Polyploidy and the Evolutionary History of Cotton

The cotton genus (Gossypium ) includes approximately 50 species distributed in arid to semi-arid regions of the tropic and subtropics. Included are four species that have independently been domesticated for their fiber, two each in Africa–Asia and the Americas. Gossypium species exhibit extraordinary morphological variation, ranging from herbaceous perennials to small trees with a diverse array of reproductive and vegetative characteristics. A parallel level of cytogenetic and genomic diversity has arisen during the global radiation of the genus, leading to the evolution of eight groups of diploid (n ¼ 13) species (genome groups A–G, and K). The evolutionary history of the genus included multiple episodes of trans-oceanic dispersal, invasion of new ecological niches, and a surprisingly high frequency of natural interspecific hybridization among lineages that are presently both geographically isolated and intersterile. Recent investigations have clarified many aspects of this history, including relationships within and among the eight genome groups, the domestication history of each of the four cultivated species, and the origin of the allopolyploid cottons. Data implicate an origin for Gossypium 5–15 million years ago (mya) and a rapid early diversification of the major genome groups. Allopolyploid cottons appear to have arisen within the last million years, as a consequence of trans-oceanic dispersal of an A-genome taxon to the New World followed by hybridization with an indigenous D-genome diploid. Subsequent to formation, allopolyploids radiated into three modern lineages, including those containing the commercially important species G. hirsutum and G. barbadense. Genome doubling has led to an array of molecular genetic interactions, including inter-locus concerted evolution, differential rates of genomic evolution, inter-genomic genetic transfer, and probable alterations in gene expression. The myriad underlying mechanisms are also suggested to have contributed to both ecological success and agronomic potential