MELANOGASTER - GENETIC DISSECTION OFFUSED IN A SUPPRESSOR OF FUSED BACKGROUND REVEALS INTERACTION WITH COSTAL-2

fused (fu) is a segment polarity gene that encodes a putative serine/t hreonine kinase. A complete suppressor of the embryonic and adult phen otypes of fu mutants, Suppressor of fused (Su(fu)), was previously des cribed. The amorphic Su(fu) mutation is viable and displays no phenoty pe by itself. We have used this suppressor as a tool to perform a gene tic dissection of the fu gene. Analysis of the interaction between Su( fu) and 33 fu alleles shows that they belong to three different classe s. Defects due to class I fu alleles are fully suppressed by Su(fu). C lass II fu alleles lead to a new segment polarity phenotype in interac tion with Su(fu). This phenotype corresponds to embryonic and adult an omalies similar to those displayed by the segment polarity mutant cost al-2 (cos-2). Class II alleles are recessive to class I alleles in a f u[I]/fu[II];Su(fu)/Su(fu) combination. Class 0 alleles, like class I a lleles, confer a normal segmentation phenotype in interaction with Su( fu). However class II alleles are dominant over class 0 alleles in a f u[0]/fu[II];Su(fu)/Su(fu) combination. Alleles of class I and II corre spond to small molecular events, which may leave part of the Fu protei n intact. On the contrary, class 0 alleles correspond to large deletio ns. Several class I and class II fu mutations have been mapped, and th ree mutant alleles were sequenced. These data suggest that class I mut ations affect the catalytic domain of the putative Fu kinase and leave the carboxy terminal domain intact, whereas predicted class II protei ns have an abnormal carboxy terminal domain. Su(fu) enhances the cos-2 phenotype and cos-2 mutations interact with fu in a way similar to Su (fu). All together these results suggest that a close relationship mig ht exist between fu, su(fu) and cos-2 throughout development. We thus propose a model where the Fu(+) kinase is a posterior inhibitor of Cos tal-2(+) while Su(fu)(+) is an activator of Costal-2(+). The expressio n pattern of wingless and engrailed in fu and fu;Su(fu) embryos is in accordance with this interpretation.