G. hirsutum Texas Maker-1 (TM-1) and Its Isogenic Lintless Mutants
The cotton genus Gossypium includes about 50 species of two ploidy levels, 2n = 26 or 52. Species with n=13 occur naturally in many tropical and subtropical regions, but genomic groups occur in a geographically related manner. For example, A and D groups originated in Africa and the New World, respectively. The most extensively cultivated cotton species, G. hirsutum (upland or American cotton) and G. barbadense (Egyptian cotton) originated 1-2 million years ago as allotetraploids containing genomes related to those now found in G. herbaceum (A1 genome) and G. raimondii (D5 genome) or perhaps G. gossypioides (D6) (Beasley, 1940; Wendel et al., 1995). This polyploidization is estimated to have occurred 1-2 million years ago. Thus, Gossypium is extremely well-suited to the study of gene evolution and genome duplication (Wendel, 2000), as well as processes involved in diploidization.
Upland or American cotton (G. hirsutum L.) and Egyptian cotton (G, barbadense L.) have distinct fiber quality and physiology. G. hirsutum L. is most widely cultivated because of its wide range of adaptation and high yield potentials, whereas G. barbadense L. has superior and unique fiber quality. For example, American cotton fibers typically range in length from 25 to 34 mm; Egyptian cotton produces fibers over 60 mm.
G. hirsutum L. cv. TM-1 is an elite inbred line (>S40) derived from the obsolete cultivar Deltapine 14. It has been widely used in research programs, isoline development, genetic and physical mapping, and QTL analysis. Several qualitative mutants in fiber development were reported (Kohel et al., 2002). The best characterized are the naked seed alleles (or fiberless mutants), N1 and n2. These mutants, discovered decades ago, lack all or most of expected fuzz and lint. N1 is phenotypically slightly more extreme and more consistent than n2, which is more susceptible to genotypic and environmental effects. Their similar phenotypic effects and linkage relationship on two homoeologous chromosomes (12 and 26) suggest that they are homoeologous loci, each is thought to be about 15 cM from its respective centromere (Samora et al., 1994). Interestingly, N1 is a dominant mutation, whereas n2 is recessive. Mutant isogenic lines have been produced by backcrossing >6 generations using TM-1 as the recurrent parent. An additional class of mutants (Kohel, 1972) is known as the lintless mutants. Ligon lintless (Li1) mutant produces very short fibers (ca. 6 mm). A mutant with normal plants but short fibers resembling Li1 was designated Ligon lintless-2 or Li2 (Kohel et al., 1992). The locus is independent of Li1 but similar fiber phenotype. Notably, Li1 also induces many phenotypic changes and pleiotropic effects on plant development. Based on the timing of fiber initiation in Li1, both lint and fuzz fibers are produced (Triplett et al., 1989). Another mutant, immature fiber (im), produces bolls that open prematurely. Fibers of the im mutant have reduced secondary wall development.
