Effect of temperature on interaction of Abruptex and recessive alleles of the Notch complex locus of Drosophila melanogaster

Interaction of recessive visible alleles ( fa g , fa no , spl , nd , nd 2 ) and different Abruptex ( Ax ) alleles of the Notch locus of Drosophila melanogaster was analysed at 18°C. 25°C and 29°C in transheterozygotes. The homozygous viable Abruptex alleles complement most of the recessives at all temperatures. The expression of hypomorphic Abruptex alleles ( Ax E2 , Ax 16172 , Ax 71d ) remains constant at all temperatures, whereas the expression of hypermorphic alleles ( Ax 28 , Ax 9B2 ) is enhanced at elevated temperatures. The homozygous lethal Abruptex alleles ( Ax 59b8.1 , Ax 59d5 ) not only complement all of the recessives at 18°C, but their own phenotypic expression is usually very strong. At 25°C they fail to complement spl and fa 8 , and their own phenotypic expression is reduced. At 29°C the lethal Abruptex alleles do not complement any of the recessives except nd , and their own expression is reduced or almost absent. At low temperatures the lethal Abruptex alleles compete efficiently with the wild‐type allele, but at elevated temperatures their competetive ability is decreased. The results strongly support the view that the Notch complex gene is a single cistron, but that the Abruptex site may possibly have three functionally different mutant forms: hypomorphic, hypermorphic, and antimorphic. The morphism may, however, he different in different functions of the gene which may suggest independence of the pleiotropic effects of the gene.