dem_hawley_recomb
A whole chromosome analysis of recombination and gene conversion in Drosophila melanogaster
Although traditional genetic assays have characterized the pattern of crossing over across the genome in Drosophila melanogaster, these assays could not precisely define the location of crossovers. Even less is known about the frequency and distribution of non-crossover gene conversion events. To assess the specific number and positions of both meiotic gene conversion and crossover events we sequenced the genomes of male progeny from females heterozygous for 93,538 X chromosomal single nucleotide and indel polymorphisms. From the analysis of the 30 F1 hemizygous X chromosomes, we detected 15 crossover and 5 non-crossover gene conversion events. Taking into account the non-uniform distribution of polymorphism along the chromosome arm, we estimate that most oocytes experience 1 crossover event and 1.6 gene conversion events per X chromosome per meiosis. An extrapolation to the entire genome would predict approximately 5 crossover events and 8.6 conversion events per meiosis. Mean gene conversion tract lengths were estimated to be 476 base pairs, yielding a per nucleotide conversion rate of 0.86 x 10-5 per meiosis. Both of these values are consistent with estimates of conversion frequency and tract length obtained from studies of rosy, the only gene for which gene conversion has been studied extensively in Drosophila. Motif enrichment analysis revealed a GTGGAAA motif that was enriched near crossover regions, but not in gene conversion regions. The low-complexity and frequent occurence of this motif may in part explain why, in contrast to mammalian systems, no meiotic crossover hotspots have been found in Drosophila.