Interactions among modifers of retrotransposon-induced alleles of the white locus of Drosophila melanogaster

Mutations in five loci that modify the phenotype of white apricot (wa), caused by the retrotransposon. copia, were examined in two-way combinations to determine whether their effects were additive or epistatic. All two-way combinations of mutations in these five loci, mottler of white (mw), suppressor of forked (su(f)), suppressor of white apricot (su(wa)), Enhancer of white apricot, (E(wa)) and Darkener of apricot (Doa), are additive in their effects on wa, implying that each second-site modifier locus affects a different process. Three other copia-induced mutations, Hw(Ua), w(hd81b25) and ct(ns) were also examined for responsiveness to mutations in these modifier loci. None clearly responded. Mutations associated with B104 insertions, including Gl, vg(ni), ct(n) and w(ric) were also examined for responsiveness to mw, mutations, which have specificity for this element as well. Both rg(nf) and w(ric) respond to mutations in mw. The former interaction demonstrates that mw is capable of interacting with B104 elements in loci other than white. The significance of the results with respect to the nature of second-site modifier loci is discussed.