Medieval legend in Europe spoke of a strange animal that could supposedly be found far off in central Asia: the vegetable lamb. According to legend, this was an animal much like an ordinary sheep except that it grew directly from a plant, to which it remained attached by the umbilical cord. The vegetable lamb would sustain itself by grazing on nearby vegetation but when this was depleted, as the lamb could not move away from the plant to which it was attached, the lamb would die. How such a pointlessly self-defeating organism was supposed to persist does not appear to have concerned the medieval lexicographers; presumably it was supposed to be allegorical of something.
Part of the reason for the legend's persistence, however, was that there was indeed a form of 'wool' that came from a plant: cotton. The cotton genus Gossypium comprises about fifty species found in tropical and subtropical regions around the world (Wendel et al. 2010). Members of the genus vary from herbaceous perennials to small trees. The genus is divided into four subgenera, most of which are geographically distinct. The subgenus Gossypium is found in Africa and Arabia, subgenus Sturtia in Australia, and subgenus Houzingenia in the Americas. These three subgenera between them include the diploid cotton species; the fourth subgenus Karpas is also found in the Americas but differs in containing tetraploid species. Genetic evidence indicates that the subgenus Karpas arose at some point in the very recent past (within the last one or two million years) from a single hybridisation event between a species of subgenus Gossypium and one of Houzingenia, probably as a result of some chance dispersal event from Africa. Gossypium seeds seem well suited to dispersal: seeds of the Hawaiian Island species G. tomentosum have apparently germinated after being kept immersed in artificial seawater for three years (Wendel et al. 2010)! This same predicection for dispersal has resulted in the tetraploid species rapidly becoming widespread despite their recent origin, and in producing two species in remote locales: the Hawaiian G. tomentosum is directly related to the mainland G. hirsutum, while the Galapagos G. darwinii is sister to the mainland G. barbadense.
Commercial cotton is grown from four species of Gossypium, which may have each been domesticated independently in prehistoric times. All Gossypium species produce seeds with a covering of fuzzy hairs, but seeds of the two Old World diploid species G. herbaceum and G. arboreum also possess an outer layer of longer, flatter hairs that can be woven into thread. It was one of these two species, or possibly some now-extinct close relative, that made the crossing over the Atlantic to become one ancestor of the tetraploid species; as a result, the tetraploid species also possess these long outer hairs. Two of the tetraploid species, G. barbadense and G. hirsutum, were also domesticated, and the latter of these is now by far the most abundant cotton species in cultivation*.
*In case you were wondering, no-one seems to have suggested that the island species related to the two American domesticates might have been human-dispersed.
Other diploid Gossypium species do not possess this longer outer hair layer, only the inner short layer, and are not sources of commercial cotton (though hybrids with some of these species have been used to breed desirable genetic traits into the commercial species). In one group of Australian species (the section Grandicalyx) found in the Kimberley region of northern Western Australia, the hair layer has become very sparse and the seeds are almost hairless. These seeds also possess fatty bodies called eliosomes that are attractive to ants, and the plants are dispersed by having hungry ants carry their seeds away. Grandicalyx species are seasonal herbs, dying off above ground during droughts only to resprout from their thick root-stock. Other Australian species include the Sturt's desert rose Gossypium sturtianum, the floral emblem of Australia's Northern Territory.
As with other plant groups, hybridisation appears to have been a recurring factor in the evolution of Gossypium. The diploid Gossypium species have been divided between eight genome groups, hybrids between which are generally not viable (though not unknown: the parents of the tetraploid lineage, for instance, belonged to separate groups). However, genetic studies of some Gossypium species have identified discrepancies where a species may possess the nuclear genome of one group, but the chloroplast genome of another. For instance, the North American species G. gossypioides resembles other New World species in its nuclear genome, but has chloroplasts related to those of G. herbaceum or G. arboreum (which it may have acquired as a result of the same hybridisation event that produced the tetraploid species*). This phenomenon, which has been called cytoplasmic introgression, may have arisen in cotton through a process called semigamy. Semigamy is a particular form of apomixis (reproduction without fertilisation) in which sperm and egg cells fuse cytoplasmically, but their nuclei remain distinct (Curtiss et al. 2011). These nuclei will eventually be segregated by cell division, resulting in offspring that are mosaics of male- and female-line genomes. Over time, selection or drift may produce a homogenous population that retains the nuclear genome of one ancestor, but the cytoplasmic heritage of the other.
*The American parent of the tetraploids has more usually been identified as G. raimondii, a South American species, but G. raimondii is the direct sister species of G. gossypioides. It may be that G. gossypioides is the true parent of the tetraploids, or it may be that it too is derived from G. raimondii or its parent stock).
REFERENCES
Curtiss, J., L. Rodriguez-Uribe, J. McD. Stewart & J. Zhang. 2011. Identification of differentially expressed genes associated with semigamy in Pima cotton (Gossypium barbadense L.) through comparative microarray analysis. BMC Plant Biology 11: 49.
Wendel, J. F., C. L. Brubaker & T. Seelanan. 2010. The origin and evolution of Gossypium. In: Stewart, J. McD., D. Oosterhuis, J. J. Heitholt & J. R. Mauney (eds) Physiology of Cotton pp. 1-18. Springer.
Great article, Christopher!
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