The nature, extent, and consequences of polyglutamine tract variation in Notch in Drosophila

The Notch receptor is a key signaling protein that also acts as a transcriptional co-activator in numerous cell fate decisions in animals, including Drosophila melanogaster. Like many other transcriptional activators interacting on the DNA, the Notch protein carries a polyglutamine tract encoded by the opa repeats of the Notch gene. Here, I show that considerable variation exists within this tract across populations from the United States and Malawi. This variation is distributed asymmetrically across the range of possible alleles, with a peak in each population at opa31 (typically encoding Q₁₃HQ₁₇) and/or opa32 (typically encoding Q₁₃HQ₁₈) and a tendency towards a large tail of longer alleles and few shorter alleles. Variation in this pattern between populations may be a result of certain tracts being less harmful in certain backgrounds, or it may be due to the ancestry of these populations. This variation has real effects, such that lines bearing alleles longer or shorter than the common 31- and 32-codon alleles exhibit abnormal phenotypes, gene expression disruption, and decreased viability, effects that persist even when the Notch gene is outcrossed or recombined into other backgrounds. I also describe lines bearing CRISPR-Cas9-edited opa repeats and highlight a potential interaction between Notch and the transcriptional activator Dorsal. Dorsal also exhibits variation in the length of its polyglutamine tract, and short tracts at Dorsal most frequently appear alongside short tracts at Notch, while long tracts of each also frequently co-occur.