¹ Department of Botany, Iowa State University, Ames, IA 50011 USA

² US Forest Service, Pacific NW Research Station, 3200 SW Jefferson Way, Corvallis, OR 97331

The cotton genus (Gossypium) includes approximately 50 species with a global distribution in arid to semi-arid regions of the tropic and subtropics. Gossypium species exhibit extraordinary morphological diversity, ranging from herbaceous perennials to small trees and having a great variety of floral and vegetative features. A parallel level of cytogenetic and genomic diversity has evolved during the global radiation of the genus. Four species have independently been domesticated for their fiber, two each in the Old and New World. Recent molecular phylogenetic investigations have elucidated many aspects of the evolutionary history of Gossypium, including its origin within the small Malvaceous tribe Gossypieae. These data show that Gossypium is monophyletic and phylogenetically sister to the Hawaiian genus Kokia and the east African/Madagascan genus Gossypioides. Based on molecular clock calculations, we suggest that Kokia and Gossypioides diverged from each other in the Pliocene. Phylogenetic relationships among species and genome groups in Gossypium have been assessed using multiple data sets based on both nuclear and cpDNA sequences. These data confirm the monophyly of each genome group, with the exception of some taxa that have reticulate evolutionary histories. Our analysis suggests that the genus originated 5 - 15 million years ago, and that the major genome groups arose in rapid succession following formation of the genus. The evolutionary history of the genus has included multiple episodes of trans-oceanic dispersal, invasion of new ecological niches, acquisition of specialized reproductive syndromes, and a surprisingly high frequency of natural interspecific hybridization among lineages that presently are inter-sterile.