The Extraordinary Evolutionary History and Natural Diversity of the Cotton Genus, Gossypium.

Abstract. The cotton genus (Gossypium) is remarkable both for its extraordinary natural diversity and its importance to humankind. The genus contains more than 50 recognized species, including several recently described, 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. This repeated domestication of different wild progenitors represents a remarkable case of human-driven parallel evolution. Morphological variation in Gossypium is extensive; growth forms in the genus range from sprawling herbaceous perennials to ~15 m tall trees, representing a notable array of reproductive and vegetative characteristics. Equally impressive is the striking cytogenetic and genomic diversity that emerged as Gossypium diversified and spread worldwide, ultimately spawning eight groups of closely related diploid (n = 13) species (i.e., genome groups A through G, and K). DNA sequence data place the origin of Gossypium at about 5-10 million years ago, after which it rapidly diversified into the major genome groups presently recognized. Allopolyploid cottons originated within the last 1-2 million years, a consequence of an improbable trans-oceanic dispersal of an A-genome taxon to the New World and subsequent hybridization with an indigenous D-genome diploid. Diversification of the nascent allopolyploid gave rise to three modern lineages containing seven species, including the agronomically important G. hirsutum and G. barbadense. The extraordinary diversity in the genus represents a largely untapped genomic reservoir for agronomic exploration.