Chromosome structural changes in diploid and tetraploid A genomes of Gossypium

The genus Gossypium, which comprises a divergent group of diploid species and several recently formed allotetraploids, offers an excellent opportunity to study polyploid genome evolution. In this study, chromosome structural variation among the A, A(t), and D genomes of Gossypium was evaluated by comparative genetic linkage mapping. We constructed a fully resolved RFLP linkage map for the diploid A genome consisting of 275 loci using an F2 interspecific Gossypium arboreum x Gossypium herbaceum family. The 13 chromosomes of the A genome are represented by 12 large linkage groups in our map, reflecting an expected interchromosomal translocation between G. arboreum and G. herbaceum. The A-genome chromosomes are largely collinear with the D genomes, save for a few small inversions. Although the 2 diploid mapping parents represent the closest living relatives of the allotetraploid A(t)-genome progenitor, 2 translocations and 7 inversions were observed between the A and A(t) genomes. The recombination rates are similar between the 2 diploid genomes; however, the A(t) genome shows a 93% increase in recombination relative to its diploid progenitors. Elevated recombination in the D(t) genome was reported previously. These data on the A(t) genome thus indicate that elevated recombination was a general property of allotetraploidy in cotton.