Genetic Basis for Two Types of Recessive Lethality at the Notch Locus of Drosophila

OTCHES, symbolized as N , are inherited as sex-linked dominant mutations N that are lethal in the homozygous and hemizygous condition.In heterozygotes N / f , typical females have notched wings, thickened wing veins and an irregular distribution of thoracic hairs.Since many Notch mutants are associated with a visible cytological deficiency for salivary band 3C7, it has been concluded that the Notch gene is contained within this single band (MOHR 1919(MOHR , 1923;;SLIZYNSKA 1938;DEMEREC 1939).In addition to these deficiency types of Notches, many N's are cytologically normal and phenotypically similar to or, in some instances, indistinguishable from the deficiency type; they have been used to demonstrate the linear arrangement of mutant sites in the Notch gene within salivary band 3C7 (WELSHONS 1958(WELSHONS , 1965;;WELSHONS and VON HALLE 1962).Interspersed in the linear array of N's, are mutant sites inherited as recessive visibles.The phenotypes of these recessives fall into two catagories: Those that roughen the eye are facet ( f a ) , facet-glossy (fag) and split ( s p l ) .Those that produce wing notching or alterations in wing venation are facet-notchoid (fa""), notchoid ( n d ) , and notchoid-2 ( n d 2 ) .Additional details relevant to these mutants will be found in LINDSLEY and GRELL (1968).In heterozygotes of a N (deficiency or nondeficiency type) with fa, fag, spl, nd, and nd2.there is a phenotypic expression of the recessive in addition to that of the dominant Notch.In the case of N/fano, depending upon the N mutant used, heterozygotes are very inviable or completely lethal, although the recessive in homozygous or hemizygous condition has good viability and fertility.Excepting fa"" for the moment, the pseudodominant manifestation of recessive visibles when combined with Notch suggests that these recessive mutant sites permit the synthesis of a gene product that is functional but slightly aberrant, whereas the N's totally lack a product or fashion one that is largely or completely impaired at the functional level.The mutant fano, like some of the other recessives, seems to be a hypomorph ( WELSHONS 1965), and the lethality of N/fano evidently results from the hypomorphism of fano combined with the amorphism of Notch, yielding a heterozygous hypomorphic condition more extreme than in fano homozygotes.There is in Drosophila stocks an X chromosome tandemly duplicated for a series of salivary bands, one of which is 3C7; the chromosome is symbolized as Dp(1;l)Co.When heterozygous with a normal X , the flies express a Confluens