The Evolution of Cotton (2024)

The A and D genomes of the two parent plants were similar enough that they could join together. But they couldn't combine in the normal way, where one parent contributes one copy of each chromosome. The chromosomes were too different in size, and their genes were arranged in a different order. So a strange thing happened that plants sometimes do. The offspring ended up with all of the parents' chromosomes. It had two copies of the entire A genome, plus two copies of the entire D genome. Instead of having two copies of each gene (one from each parent), it had four (two from each parent). In other words, it was tetraploid.

Importantly, the new AD offspring's traits were not a blend of its parents'. It was different from both parents, and better-suited to the hot, dry environment along the coast. Unlike either parent, it could grow in sand, and it could withstand salt spray and flooding. Its salt-resistant seeds floated easily. They took root on islands throughout the Caribbean, and even Hawaii and the Galapagos.

DNA evidence suggests that the A and D genomes got together just one time. Then over 1–2 million years, it diversified into at least 6 species, each suited to a different habit. Two were later shaped into the most highly produced domesticated species—upland and Pima cotton. These species' unique genetic heritage has led to some of their best traits, most notably their abundant, high-quality fibers.