ALLELIC NEGATIVE COMPLEMENTATION AT THE ABRUPTEX LOCUS OF zyxwvu DROSOPHILA MELANOGASTER PETTER PORTIN Department of Genetics, University of Turku, SF-20500, Turku 50, Finland Manuscript received September 3, 1974 Revised copy received February zyxwv 24, 1975 ABSTRACT The mutations of the Abruptex locus in Drosophila melanogaster fall into three categories. There are recessive lethal alleles and viable alleles. The latter can be divided into suppressors and nonsuppressors of Notch mutations. The recessive lethals are lethal in heterozygous combination with Notch. As a rule the recessive lethals are lethal also in heterozygous combination with the viable alleles. Heterozygous combinations of certain viable alleles are also lethal. In such heterozygotes, one heteroallele is a suppressor of Notch and the other is a nonsuppressor. Other heterozygous combinations of viable alleles are viable and have an Abruptex phenotype. The insertion of the wild allele of the Abruptex locus as an extra dose (carried by a duplication) into the chro- mosomal complement of the fly fully restores the viability of the otherwise lethal heterozygotes if two viable alleles are involved. The extra wild allele also restores the viability of heterozygotes in which a lethal and a suppressor allele are present. If, however, a lethal and a nonsuppressor are involved, the wild allele only partly restores the viability, and the effect of the wild allele is weakest if zyxwvut two lethal alleles are involved. It seems likely that of the viable alleles the suppressors of Notch are hypermorphic and the nonsuppressors are hypomorphic. The lethal alleles share properties of both types, and are possibly antimorphic mutations. It zyxwv is suggested that the locus is responsible for a single function which, however, consists of two components. The hypermorphic mutations are defects of the one component and the hypo- morphic mutations of the other. In heterozygotes their cumulative action leads to decreased viability. The lethal alleles are supposed to be defects of the function as a whole. The function controlled by the locus might be a regulative function. ENETIC complementation is of fundamental importance in the definition G of genes and the analysis of gene function. Complementation is defined as the complementary action (cooperation) of homologous sets of genetic material involving the interaction of mutant genes, or their products, in double mutants. Those combinations that result in marked improvement in the function under study or in the development of a character which cannot be realized by the in- dividual action of single mutants are said to complement each other. (FINCHAM 1966; RIEGER, MICHAELIS and GREEN 1968). In particular the phenomenon of intracistronic complementation, i.e. complementation between heterozygous pairs of mutations which are on the basis of other criteria mutations within the same cistron, is a central phenomenon in the analysis of functioning of genes. Genetics a1 : 121-133 September, 1975