The I-R system of hybrid dysgenesis in Drosophila Melanogaster: Are I factor insertions responsible for the mutator effect of the I-R interaction?

Summary

When Drosophila melanogaster males coming from a class of strains known as inducer are crossed with females from the complementary class (reactive), a quite specific kind of sterile female (SF) is obtained that exhibits other dysgenic traits such as non-disjunctions and non-randomly distributed mutations. This syndrome is caused by the interaction of the ‘I factor’ linked to inducer chromosomes with the maternally inherited reactive state ‘R’. This I-R interaction is also responsible for ‘chromosomal contamination’ that is likely to result from very frequent I factor insertions into reactive chromosomes. Such insertions might be responsible for the I-R induced mutations and some data concerning this hypothesis are reported here.

Out of nine I-R-generated mutants one, the white ^IR1 (w ^IR1) allele, is closely linked to an I factor, which maps either at the site of the mutation or within less than 0.02 map units. In addition, w ^IR1 is somewhat unstable when transmitted through SF females.

In contrast, the typical I factor does not seem to be associated with any of the eight other mutants as judged by their inability to induce the female sterility characteristic of the I-R syndrome. The possibility is discussed that most of I-R-induced mutations are nevertheless caused by insertions of either undetectable I factors or other transposable elements, not related to I, whose transposition is dependent on the I-R interaction.