Gerald Myers | 10/26/2004 10:25:00 PM
Gerald O. Myers and Muhanad Akash
Cotton is the most important textile fiber crop and the world’s second-most important oil-seed crop after soybeans. Cotton is grown commercially in the temperate and tropical regions of more than 50 countries. In the United States, cotton is a major agricultural crop and was grown on more than 12.2 million acres in 2002. In Louisiana in 2002, cotton was produced on about 490,000 acres, which was well below the 50-year average of about 680,000 acres.
Although traditional plant improvement efforts have successfully modified the crop to meet the needs of both producers and consumers, genetic engineering has been used to address several important pest problems such as weeds and lepidopterous (caterpillar) pests. Future improvements in cotton will depend upon the concerted application of traditional plant breeding, genetic engineering and molecular genetic tools to increase yield and fiber quality. Yield remains the single most important criterion for the development of new varieties in modern cotton improvement programs, yet modern spinning technologies require that similar efforts be placed upon improving fiber quality.
Fiber quality is evaluated by a combination of traits: micronaire (fiber maturity), length (longer fiber can be spun into finer yarn), strength, elongation (elasticity) and uniformity. Genetic linkage maps are essential to locate the genes involved in the expression of these traits. This can easily be done for simple heritable traits based on one gene, but it is also possible for complex traits based on more genes (quantitative trait loci or QTL).
QTL involved in the expression of the five standard fiber quality traits have been identified by means of amplified fragment length polymorphisms (AFLP) markers. This is the marker system of choice because of the low amount of polymorphism (variation) detectable by other DNA marker technologies.
The initial step to identifying QTL for fiber quality traits involved the construction of a genetic linkage map in upland cotton. Upland cotton is the type grown in most of the United States, and the only type grown in Louisiana. In 2001, this was done using AFLP technology from a cross between a high fiber quality line, PeeDee 2165 and Paymaster 54, a historically important high-yielding cotton variety.
In 2002, these upland cotton plants were grown at the Dean Lee Research Station in Alexandria, La., and the Ben Hur Research Farm in Baton Rouge, La., to collect fiber for analysis. The fiber was analyzed using both interval analysis and composite interval analysis methods to detect QTL for the five fiber quality traits and to place them on the previously generated AFLP linkage map. Multiple QTL methods were used to eliminate any biased estimates of effects of size and location that can be introduced by using single QTL methods.
The genetic linkage map generated contained 143 AFLP markers assigned to 13 major and 15 minor linkage groups and covered 37.7 percent of the cotton genome. Linkage analysis between these markers and the studied fiber quality traits indicated the following, in brief:
One QTL for fiber elongation explained approximately 50 percent of the variation.
Five QTL for fiber length were detected via interval analysis and seven via composite interval analysis. Only one of these QTL was detected by both methods and had a major impact on fiber length.
Two marker intervals were detected as being significant by both interval mapping and composite interval mapping, with one explaining about 18 percent and the other about 53 percent of the variation in fiber uniformity.
Eleven QTL for fiber strength were detected, five by interval mapping and six by composite interval mapping. Of these, three QTL for fiber strength were detected by both methods and explained from 14 percent to 31.4 percent of the observed variation. This number of QTL for fiber strength was in close agreement with previous studies.
QTL detected for micronaire numbered three by interval mapping and six by composite interval mapping. A comparison of the two mapping methods discovered two QTL common to both. One of these had a relatively minor effect, explaining about 9 percent of the variation, whereas the other had a more pronounced effect, explaining about 29 percent of the variation observed.
The results from these experiments confirm the quantitative nature of inheritance for these five major fiber quality traits. As a consequence, the improve-ment of cotton for these characteristics cannot be easily transferred from one variety to another and will take time. The detection of a few QTL that had a relatively larger effect on fiber quality traits makes these prime candidates for further investigation, primarily through the development of more detailed genetic linkage maps. If these QTL can then be located physically, then marker-assisted selection to effect more rapid advances in cotton fiber quality will be possible.
(This article was published in the fall 2003 issue of Louisiana Agriculture.)