The most commonly cultivated species is G. hirsutum L. Other cultivated species are G. arboreum L., G. barbadense L., and G. herbaceum L.
Four species of Gossypium occur in the United States (Fryxell, 1979; Kartesz and Kartesz, 1980). G. hirsutum is the primary cultivated cotton. G. barbadense is also cultivated. The other two species, G. thurberi Todaro and G. tomentosum Nuttall ex Seemann, are wild plants of Arizona and Hawaii, respectively. G. tomentosum is known from a few isolated locations very close to the ocean.
C. Genetics of Cotton
At least seven complete sets of genes, designated A, B, C, D, E, F, and G, are found in the genus (Endrizzi, 1984). Diploid species (2n=26) are found on all continents, and a few are of some agricultural importance. The A genome is restricted in diploids to two species (G. arboreum and G. herbaceum) of the Old World. The D genome is restricted in diploids to some species of the New World, such as G. thurberi.
By far the most important agricultural cottons are G. hirsutum and G. barbadense. These are both allotetraploids of New World origin and presumably resulted from an ancient cross between Old World A genomes and New World D genomes. How and when the original crosses occurred are speculative. Euploids of these plants have 52 somatic chromosomes and are frequently designated as AADD. Four additional New World allotetraploids occur in the genus, including G. tomentosum, the native of Hawaii. G. tomentosum has been crossed with G. hirsutum in breeding programs.
The New World allotetraploids are peculiar in the genus because the species, at least in their wild forms, grow near the ocean as invaders in the constantly disturbed habitats of strand and associated environs. It is from these “weedy” or invader species that the cultivated cottons developed (Fryxell, 1979).
SAMPLE PETITION—Herbicide-Tolerant Plants