High frequency recombination in centromeric and histone regions of Drosophila genomes

Abstract

DNA SYNTHESIS in eukaryotes is asynchronous, with heterochromatin replicating later than euchromatin¹. In Drosophila, autoradiographic studies of metaphase chromosomes from embryonic² and cultured³ cells have shown that the centromeric heterochromatin of chromosome 2 is late replicating. The histone genes have been localised to subsections 39D and 39E in the left arm chromosome 2, close to the centromere⁴, and may be part of this late-replicating region for the following reasons. First, the five tandemly arranged histone coding sequences are reiterated approximately 50 times, and late-replicating DNA has been shown to contain significantly more repeated (but non-satellite) sequences than the genome as a whole⁵. Second, the histone genes are transcriptionally active in all but the final hours of the S phase, suggesting that their replication may be delayed⁶. Third, they are in close proximity to the centromere. Asynchrony has been detected in the recombination process as well. Heat treatment of Drosophila oocytes at sequential intervals during premeiotic-S phase elicits increases in recombination that follow a well-ordered pattern⁷. To investigate the possibility that the pattern of heat response parallels that of replication, the centromeric heterochromatin of chromosome 2 and the contiguous histone region have been examined for the time of their recombinational response to heat during the S phase. We present here evidence for late response in both regions. In addition, a level of response roughly two orders of magnitude greater than the average encountered in the genome has been found in these regions.