INCREASED FREQUENCY OF MITOTIC CROSSING-O VER IN HETEROCHROMATIN DURING THE FIRST CLEAVAGE DIVISION IN DROSOPHILA-MELANOGASTER

It is shown that, although no compaction of paracentromeric heterochro matin occurs during the first cleavage division in Drosophila melanoga ster, the frequency of mitotic crossing-over in corresponding chromoso me regions is increased, as compared to that in euchromatin. Because a similar situation is observed at later stages of Drosophila developme nt, at which compact chromatin regions become well-manifested, it is c oncluded that the effect of heterochromatin on the frequency of crossi ng-over does not depend on its packing. A positive correlation between crossing-over events in paracentromeric heterochromatin and euchromat in was observed. This effect is probably due to the formation of a con tinuous region of somatic synapsis, which facilitates the process of m itotic crossing-over. On this basis, it is proposed that the effect of heterochromatin on mitotic crossing-over is associated with preferent ial chromosome pairing in the corresponding regions.