Relationship between sister chromatid exchanges and DNA replication in somatic cells of Drosophila melanogaster.

In Drosophila melanogaster cell lines and larval neuroblast cells, two aspects of the phenomenon of sister chromatid exchanges were analyzed: (1) the frequency of SCEs in relation to the ploidy level (comparing diploid and tetraploid cells) and in relation to the cell type (comparing embryonic and larval cells) (2) the localization of the sites of exchange with reference to eu- and heterochromatin. A good correlation between SCE frequency and genome size in the same cell type (in distant species also), but a significant difference in the SCE rate between different cell types within the same species, were found. The results confirmed also the non-random distribution of SCEs in the different portions of the genome since a preferential localization in the euchromatin was clearly demonstrated. Moreover, a direct proportionality between SCE frequency and the length of the S phase was supposed, favouring the hypothesis of a relationship between the phenomenon of sister chromatid exchanges and DNA replication.