1 Taxonomy and Evolution of the Cotton Genus

We present an overview of the taxonomy of Gossypium L. (the cotton genus) and its evolutionary history. Gossypium contains more than 50 recognized species, including several recently described, distributed in arid to semiarid regions of the tropics and subtropics. Diversity in Gossypium has been promoted by two seemingly unlikely pro-cesses: transoceanic, 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. Mor-phological 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 cyto-genetic 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 Gossy-pium at about 5 to 10 million years ago (mya), which rapidly diversified into these major genome groups shortly thereafter. Allopolyploid cottons appeared within the last 1 to 2 million years, a consequence of the improbable transoceanic 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 L. and G. barbadense L. Because of its economic importance, the cotton genus (Gossypium L.) has been of interest to agricultural scientists, taxonomists, and many other kinds of biologists. Accordingly, a considerable amount is understood regarding the ori-gin and diversification of the genus, its basic plant biology, and its properties a