Cotton Breeding for Fiber Quality Improvement - Industrial Crops: Breeding for BioEnergy and Bioproducts - page 192

respectively. These traits have revolutionized cotton pest and weed control and
made life considerably simpler for cotton growers. As of yet, there are no GM traits
for fiber quality that have been commercialized.
In this chapter, we will focus on the background, methods, and future opportu-
nities for simultaneously improving yield and fiber quality in Gossypium hirsutum
(“upland”) cotton. We will particularly highlight opportunities and potential for
integrating conventional breeding with future biotechnology traits and molecular
tools.

Taxonomy, Early Domestication, and Selection in Cotton

Cotton taxonomy has been widely studied since the mid-nineteenth century because
of its industrial importance with 50 or more Gossypium species believed to have
appeared 10-20 million years ago in three centers of diversity: Australia, Africa-
Arabia and, Central America [ 1 , 2 ]. Recent comprehensive reviews on history and
taxonomy of Gossypium species include Fryxell [ 3 , 4 ], Brubaker et al. [ 5 ], and
Percival et al. [ 6 ]. These reviews are recommended for more detail than presented
below.
Cotton is divided into eight diploid genome groups with 2 n 26 chromosomes

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plus five tetraploid species (2 n 52) (Table 10.1 ). The A genome diverged from

genomes B, E, and F, 4-9 million years ago in Africa-Arabia [ 2 ], and the two
important A genome species with spinnable fibers, G. arboreum and G. herbaceum
appeared over 1 million years ago [ 8 ]. The tetraploid species appeared in Mexico-
Guatemala, 1-2 million years ago from a chance hybridization and chromosome
doubling between A and D genome ancestors of G. arboreum and G. raimondii ,
respectively [ 9 ], and subsequently radiated into the five tetraploid species,
G. hirsutum , G. barbadense , G. mustelinum , G. darwinii , and G. tomentosum of
which only the first two are used in agriculture.
There are therefore only four species grown commercially because they produce
fibers long enough to be spun into yarn for textiles, and these are G. hirsutum
(upland cotton), G. barbadense (Pima, Egyptian or Sea Island cotton), G. arboreum
(desi cotton), and G. herbaceum (Levant or Arabian cotton). G. hirsutum , because
of its higher fiber yields and greater adaptability, is by far the most widely grown
with over 90 % of world production from this species, followed by G. barbadense
(6 %) and G. arboreum and G. herbaceum (less than 2 % each).
The cultivated diploid and tetraploid species differ significantly in their mor-
phology and agronomic performance, but also in their fibers, although that distinc-
tion is narrowing with focused genetic improvement of all commercial species.
Table 10.2 shows the typical fiber properties of the two cultivated tetraploid species
compared with fiber from the diploid G. arboreum . Although there is variation
between cultivars within each species, the shorter, weaker, and coarser fiber of
G. arboreum compared with the long fine fibers of G. barbadense dictates different

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