The Evolution of Cotton

Cotton was domesticated from wild ancestors

Like all modern crops, the cotton plants that grow on farms today are descended from species that lived in the wild—and still live in the wild today. These wild cousins are not all that different from domesticated cotton. They grow in warm, dry areas of the world, and they make a big poufy seed pods, filled with seeds that are covered with long, silky fibers.

Thousands of years ago, ancient people discovered that the fibers from wild cotton plants could be spun into ropes or yarn and woven into fabric, and they began farming cotton. As early farmers did with many types of crops, they took advantage of natural variations in the cotton plants. They noticed that some plants were more useful than others—maybe their fibers were longer or stronger, which made for a better yarn. Or maybe some produced bigger seed pods with more fibers. The farmers knew that traits passed through seeds from parent to offspring, so they collected seeds from the best plants and used them for the next year's crop. This process, known as selective breeding, gradually changed wild cotton into a domesticated form that was even more useful.

wild vs domestic

Domesticated cotton has more plentiful and longer fibers that are more easily removed from the seed than wild varieties. (Shown are domesticated upland cotton, cult. AD1, and its closest wild relative. Images courtesy Jonathan Wendel, used with permission.)

Cotton was domesticated in four places

map

Genetic and archaeological evidence suggests that the 4 commercially grown species of cotton—and possibly a few others—were domesticated independently in at least 4 different places around the world (see map). Because each group of people valued many of the same characteristics, all domesticated cotton varieties look similar. Yet each group began with a different wild ancestor, so each type of domesticated cotton has some unique characteristics—like the long, silky fibers of Pima cotton, and the abundant yields of upland cotton.

The mysterious genetics of cotton

Domesticated cotton species and their wild cousins all belong to the genus Gossypium. This genus has more than 50 members in all, which live all around the world. The members are very diverse, including tree-sized plants in Mexico, and small, shrubby plants with nearly bald seeds in Australia.

Way back in the old days, plant biologists began grouping plants into the Gossypium genus based on the characteristics that they had in common: similarities in their leaves, branching pattern, flowers, fruits. By the 1930s, DNA came into play. When geneticists started studying Gossypium chromosomes under the microscope, they noticed something curious. Though the chromosomes were often vastly different in size between species, most had 13 pairs of chromosomes. However, they found that a handful of species in the Americas (including upland and Pima cotton) had 26 pairs chromosomes—or double what is typical of the genus.

After further study, scientists learned that these species have two complete genomes. The genomes are very different in size, and they came from two parent species that are quite distantly related. Researchers wanted to know—when and how did these two parents, one native to Africa and the other to the Americas, get together?

chromosomes

Most Gossypium species have 13 pairs of chromosomes (26 in all). Upland and Pima cotton have 26 pairs of chromosomes (52 in all)—amounting to two complete genomes.

The natural history of cotton

Using a combination of genetic, fossil, and archaeological evidence, along with some deductive reasoning, scientists pieced together the most likely evolutionary history of cotton.

  • map 1
    All living Gossypium species share a common ancestor that lived in Africa about 5-10 million years ago.
  • map 2
    The seeds of this ancestor spread across land and water, establishing new populations.
  • map 3
    The descendants gradually changed over time into different species—including A-genome species in Africa and D-genome species in the Americas.
  • map 4
    After several million years of separation and independent evolution, an A-genome plant traveled across the ocean to the Americas.
  • map 5
    There, it interbred with a D-genome plant to make an AD genome hybrid—the ancestor to both upland and Pima cotton.

When A meets D: More is better

chromosomes

The A and D genomes of the two parent plants were similar enough that they could join together to make an offspring. But the genomes were too different to combine in the normal way, where one parent contributes one copy of each chromosome. Since the chromosomes were very different in size and had different arrangements of genes, a strange thing happened that plants sometimes do: the offspring ended up with all of the parents' chromosomes, which made it tetraploid. 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).

Importantly, rather than having traits that were somewhere in between, the AD offspring looked different from both of its parents. It was better-suited to the hot, dry environment along the coast. Unlike its parents, it could grow in sand, and it could withstand salt spray and flooding. Its salt-resistant seeds floated easily and took root in islands throughout the Caribbean, and even Hawaii and the Galapagos islands.

DNA evidence suggests that the A and D genomes got together just one time. The new tetraploid cotton quickly made its way over land and sea, diversifying into multiple species that were best suited to different habits. In the 1-2 million years since the A and D parents got together, the new AD offspring gave rise to at least 6 species. Two of those were shaped into the most highly produced domesticated species—upland and Pima cotton. These species' unique genetic heritage has led to some of their most desirable traits, most notably their abundant, high-quality fibers.

Related links

To learn more about cotton and some of the research that is being done on it, visit the lab websites of Dr. Joshua Udall and Dr. Jonathan Wendel.

References

References

Beasley, J. O. (1942). Meiotic chromosome behavior in species, species hybrids, haploids, and induced polyploids of Gossypium. Genetics, 27(1), 25-54.

Flagel, L. E., Wendel, J. F. & Udall, J. A. (2012). Duplicate gene evolution, homoeologous recombination, and transcriptome characterization in allopolyploid cotton. BMC Genomics, 13:302. doi: 10.1186/1471-2164-13-302

National Cotton Council of America (n.d.). The World of Cotton. Retrieved December 7, 2016, from http://www.cotton.org/econ/world/

National Cotton Council of America (2016). Production & Acreage Information. Retrieved December 7, 2016, from http://www.cotton.org/econ/cropinfo/production/index.cfm

Wendel, J. F. & Grover, C. E. (2015). Taxonomy and evolution of the cotton genus, Gossypium. In Fang, D. D. & Percy, R. G. (Eds.) Cotton (pp. 25-44). Madison, WI: American Society of Agronomy, Crop Science Society of America, and Soil Science Society of America, Inc.


APA format:

Genetic Science Learning Center. (2010, December 9) The Evolution of Cotton. Retrieved December 14, 2017, from http://learn.genetics.utah.edu/content/cotton/evolution/

CSE format:

The Evolution of Cotton [Internet]. Salt Lake City (UT): Genetic Science Learning Center; 2010 [cited 2017 Dec 14] Available from http://learn.genetics.utah.edu/content/cotton/evolution/

Chicago format:

Genetic Science Learning Center. "The Evolution of Cotton." Learn.Genetics. December 9, 2010. Accessed December 14, 2017. http://learn.genetics.utah.edu/content/cotton/evolution/.