The effect of heat shock on certain meiotic parameters in Drosophila melanogaster was studied in the cv – v – f region of the X chromosome of females homozygous for the mus309 mutation, deficient in DNA double-strand break repair, or those of wild type. The heat shock in the wild females caused the frequencies of the single crossovers and double crossovers and all the map lengths to decrease while crossover interference remained unchanged. In the mus309 mutants all parameters, crossover interference included, remained unchanged despite the heat shock treatment. However, the mus309 mutation had a significant effect on all meiotic parameters both in the females not given the heat shock and in the heat shocked females with the exception that the recombination frequency of the v and f markers was the same in both genotypes in the females not given the heat shock. It seems that the heat shock treatment has an effect on crossing over which is independent on the mus309 gene and affecting the occurrence of crossing over itself. On the other hand, the mus309 gene has an effect on crossing over which is independent of the heat shock treatment and affects some precondition of crossing over. This precondition is probably the choice between two routes of the repair of double-strand DNA breaks known to be controlled by the mus309 gene. As explained in the discussion, the results are in accordance with the genetic models of interference in which interference depends on genetic distance between the crossover points, but in contradiction with physical models where interference is dependent on physical distance between the crossover points.
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