Site-specific breaks induced by the male recombination factor 23.5 MRF in Drosophila melanogaster.

The male recombination second chromosome 23.5 MRF was isolated from the same Greek population (Southern Greece) in which some years ago 31.1 MRF was discovered. A cytological analysis was carried out on the salivary gland chromosomes of the third-instar larvae deriving from crosses of males heterozygous for the 23.5 MRF chromosome II with various laboratory strains, with the following findings. (a) 23.5 MRF caused all types of chromosome rearrangements most of which involved chromosome II. (b) Discernible aberrations were not detected on the X-chromosome or the left arm of chromosome II. (c) The distribution of the break-points involving the second chromosome showed that 23.5 MRF induced breaks at specific sites on this chromosome. (d) The rearrangements induced were not stable; some were lost during the study, and new arrangements were generated. (e) Evidence was provided that 23.5 MRF was able to induce chromosome breakage in mitotic cells of salivary-gland chromosomes as well. (f) A comparison of the present results with data from the same natural population was made, and hypotheses explaining the present results are discussed.