Nucleotide diversity in the two co-resident genomes of allopolyploid cotton

Genetic diversity within and among populations lies at the heart of evolution. Unraveling the extent to which each intrinsic or extrinsic factor determines levels of diversity among genes, populations, and species is challenging, given the difficulty of isolating any single potentially important variable from all others. Allopolyploid species provide an opportunity to disentangle external and intrinsic factors, as the two (or more) homoeologous genomes co-occur in the same nucleus, often exhibiting high collinearity along homoeologous chromosomes. Here we evaluate the pace of molecular evolution and intraspecific, intragenomic diversity in two species of allopolyploid Gossypium, G. hirsutum and G. barbadense, using several hundred genes sequenced from multiple accessions of each species. Genic diversity in both species is low, having been influenced both by the polyploid bottleneck and a domestication bottleneck (for cultivated accessions), but with a directional bias in homoeolog diversity favoring the same genome in both allopolyploids. Total diversity is remarkably similar for the two homoeologous genomes overall, but the two copies of many gene pairs have accumulated statistically different diversity levels, and in a biased fashion with respect to genome. Domesticated accessions show reduced diversity in both genomes, as expected, but with a much greater reduction in one of the two homoeologous genomes. Furthermore, this biased reduction affects opposite homoeologous genomes in the two species. Interspecific introgression has played a role in shaping diversity within each species. Introgression was only detected for certain accessions, and only from G. barbadense into G. hirsutum in one of the two co-resident genomes.