Abstract Back to Table of contents
Molecular markers (MM) are a new genetic tool which have the potential to enhance selection efficiency in cotton (Gossypium species) breeding. There are about 80 morphological traits that have been used to identify 17 linkage groups in G. hirsutum L. Besides not producing a complete linkage map, most of these morphological markers have major effects on other important quantitative traits and have very limited usefulness in selection. In many cases the heterozygous condition is not identifiable. In contrast, MM have the potential of being available in very large numbers; they generally have no effect in themselves on the phenotype, and the heterozygote of MM can be identified. The three most used MM are RAPDs, isozymes, and RFLPs. Each MM has its place as determined by the following factors: research objective, laboratory accessibility, detectable genetic variability, repeatability, co-dominance expression and cost.
The association of RFLPs with 11 quantitative traits in a population of 124 F2/F3 progenies from the cross of ‘Prema” X MD5678ne was evaluated at Stoneville, MS. Prema is a Western USA cultivar that has low yield and high fibre strength and fineness when grown in the Mississippi Delta. In contrast, MD5678ne is a near isoline of ‘DES56’, a Delta cultivar, which has high yield and low fiber strength and fineness when grown in the Delta. The progenies were grown at two Delta locations with two replications each in 1991. A total of 113 RFLPs exhibiting polymorphism for 118 loci associated with yield, yield components, and fiber properties. Significant associations, P£0.05, were detected for all traits. There were 13 significant RFLP effects for yield, which individually accounted for 4.2 to 14.2% of the yield variation. Significant over-dominance and dominance effects for yield were detected for one and four RFLPs, respectively. Eight significant positive effects for yield were associated with bands from the Delta parent and five from Prema, indicating some useful yield genes could be obtained from Prema to increase yield in Delta cottons. About 38 RFLPs were associated significantly with fiber strength and fineness, and 90% of the positive effects were from the Prema parent.
Selection procedures for breeding improvement range from simple backcrossing to multivariate and selection indices. Once chromosome regions are identified fro important quantitative traits, MM should be a tremendous aid in selecting parents. Also, MM can be useful in “fingerprinting” cultivars and detecting variability within and among cultivars and genetic populations.
Molecular markers are a valuable new tool which holds much promise of being used in future cotton genetic-breeding investigations. It offers a relatively simple method to trace genetic sources of useful variability. Specific chromosome regions with important QTL can be identified and appropriate selection strategies developed. MM have been used to describe the genomic relationships of Gossypium species and describe the genetic structure of USA cultivars. MM were not a good tool to select good combiners in hybrid studies. MM have been shown to be closely associated with QTL with major effects. Limitations to MM use are cost, availability of quality MM, a G. hirsutum x G. hirsutum map, appropriate software, and the polyploid nature of cotton. Use of MM in cotton is in its infancy and as more research is conducted many of the limitations will be reduced.
Back to Table of contents