Sister chromatid exchanges in Drosophila melanogaster cell lines in vitro

The frequency of sister chromatid exchanges (SCEs) in two cell lines of Drosophila melanogaster with different karyotypes (XX and XY) was determined, considering (1) the distribution of SCEs within each chromosome, with reference to eu- and heterochromatin and (2) the distribution of SCEs in different chromosomes. A comparison was made between chromosome pairs within each karyotype and between the two different karyotypes. The following results were obtained. The SCEs are not randomly distributed along chromosomes, since exchanges were never observed in heterochromatin. SCEs are more frequent in XY than in XX cells; moreover, in both cell types there exists a significantly higher frequency of SCEs in the X chromosome than in the autosomes. These findings are discussed in relation to chromosome aberrations and mitotic recombination.