The Meiotic Recombination Frequency (MRF) assay. (A) Crossing scheme: At - Arabidopsis thaliana, Aa - Arabidopsis arenosa, As - Arabidopsis suecica. G = Green Fluorescent Protein (GFP) and R = Red Fluorescent Protein (RFP) are transgenic markers on the upper arm of chromosome 3 in the Arabidopsis thaliana genome. MRF assay in diploid (grey), autotetraploid (yellow) and allotetraploid (blue) background. B and C denote the corresponding transgene-free location, D and E indicate the homoeologous positions in the Arabidopsis arenosa genome. The copy number is shown as number of letters for each position. Red arrow; yellow seeds carrying a single copy of each marker were selected; crossed circle - self-pollination; rec - reciprocal cross. (B) Examples for seed phenotypes obtained from mother plants with a single copy of the meiotic tester chromosome pollinated by wild type (female recombination). Scale bar, 1 mm.