ABSTRACT

The existence of a detailed RFLP map of the cotton genome (1) permits cotton geneticists to exploit methods devised in other species for mapping genes controlling agronomically important traits (2-5).

These new tools are being employed in two different approaches to mapping agronomically and economically important traits in cotton. One approach involves analysis of introgression from G. hirsutum into a total of 91 cotton cultivars and breeding lines generally classified as G. barbadense. All G. barbadense cultivars showed appreciable amounts of introgression from G. hirsutum, with most ranging from 10-20% G. hirsutum. Pima S2 showed was an exceptional case, with 83.9% of RFLP alleles showing the G. hirsutum genotype. The degree of G. hirsutum introgression into these genotypes shows significant negative correlation with both fiber length ("T1"; r=0.502) and fiber strength ("mean"; r=O.402) values published in the National Collection of Gossypium Germplasm (6).

Our second approach to identifying agriculturally important genes involves using a subset of the mapped cotton RFLPs to perform "QTL" (quantitative trait locus) mapping, as described in several recent investigations (3-5). Fiber quality parameters show a continuous distribution of phenotypes in crosses between Upland (G. hirsutum) and Pima (G. barbadense) cultivars. Using RFLPS, many of these phenotypes can be attributed to discrete Mendelian loci, and RFLP markers near these QTLs can be used to manipulate fiber quality and other parameters in breeding of improved cultivars. Marker-assisted selection has the potential to accelerate breeding progress, by permitting selections to be made at the seedling stage, and in environments where the trait is not expressed.

The utility of QTL mapping is not limited to fiber quality -- rather, it is applicable to any trait which can be measured reliably and which shows variation among different genotypes (2). Studies are in progress to map cotton genes affecting tolerance of various biotic and abiotic stresses, including insect damage. Beyond the immediate gains which can be accomplished by marker-assisted selection, RFLP mapping provides a starting point for potential "map-based cloning" of genes with extraordinary importance in cotton improvement (7).