A PRELIMINARY INVESTIGATION OF PSEUDOALLELISM AT THE NOTCH LOCUS OF Drosophila Melanogaster.

In order to place the purpose and results of this investigation in their proper perspective, it will be necessary to present a short summary of the existing information concerning the Notch locus. In Drosophila melanogaster this locus has yielded both dominant and recessive mutants. The dominant class of Notch mutants will be discussed first. Then some pertinent information concerning four recessive mutants and their interaction with the dominant Notches will be presented. Notch is a sex-linked dominant which is lethal in hemizygous males. Heterozygous females usually display notched wings, thickened wing veins, and certain bristle abnormalities. Many Notch mutations are associated with deficiencies' 2, translocations, or inversions in which the cytological break points are in the vicinity of salivary chromosome band 3C7; some of these mutants are variegatedtype Notches.3 Other Notch mutations are deficiencies for various lengths of the salivary chromosome. All but a few of these, which may be stable-type positioneffect Notches, lack at least the 3C7 band, and some are deficient for only 3C7.3' 4 The correspondence between the loss of band 3C7 and the production of a Notch phenotype has led to the conclusion that the locus of Notch is probably contained within this single band.' Although an appreciable number of Notch mutants exist which do not possess a visible deletion, it is possible that these mutants are subvisible deficiencies for a portion of band 3C7. In addition to the variety of Notch mutants, there exists at approximately the same genetic and cytological location a series of recessive mutants called facet (fa), facet-notch (fan), facet-notchoid (fan0), and split (spi). Both facet and split affect the eyes of Drosophila, but the two phenotypes differ in detail and can be separated with ease.5' 6 Facet-notch and facet-notchoid are wing mutants, but again the phenotypes are separable.5 6 The heterozygous combinations between fa, fan, and spl have been reported to result in a wild phenotype;5 6 one would presume that the mutants are non-allelic. However, an interpretation of allelism is favored by the fact that recombination between these mutants had not been reported. Consistent with this interpretation is some evidence obtained by Glass5 for an allelic relationship between fa and fan, and the results obtained by Bauer,6 which suggest an allelic relationship between fan and fa"0. In fact the detailed experiments of the latter investigator indicated at least a slight degree of interaction between all four of the mutants fa, fan, fano, and spi. Bauer went so far as to postulate the presence of a gene doublet which had arisen in the course of evolution in the form of a tandem duplication involving a part of salivary band 3C7. He suggested that the mutants fa, fan, and fano were associated with one locus and the spl mutant with the other. It should be noted that, in order to explain his results, Bauer had invoked a pseudoallelic interpretation, although the term "pseudoallele" was not used because it had not yet acquired its present connotation.