Blackpatch, a neural degeneration mutation that interacts with the Notch locus in Drosophila.

We have identified a gene in Drosophila melanogaster that is involved in the development of the adult eye and optic lobe of the brain and that interacts with facet alleles at the Notch locus. We have named this locus Blackpatch (Bpt). Mutant alleles of Bpt produce a variety of abnormal phenotypes in the presence of facet alleles. These phenotypes include neural degeneration in the eye and in the optic lobe of the adult brain that begins 60 hr after pupariation and produces a dark, necrotic eye spot in the adult eye. Other phenotypes include recessive embryonic lethality, pharate adult lethality, and premature adult death. We have isolated and characterized 10 Bpt alleles, all of which yield the neural eye/brain degeneration phenotype in individuals who are also homozygous or hemizygous for facet mutations. Only some of the facet alleles interact with Bpt. Bpt mutations also interact with the split mutation but do not interact with other types of Notch mutation. Somatic mosaic analysis and imaginal disc transplantation experiments suggest that the optic lobe of the brain may be the focus of Bpt action. We conclude that the Notch and Bpt genes have important functions during the interaction between the retina and the optic lobe of the brain.

Phenotypic and molecular analysis of the facets, a group of intronic mutations at the Notch locus of Drosophila melanogaster which affect postembryonic development.

The function of the Notch locus of Drosophila melanogaster is essential for normal development both during embryogenesis and during postembryonic stages. In the embryo its function is necessary for the correct segregation of neural from epidermal lineages. During postembryonic stages Notch exhibits pleiotropic effects that are both tissue- and stage-specific. Here, we examine a group of six recessive mutations, the facets (fa, fa3, fag, fag-2, fafx and fasw), which affect eye morphology and have been previously shown to be associated with the insertion of transposable elements in an intronic region of Notch. The analysis of revertants has shown that the mutant phenotype depends on the presence of the transposable element and that the disruption of the wild-type sequence organization per se is not its cause. Four of these alleles, even though they are associated with the insertion of the same transposable element, display considerably different phenotypes. Therefore, no simple correlation exists between the mutant phenotype and the type of inserted element. A comparison of the tissue localization of the Notch and the transposable element transcripts revealed that in the third larval instar the elements are transcribed in both orientations in tissues in which Notch is also transcriptionally active. The complexity of the defects associated with the facet alleles, as well as the findings of the transcriptional analysis, indicate that a mutational mechanism based solely on transcriptional interference is not sufficient to explain the nature of the mutational event. It is likely that in these mutations alterations, in the temporal and/or spatial context caused by transcriptional and perhaps posttranscriptional interference mechanisms by the inserted elements, may be responsible for the mutant phenotype.

Enhancement and Suppression of a Euchromatic Position Effect at NOTCH in Drosophila.

The recessive visible rough-eye mutant facet-strawberry, fa(swb), is caused by the deletion of 0.8 kb of base sequences from the 5' end of the Notch locus. Visible deficiencies adjacent to fa(swb) suppress this mutant effect of the Notch locus, and in the same region (between salivary bands 3C1 and 3C7), we have demonstrated the presence of at least one partial suppressor and one enhancer of the fa(swb) position effect at Notch.-The enhancer seems to be a small inversion approximately equal to the salivary-band doublet 3C2, 3, and the partial suppressor lies between the inversion in 3C2, 3 and the small deletion in fa(swb) immediately distal to 3C7. Neither the enhancer, e(fa(swb)), nor the partial suppressor, su(fa(swb)), can be detected except when linked in cis to fa(swb). The e(fa(swb)) and the su(fa(swb)), in unison, act antagonistically on the fa(swb) position effect.-The fa(swb) mutant is interpreted to be a nonvariegating position effect at the Notch locus resulting from a novel euchromatic-euchromatic association of base sequences caused by the small deletion.

Suppression of the facet-strawberry position effect in Drosophila by lesions adjacent to notch.

The recessive visible rough eye mutant effect of fa( swb), a small deletion at the 5' end of the Notch locus, is suppressed when fa(swb) is coupled to five different closely linked deficiencies distal to salivary band 3C7. In addition, an inversion with a proximal breakpoint between 3C3 and 3C5 similarly suppresses the mutant effect. The data support the position effect interpretation of fa( swb): The small deletion allows functions distal to Notch to interfere with functions at Notch, and when the interference is eliminated, the fa(swb)-mutant effect disappears.-The fa(swb) deletion also interacts with another recessive visible rough-eye mutant at Notch called fa(g). In the cis condition, fa(swb) fa( g) double mutants have a mutant-eye phenotype like fa( g) (similar to the mutant effect of fa(swb)) and, in addition, express an accessory phenotype (thickened wing veins). Although the mutant-eye effect of fa(swb) can be suppressed by lesions adjacent to Notch, the accessory phenotype of the coupled mutants is not suppressed. It is suggested that the fa(swb) deletion has two observable effects: One is a modifiable position effect causing the fa(swb) rough-eye phenotype; the other is a stable effect exerted upon a 5-kb insertion that is the probable cause of the fa(g) mutant expression, thus resulting in a wing effect that accompanies the eye effect of fa( g).

The molecular genetics of the Notch locus in Drosophila melanogaster.

The Notch locus of Drosophila melanogaster profoundly affects differentiation of the central nervous system in the early embryo. Previous molecular studies suggested that the locus spans 40 kb of DNA and encodes a 10.5-kb poly(A)+ RNA. The results of genetic, cytogenetic, and molecular studies of newly induced and preexisting Notch alleles are reported. Molecular analysis of 5' flanking mutations defines a distal limit for the locus, and transcriptional activity of Notch in relation to that of flanking transcription units provides evidence regarding the proximal limit. Examination of a set of mutable alleles implicates a foldback transposable element as the basis of chromosomal instability, and shows that insertion sequences within the locus do not necessarily result in a mutant phenotype. The results are discussed with regard to existing developmental and genetic analyses, and a molecular model is proposed that attempts to explain the pleiotropic action of Notch.

The recombinational analysis of aberrations and the position of the notch locus on the polytene chromosome of Drosophila.

The recombinational analysis of heterozygotes for a point-mutant N and a deficiency N suggests that the map region approximated by the interval fa to nd2 is at the right edge of salivary band 3C7 or in the interband to the right. The map region N55ell to fa can be anywhere between the left-interband and the right edge of 3C7. We discovered that small inversions also can be used in the recombinational analysis, and the inversion data support the conclusions already described. The reactivation of latent mutability in a Notch inversion resulted in reinversion of the original aberration, followed by reversion of N to N+. From the same Notch inversion, we isolated a spontaneous deficiency superimposed upon the original aberration, which supported our hypothesis that two of our w to N deficiencies probably originated as deficiencies superimposed upon inversions.

The cytogenetics of a recessive visible mutant associated with a deficiency adjacent to the notch locus in Drosophila melanogaster.

The recessive visible faswb allele in Drosophila is an interband deletion between salivary band 3C5, 6 and 7. Heterozygosity for the deletion does not suppress recombination between faswb and mutant sites at Notch adjacent to it.--Df(1)w67k30, deficient for salivary bands 3C2 to 6, is the left of faswb. By crossing over within the homologous bit of interband retained in w67k30 and faswb, the two deficiencies can be linked. Cytologically, 3C7, "fused" to 3C5,6 in faswb, becomes "fused" to 3C1 when the two are coupled. In the double deletion, the recessive visible phenotype of the faswb "allele* is suppressed. Both w67k30 and faswb can be recovered by uncoupling the two deficiencies.--The data suggest that the mutant faswb does not represent a lesion at Notch; the entire gene or locus seems to be present. The interband deletion in faswb has secondarily moved an intact Notch locus to a foreign environment that interferes with its normal function. When faswb is linked to w67k30, the interference is eliminated and normal Notch functions resume.--The position of Notch on the salivary gland chromosome is reviewed in relation to the information obtained in these experiments.

Intragenic deletions and salivary band relationships in Drosophila.

In the absence of assumptions pertaining to the organization and function of chromomeric DNA, the cytogenetic analysis of intragenic deletions that start at Notch and spread to the right or left of the locus suggests that the recombinational gene is bilaterally associated with salivary band 3C7. Either there are two genes resolved as a single cistron, or one must seek an alternative interpretation that allows some modicum of independent in the relationship between gene and band. Although we momentarily lean toward the hypothesis that gene and salivary band are separate entities on a binemic chromosome, alternative views can be devised, and the data must remain open to reinterpretation.--The recessive visible allele fa-swb behaves as a point mutant at the left end of the map and seems to be a deletion in the interval 3C6 to 7; we suspect some part of the band is missing. We have used the aberration in fa-swb as a cytological marker, isolated intragenic recombinants, and subjected them to examination. The analysis indicates that the chromosomal interchanges occurred to the right of 3C7.

The cytogenetic analysis of a fractured gene in Drosophila.

The data presented in this study are derived from the analyses of Notch mutants known to be associated with visible cytological deficiencies. One mutant, Df(1)N(62b1), described as a right-side deficiency, bears a deletion that apparently initiates within the Notch locus and extends to the right as far as the locus of dm. Recombination experiments using heterozygotes of Df(1)N(62b1) with a series of intragenic point mutants within the Notch cistron suggest that this deficiency represents a deletion for the right-end portion of the gene. A consideration of the cytology of Df(1)N(62b1) supports the cytogenetic inference that, if a Notch locus-3C7 relationship is valid, the missing portion of the gene as assayed by recombination experiments has an interband position between 3C7 and 8.-The data derived from two left-side deficiencies with a genetic lesion in Notch and a deletion extending to w are somewhat equivocal, but they do support the presumed Notch locus-3C7 band relationship and thereby enhance the likelihood that Df(1)N(62b1) is correctly interpreted.-Cytogenetic information presently available suggests that, although a significant portion of the Notch cistron has a position on the salivary map identified as interband 3C7 to 8, the 3C7 band is part of the total picture of the Notch gene.