EPISTATIC INTERACTIONS BETWEEN SMELL-IMPAIRED LOCI IN DROSOPHILA-MELANOGASTER

Odor-guided behavior is a polygenic trait determined by the concerted expression of multiple loci. Preciously, P-element mutagenesis was use d to identify single P[ArB] insertions, in a common isogenic backgroun d, with homozygous effects on olfactory behavior. Here, we hale crosse d 12 lines with these smell impaired (smi) mutations in a half-diallel design (excluding homozygous parental genotypes and reciprocal crosse s) to produce all possible 66 doubly heterozygous hybrids with P[lArB] insertions at two distinct locations. The olfactory behavior of the t ransheterozygous progeny was measured using an assay that quantified t he avoidance response to the repellent odorant benzaldehyde. There was significant variation in general combining abilities of avoidance sco res among the smi mutants, indicating variation in heterozygous effect s. Further, there was significant variation among specific combining a bilities of each cross, indicating dependencies of heterozygous effect s on the smi locus genotypes, i.e., epistasis. Significant epistatic i nteractions were identified for nine transheterozygote genotypes, invo lving 10 of the 12 smi loci. Eight of these loci form an interacting e nsemble of genes that modulate expression of the behavioral phenotype. These observations illustrate the power of quantitative genetic analy ses to detect subtle phenotypic effects and point to an extensive netw ork of epistatic interactions among genes in the olfactory subgenome.