Molecular Marker Diversity and Field Performance in Commercial Cotton Cultivars Evaluated in the Southwestern USA

Gutierrez et al., 2002; Lu and Myers, 2002). Genetic distances could be as low as 1 to 3%. Research in AustraGenetic diversity in modern upland cotton cultivars (Gossypium lia, China, and Pakistan obtained similar results (Multani hirsutum L.) is thought to be narrow, thus limiting genetic advance. Robust information on the genetic relatedness among currently grown and Lyon, 1995; Zuo et al., 2000; Rahman et al., 2002). cotton cultivars is lacking. The objectives of the present study were The hypothesized narrow genetic base of upland cotto field test a sample of elite commercial cotton cultivars, including ton germplasm used in breeding has been considered many transgenic cultivars representing the major cottonseed compaas one of the reasons contributing to the lack of progress nies, and to evaluate their genetic divergence using simple sequence in the improvement of cotton cultivars to meet the needs repeat (SSR) markers. Eighty-eight SSR primer pairs were chosen of cotton growers and industry in the USA during the for genotyping that provided 177 SSRs. Jaccard’s genetic similarity last 15 yr (Meredith, 2000; Lewis, 2001). A series of coefficients among 24 genotypes ranged from 0.694 to 0.936, with an studies on pedigrees, coefficients of parentage (CPs), average of 0.772, indicating that sufficient genetic diversity does exist and genetic diversity for 260 cotton cultivars released within our sample of commercial upland cotton. Genetic similarities among cultivars from the same seed companies were generally higher in the USA between 1970 and 1990 were conducted than the mean of all cultivars and grouped into six major groups: two (Bowman et al., 1996, 1997; May et al., 1995; van EsDeltapine (DP), one Stoneville (ST), one FiberMax (FM), and two broeck et al., 1998; Bowman et al., 2003). The mean CP New Mexico (NM) Acalas. One California Acala cultivar of New suggested wide genetic diversity among the 260 cultivars Mexico origin, developed by Phytogen (PHY), did not group with (Bowman et al., 1996), but that diversity was declining New Mexico Acala germplasm. Texas High Plains stripper type cultidue to the frequent use of a few parents combined with vars were distant from picker types and formed independent groups. reselection within cultivars and elite germplasm to deUnder New Mexico growing conditions, DP and ST cultivars yielded velop new cultivars (van Esbroeck et al., 1998). Surprishigher but produced lower fiber quality, while NM Acala cotton had ingly, the introduction of transgenic cotton cultivars has lower yield but higher fiber quality. The PHY and FM cultivars were intermediate in cotton yield and fiber quality. Six SSR markers were significantly reduced field genetic uniformity since the identified to be significantly correlated with fiber yield or quality percentage of the crop planted to a few cultivars has among the cultivars tested, providing impetus to validate the marker– declined (Bowman et al., 2003), even though all the trait associations. transgenic cotton cultivars were developed via backcrossing using popular nontransgenic cultivars as recurrent parents. However, multivariate analysis of agroH germplasm in cotton breeding had nomic and fiber traits of ancestral cultivars detected been openly shared between public and private high similarity (van Esbroeck et al., 1999), supporting breeders in the USA, but this exchange of germplasm the conclusion that modern cotton cultivars have a narhas been curtailed in recent years as public breeding row genetic base when evaluated with isozyme and programs were eliminated and seed companies assumed DNA markers (Wendel et al., 1992). This suggested that the nearly exclusive role of cultivar development. Prespedigree analysis may overestimate genetic distance ently, the focus of the few remaining public breeding among modern cultivars (van Esbroeck et al., 1999). programs has shifted to germplasm development and Therefore, genetic diversity among modern commercial developmental breeding, while commercial cultivars have cotton cultivars needs to be assessed using more prebeen solely released through seed companies. A number cise methods. of studies have suggested that cultivated upland cotton It is known that cotton cultivars released by different germplasm possesses a low level of genetic diversity when developers and adapted to the same region appear highly evaluated by isozymes, random amplified polymorphic morphologically similar, but they could perform very DNA (RAPD), amplified fragment length polymorphism differently from unadapted cultivars introduced from (AFLP), restricted fragment length polymorphism, and other regions or countries. For example, New Mexico SSRs (Wendel et al., 1992; Tatineni et al., 1996; Pilley Acala cultivars, known for their high fiber quality, good and Myers, 1999; Abdalla et al., 2001; Iqbal et al., 2001; Verticillium wilt (Verticillium dahliae Kleb) tolerance, and large boll size (Smith and Cothern, 1999), are J.F. Zhang and Y. Lu, Dep. of Agronomy and Horticulture, Box adapted to the southwestern growing region of the U.S. 30003, New Mexico State Univ., Las Cruces, NM 88003; R.G. Cantrell, Cotton Incorporated, 6399 Weston Parkway, Cary, NC 27531; E. Hughs, Cotton Belt. Even though they are very tall and late USDA-ARS, Southwestern Cotton Ginning Research Laboratory, maturing with low yield when grown in other regions Mesilla Park, NM 88003. Received 2 Oct. 2004. *Corresponding author ([email protected]). Abbreviations: AFLP, amplified fragment length polymorphism; CP, coefficient of parentage; DP, Deltapine; FM, FiberMax; JC, Jaccard’s Published in Crop Sci. 45:1483–1490 (2005). Crop Breeding, Genetics & Cytology coefficient; NM Acala, Acala of New Mexico origin; PHY, Phytogen; QTL, quantitative trait loci; RAPD, random amplified polymorphic doi:10.2135/cropsci2004.0581 © Crop Science Society of America DNA; SM, simple match coefficient; SSR, simple sequence repeat; ST, Stoneville. 677 S. Segoe Rd., Madison, WI 53711 USA 1483 Published online June 24, 2005