Genetic Analysis of Delta, a Neurogenic Gene of Drosophila melanogaster.

We report here the results of a genetic analysis of the gene Delta (Dl) of Drosophila melanogaster. Dl has been mapped to the band 92A2, on the basis of two pieces of evidence: (1) this band is the common breakpoint of several chromosomal aberrations associated with Dl mutations and (2) recombination mapping of alleles of five different lethal complementation groups that are uncovered by Df( 3R)Dl(FX3) (breakpoints at 91F11; 92A3). Dl was found to map most distally of all five complementation groups. The analysis of a large number of Dl alleles demonstrates the considerable genetic and functional complexity of Dl. Three types of Dl alleles are distinguishable. Most alleles behave as amorphic or hypomorphic recessive embryonic lethal alleles, which in addition cause various defects in heterozygosity over the wild-type allele. The defects are due to haplo-insufficient expression of the locus and can be suppressed by a duplication of the wild-type allele. The second class is comprised of three alleles with antimorphic expression. The phenotype of these alleles can only be reduced, rather than suppressed, by a duplication of the wild-type allele. The third group is comprised of three visible, predominantly hypomorphic alleles with an antimorphic component of phenotypic expression. The pattern of interallelic complementation is complex. On the one hand, there is a group of hypomorphic, fully penetrant embryonic lethal alleles which complement each other. On the other hand, most alleles, including all amorphic alleles, are viable over the visible ones; alleles of antimorphic expression, however, are lethal over visible alleles. These results are compatible with a rather complex genetic organization of the Dl locus.