Cotton Breeding for Fiber Quality Improvement - Industrial Crops: Breeding for BioEnergy and Bioproducts - page 201

cultivars or closer relatives. The nectariless trait, for example, has been introgressed
from G. tomentosum [ 38 ].
The secondary gene pool for G. hirsutum cotton is represented by a number of
diploids with compatible A, D, B, or F genomes, and there are ongoing efforts in a
number of public and private programs to introgress unique pest or disease resis-
tance from these species via the synthetic tetraploid route or via synthetic hexaploid
bridging species [ 6 ]. Such approaches require substantial investment in time and
resources (often 10-15 years) to achieve a usable outcome, but there are a number
of successful examples, including cytoplasmic male sterility from G. harknessii
[ 39 ] and reniform nematode resistance from G. longicalyx [ 40 ]. Beasley sourced
improvements in cotton fiber strength (reportedly from the D genome contributor),
with his triple hybrid (( G. thurberi

G. hirsutum) that has made a
contribution to the high fiber strength of modern cultivars. The use of molecular
markers to accelerate the introgression of traits from these more distant sources is
clearly warranted.
The tertiary gene pool consists of those other diploid species with a completely
different genome type such as C, E, G, or K that show relatively poor or no
recombination with the A or D genome. These include a number of the
Australian endemic species some of which have unique traits such as glandless
seed-glanded plant that have been the focus of introgression from the C-genome
species G. sturtianum [ 41 ], although without much success and suggest that, in the
long run, GM approaches may be more useful for transferring such traits into the
cultivated species.
Finally, modern biotechnology, sometimes dubbed the quaternary gene pool,
allows the transfer of genes from any organism, for example, Bt cotton expressing
the insecticidal genes from the bacterium Bacillus thuringiensis . Such techniques
can also be used to transfer any trait identified in any plant, including other sexually
or nonsexually compatible Gossypium species (especially tertiary but also second-
ary gene pools), into the cultivated forms of cotton, regardless of any ability to
generate a viable hybrid between those species. This process is still considered
recombinant DNA by GM regulators, so it would still carry a large cost for
registration of the traits in global markets, but perhaps lower than for genes from
non-plant sources. To this time biotechnology traits have, however, been limited to
high-value trans-kingdom transgenes, such as pest and herbicide tolerance, so the
likelihood of plant to plant gene transfers being commercialized will depend on the
ability of the biotech developers to recapture their investments through trait licenses
to seed companies and cotton growers.
Mutants have played a significant part in increasing our understanding of gene
action and in dissecting biochemical and developmental pathways in model plants
such as Arabidopsis and rice and are also being used in cotton. Natural mutations in
fiber development, for example, have been critical to the discovery of the key
regulatory genes in fiber initiation through genome scale gene expression compar-
isons between fiberless mutants and wild-type cotton seeds [ 42 ]. While induced
mutation in breeding was a fad of the nuclear age and did deliver some variation to
breeding programs,

G. arboreum)

it was never widely successful. However, with the new