Studies on the chromosomal rearrangements induced by the male recombination factor 31.1 MRF in Drosophila melanogaster

A cytological analysis was carried out on the salivary-gland chromosomes of third-instar larvae deriving from appropriate crosses of the 31.1/CyL⁴ strain with the dp b cn bw and dp b cn bw; ve strains.(1) The 31.1 MRF induced chromosomal rearrangements in both male and female germ cells. (2) The aberrations that occurred in the male germ cells were of three types — inversions, duplications and deficiencies — whereas those that occurred in the female germ cells were of all types. (3) The distributions of the break-points among the 4 major autosomal arms between the 1F₂ and 2AF₂ larvae were different. (4) The total distribution of the break-points along the polytene chromosomes showed that the factor induced breaks in all major chromosomes (X, II, III), and that 51.81% of them occurred in sites showing late replication and/or ectopic pairing. (5) A relationship existed between the frequencies of the second chromosomal aberrations and male recombination.A comparison of the break-points induced by the factor was made with those found in the rare endemic inversions of the same natural population from which the 31.1 MRF was isolated. The possibility that the factor acts in the natural population is discussed.