ABSTRACT

Gossypium hirsutum has a large indigenous range encompassing most of Mesoamerica and the Caribbean, where it exhibits a diverse array of morphological forms spanning the wild-to-domesticated continuum. Modern, highly improved varieties ("Upland cotton") are day-length neutral annualized plants derived from sub-tropical, perennial antecedents. Molecular markers were employed to: (1) assess levels and patterns of genetic variation in the species, (2) elucidate the origin of Upland cotton, and (3) quantify the amount of genetic diversity contained in the gene pool of modern cultivars. 538 accessions representing the full spectrum of morphological and geographical diversity were analyzed for allozyme variation at 50 loci, and a subset of these (84) were also examined for variation at 205 RFLP loci. Genetic relationships among accessions collected from throughout the species' range are correlated with geography, although influences of post-Columbian germplasm exchange are evident in some cases. Two centers of genetic diversity are indicated, one in Mexico-Guatemala and the other in the Caribbean. In comparison with other plant species, G. hirsutum contains an average amount of genetic diversity; modern Upland cultivars, however, are genetically depauperate. The Upland gene pool appears largely to have been derived from Mexican highland stocks, which, in turn, were derived from material originally from southern Mexico and Guatemala. The majority of the genetic base that existed prior to the introduction of Mexican highland stocks has been lost. Introgression of G. barbadense genes into G. hirsutum has been common in a broad area of sympatry in the Caribbean, especially in race 'marie-galante'. Contrary to information derived from breeding history, the genetic data indicate that Upland cotton contains little introgressive material from any other species, suggesting that retention of genes from trans-specific sources has been minimal. The data also demonstrate that G. lanceolatum (= G. hirsutum race 'palmeri') should not be recognized as a distinct species.