Biodiversity and the evolution of the cotton genome

The cotton genus (Gossypium) is remarkable for its extraordinary natural diversity as well as its importance to humankind. More than 50 species are recognized, including several recently described, which collectively are distributed in arid to semi-arid regions of the tropics and subtropics. Diversity in Gossypium has been promoted by two seemingly unlikely processes: trans-oceanic, long-distance dispersal, and wide hybridization among lineages that presently are widely separated geographically. Included are four species that were independently domesticated for their seed fiber, two diploids from Africa-Asia and two allopolyploids from the Americas. As Gossypium spread worldwide, it experienced remarkable morphological, cytogenetic and genomic diversification, spawning eight monophyletic groups of diploid (n = 13) species (“genome groups” A through G, and K) and 8 allopolyploids (n = 26; AD-genome), the latter resulting from an improbable trans-oceanic dispersal of an A-genome taxon to the New World 1-2 million years ago and subsequent hybridization with an indigenous D-genome diploid. The extraordinary diversity in the genus represents a largely untapped genomic reservoir for agronomic exploration. From a genomic perspective, cotton has a marvelously complex evolutionary history. We now understand that modern allopolyploid species are descended from diploid ancestors which themselves were once polyploid, with this cycle of polyploidization followed by diploidization being repeated many times. This history of “genomic superimpositions” will be described, as will some of the many implications that derive from this understanding.