THE ORIgIn anD EvOlUTIOn OF GOSSYPIUM

The genus Gossypium has a long history of taxonomic and evolutionary study. Much of this attention has been stimulated by the fact that the genus includes four domesticated species, the New World allopolyploids G. hirsutum and G. barbadense (2n = 52), and the Old World diploids G. arboreum and G. herbaceum (2n = 26). These cultivated species embody considerable genetic diversity, but this diversity is dwarfed by that included in the genus as a whole, whose 50 species have an aggregate geographic range that encompasses most tropical and subtropical regions of the world. A remarkable morphological diversification accompanied the global radiation of Gossypium in response to the demands of particular ecological settings and selective environments. Plant habit, for example, ranges from fire-adapted, herbaceous perennials in northwest Australia to small trees in southwest Mexico that “escape” the dry season by dropping their leaves. Corolla colors embrace a rainbow of mauves and pinks (“Sturt”s Desert Rose”, G. sturtianum, is the official floral emblem of the Northern Territory, Australia), whites and pale yellows (Mexico, AfricaArabia) and even a deep sulphur-yellow (G. tomentosum from Hawaii). Seed coverings range from nearly glabrous (e.g., G. klotzschianum and G. davidsonii), to short, stiff, dense brown hairs that aid in wind-dispersal (G. australe, G. nelsonii), to long, fine white fibers that characterize highly improved forms of the four cultivated species. There are even seeds that produce fat bodies to facilitate ant-dispersal (section Grandicalyx cottons from NW Australia). Much of this morphological diversity is lucidly detailed in Fryxell Chapter 1