Germline hypermutability in Drosophila and its relation to hybrid dysgenesis and cytotype.

In its hypermutable state, an unstable singed allele, sn(w), mutates in the germline to two other alleleic forms at a total frequency usually between 40 and 60%. In its stable state, the mutation rate of sn(w) is essentially zero. Its state depends on an extrachromosomal condition indistinguishable from a property called cytotype previously studied as a component of hybrid dysgenesis. Of the two known systems of hybrid dysgenesis, denoted P-M and I-R, sn(w) hypermutability is determined by the P-M system and appears to be independent of the I-R system. Cytotype, as defined by the control of sn(w) mutability, is self-reproducing in the cytoplasm or nucleoplasm of the germline through at least two generations. However, it is not entirely autonomous, being ultimately determined by the chromosomes after sufficiently many generations of backcrossing. This combination of chromosomal and extrachromosomal transmission agrees well with previous studies on cytotype. Temperature differences have little effect on the mean mutation rates, but they have a pronounced effect on the intrinsic variance among individuals. The latter effect suggests that high temperatures reduce germ-cell survival during the development of dysgenic flies. Chromosomal rearrangements produce no apparent effects on the behavior of sn(w). Hypermutability is thought to be caused by the excision or other alteration of an inserted genetic element in the sn(w) gene. This element might be a copy of the "P factor," which is though to be a mobile sequence capable of causing female sterility and other dysgenic traits in the P-M system.