Genetics of Cotton

At least seven genomes, designated A, B, C, D, E, F, and G, are found in the genus. 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 of ancient cross between Old World A genomes and New World D genomes. How and when the original crosses occurred have been subject to much speculation. 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, a native of Hawaii. Gossypium 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 sandy habitats and associated environs. It is from these "weedy" or invader species that the cultivated cottons developed.

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