Nonrandom segregation of centromeres following mitotic recombination in Drosophila melanogaster

Mitotic recombination is widely used in Drosophila as a technique to study genetic and developmental problems. It has been generally assumed that, following mitotic exchange between homologous chromatids during the G2 stage, the centromeres attached to the chromatids involved in the exchange segregate randomly. As a result, two equally frequent types of segregation, yielding genetically different products, are produced. However, when epidermal or enzymatic cell-marker mutants are used, only one type of segregation gives rise to marked cells. In the present work we test this assumption of random segregation using cytological markers. With cytological markers, larval neuroblast cells resulting from mitotic recombination exhibit recognizably all possible products from mitotic recombination. We find that one type of segregation is favored, in that, after mitotic recombination, the centromeres attached to the chromatids involved in the mitotic exchange preferentially migrate to opposite poles during anaphase. This favored segregation could be the result of exchange between previously oriented chromatids or could be due to the effect of the exchange upon subsequent orientation of homologous chromosomes. In either case, frequencies of mitotic recombination have been overestimated in the past.