Sex-specific crossover rates did not change with parental age in Arabidopsis

Crossing over, the exchange of DNA between the chromosomes during meiosis, contributes significantly to genetic variation. The rate of crossovers (CO) varies depending upon the taxon, population, age, external conditions, and also, sometimes, between the sexes, a phenomenon called heterochiasmy. In the model plant Arabidopsis thaliana, the male rate of crossovers (mCO) is typically nearly double the female rate (fCO). With increasing parental age, it has been reported that the disparity decreases, because fCO rises while mCO remains stable. That finding, however, is based on chromosome-based averaging, and it is unclear whether all parts of the genome respond similarly. We addressed this point by examining how the level of heterochiasmy responded to parental age in eight genomic intervals distributed across the five chromosomes of Arabidopsis. Unlike the previous work, in each of the eight intervals, the level of heterochiasmy did not change with age, that is, the ratio mCO:fCO remained stable. As expected, though, amongst the intervals, the levels of heterochiasmy at any of the four ages examined, did vary. We propose that while the levels of heterochiasmy in Arabidopis might decrease with age on a chromosomal basis, as reported earlier, this is not true for all locations within each chromosome. This has practical implications for plant breeding research, a major aim of which is identifying ways to induce local increases in CO rates.