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Genetic and developmental study of the notch locus of Drosophila melanogaster Foster, Geoffrey George


The sex-linked Notch locus plays an important role in embryogenesis and determination of many adult structures of the fruit fly, Drosophila melanogaster. Mutation at this locus can cause lethality in embryonic or later stages, as well as morphological abnormalities of the adult eyes, wings, bristles and legs. Alleles of the Notch locus can be broadly grouped into three classes: 1) recessive lethal Notch (N) alleles, which may be deficiencies or point mutations, 2) Abruptex (Ax) alleles, which are probably point mutations and may be either lethal or viable, and 3) viable recessive alleles with visible phenotypes, which affect a variety of traits and are point mutations. The present investigation was initiated with a view to understanding the relationships between N and Ax alleles and the nature of their role in development, and has consisted mainly of the following approaches: 1) an examination of the phenotypes of certain unusual N alleles and the phenotypic responses to alteration of the dosage of these alleles in relation to wild-type (N+), 2) an examination of the interaction of Ax alleles with N alleles and with one another, and 3) developmental studies of the conditional (temperature-sensitive) phenotypes associated with certain Notch-locus genotypes. The results of the N-allele dosage study indicate that a single mutation in the Notch locus can affect different functions associated with this locus in fundamentally different ways. Depending on the genotype and phenotype examined, the responses of various N alleles to dosage changes suggest that mutation at the Notch locus may result in reduced, increased or novel activity at the locus. Four ethyl methanesulfonate-induced Ax alleles have been examined, none of which is cytologically abnormal in salivary gland chromosome preparations, and at least three of which map within the Notch locus. Depending on culture conditions and the alleles involved, Ax/N heteroallelic combinations may be viable or lethal. All Ax/N combinations studied exhibited less severe Abruptex phenotypes (bristle loss and wing vein gapping) than the respective Ax/Ax homozygotes. However, the Ax alleles differed from one another in their effects on the wing nicking of the N alleles, in that the viable allele Ax9B2 and the semi- lethal allele AxE1 both suppressed wing nicking, whereas the two viable alleles AxE2 and AX16172 both enhanced wing nicking. Furthermore, heteroallelic combinations of Ax alleles which affected nicking in different direction, were lethal (AxE1/AxE2, AxE1/Ax16172, Ax9B2/Ax16172), whereas combinations of Ax alleles with similar effects on nicking were viable (AxE1/Ax9B2, AxE2/ Ax16172). The temperature-shift experiments have revealed an interesting pattern of temperature-sensitive periods (TSPs) for lethality or adult morphological abnormalities associated with various Notch-locus genotypes. TSPs for lethality may be monophasic occurring in the embryo (N60g11/N6g11;Dp51b7), or the second larval instar (Ax16172/n264-40), or they may be polyphasic, occurring in embryo, larval and pupal stages (N264-103/fano). On the other hand, the TSPs for all the adult morphological abnormalities examined occur during the third larval instar, including rough eyes and wing nicking (N60gll/+,N264-103/spl), leg segment fusion (N264-103/+, N264-103/spl), wing vein gapping (Ax16172/+) and disturbance of bristle numbers (N264-103/spl, Ax16172/+). Several molecular models are discussed in relation to the observations on N-allele dosage and interactions of the Ax and N alleles. The results are consistent with the hypothesis that the Notch locus is a regulator gene influencing many developmental processes, that mutations can affect the activity either of the entire gene or of various parts of the gene individually, and that N and Ax mutations usually affect this regulatory system in opposite ways from one another.

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