More than 33 million hectares of agricultural land are used to grow cotton. The value of cotton lies in the fact that it produces a smooth, spinable fibre. These fibres are the remains of the cellulose-rich walls that once surrounded giant hair cells that develop on the surface of cotton seeds.
In cultivated cottons these fibres are very long, forming clumps of 'cotton wool' when fruits open, but they are much shorter in their wild ancestors, where they only form a fuzzy covering on the seed surface. During the domestication process, humans selected rare mutant forms with long staple fibres.
Cultivated cottons evolved in the New World: Gossypium hirsutum in Central and South America and Gossypium barbadense in the Caribbean. Most cotton species have 13 pairs of chromosomes. These plants are said to be diploid; they have two of each chromosome. Diploid cottons are found in both the Old World (Asia and Africa) and the New World (North and South America).
Cultivated cottons have twice the normal chromosome number and are therefore said to be tetraploid (26 chromosome pairs). By imaging chromosomes it was clear that these tetraploid cottons are derived from a hybrid that inherited a set of large chromosomes from one cotton species and a set of smaller chromosomes from another.
The identity of the male parent was discovered over 60 years ago as a close relative of Gossypium raimondii from Peru, because its leaves and leaf hairs are most similar to those found on cultivated cottons.
To identify the female parent, the sequence of the DNA in the photosynthetic organelle, called the chloroplast, inside the plant cell was analyzed. The DNA in chloroplasts is only inherited from the female parent. By comparing the chloroplast DNA of different species, it was possible to identify the mother of all cultivated cottons. Surprisingly, the sequence of the chloroplast DNA in cultivated cotton from the New World was found to be most similar to the chloroplast DNA of Gossypium herbaceum from the Old World.
Taken together these analyses demonstrate that an Old World cotton, a close relative of the modern day Gossypium herbaceum (female parent), migrated to the New World and hybridized with Gossypium raimondii (male parent) between one and two million years ago.
None of the descendants of the mysterious female survived except the cultivated cottons, which were subsequently domesticated many times by the indigenous cultures across the New World.
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Wendel JF 1989. New World tetraploid cottons contain Old World cytoplasm. Proceedings of the National Academy of Sciences of the United States of America 86, 4132-4136.