Abstract
The eighteen species studied form an allopolyploid series (x=18). The morphology, crossing behavior, and geographical distribution of 6 diploid, 9 tetraploid, 2 octoploid, and 1 decaploid species were studied. From over 26,500 crosses, 19 hybrid combinations and several derived allopolyploids and three-species hybrids were obtained. Chromosome pairing in the hybrids showed that a minimum of 6 and a maximum of 14 well-differentiated genome groups exist in sect. Furcaia, at least two of which appear to be confined to the Old World. No evidence was found that New World genomes are represented in the Old World. The primary radiation of the diploid genomes probably occurred at about the same time as that of the diploid genomes of Gossypium, whereas the tetraploids and one of the octoploid species (H. furcatus Roxb., non Willd.) seem to be of later origin (late Pleistocene or Recent). Octoploid H. diversifolius Jacq., a circumtropical species, may be a relict of a much earlier round of polyploid evolution.
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