Genetics of inbred Drosophila melanogaster. XXI. Re-reversion in the strength of R-elements

In tests of the I-R hybrid dysgenesis system in Drosophila melanogaster we have in former tests examined the transition from strong to weak R-type by modifying the breeding protocol as regards the generation length, achieved by collecting parental flies from early or late egg-laying. To examine if independent occurrence of weak R-elements showed similar or different response at re-reversion of the generation interval, four sublines were tested. It was found that transition from long to short generation interval eventually resulted in a stepwise change to strong R-type. The change towards strong R-type occurred at a slightly slower rate than we earlier found in the opposite direction at transfer towards weak R-type. From lines bred at 14 days generation interval at 25 degrees C we kept, as a safeguard, branches at 18 degrees C. After 27 generations, tests of the strength of the R-reactivity revealed that various cytoplasms showed a depressed R-reactivity with no heterogeneity between sister lines. Return of sub-branches from 18 degrees to 25 degrees C for a couple of short generations (14 days) or a single long generation (28 days) showed results indicating heteroplasmy for weak and strong R-type elements in the branches kept at 18 degrees C. The vulnerability of the R-elements is a useful model for tests of extrachromosomal heredity, which may shed light on the problems involved in identifying other extrachromosomal systems.