A tripartite interaction among alleles of Notch, Delta, and Enhancer of split during imaginal development of Drosophila melanogaster.

A dramatic example of a phenotypic interaction that involves neurogenic loci during Drosophila imaginal development is the synergistic impact of split (spl), a recessive allele of the Notch locus, and E(spl)D, a dominant gain-of-function allele of the Enhancer of split locus, on morphogenesis of the compound eye. Screens for mutations that relieve the enhancing effect of E(spl)D on spl have yielded three classes of mutations: intragenic revertants of the E(spl)D allele, extragenic suppressors that are allelic to the neurogenic gene Delta (Dl) and unlinked extragenic modifiers. Analysis of the suppression of the spl-E(spl)D interaction by various Dl alleles indicates that this modification is sensitive to the dosage of the Dl locus. This tripartite interaction illustrates the combinatorial action of N, Dl and E(spl) during imaginal development.