Levels of Heterochiasmy During Arabidopsis Development as Reported by Fluorescent Tagged Lines

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 all crossover events (mCO) is typically nearly double the female rate (fCO). A previous, PCR-based genotyping study has reported that the disparity decreases with increasing parental age, because fCO rises while mCO remains stable. We revisited this topic using a fluorescent tagged lines approach to examine how heterochiasmy responded to parental age in eight genomic intervals distributed across the organism's five chromosomes. We determined recombination frequency for, on average, more than 2000 seeds, for each interval, for each of four age groups, to estimate sex-specific CO rates. mCO did not change with age, as reported previously, but, here, fCO did not rise, and thus the levels of heterochiasmy were unchanged. We can see no methodological reason to doubt that our results reflect the underlying biology of the accessions we studied. The lack of response to age could perhaps be due to previously reported variation in CO rate among accessions of Arabidopsis.