**1. Introduction**

Global competition of cotton fibers has changed the U.S. cotton industry in the last decade from a mainly domestic consumer to exporter of cotton fibers in the world market. In 2012, the U.S. produced 17.3 million bales of cotton, 14.4 % of the world production. Only 3.4 million bales, 19.6 % of the total production, were used in the domestic textile industry with 13 million bales exported (USDA-FAS, 2013). Foreign customers demand stronger, longer and more uniform fibers, and less short fiber and impurity contents of raw fibers than domestic markets. Meanwhile, domestic surviving textile industries raised spinning speed by updating to modern high speed spinners in order to improve their competitive ability in the global market. Fiber quality standards were also raised in order to operate at maximum efficiency using the modern spinners. According to Estur (2004), the modern high-speed spinners require micron‐ aire between 3.8 and 4.4, minimum 27.4 mm for 2.5% span length, 28 g/tex for strength, and 6 % for elongation, and maximum of 5 % short fiber content and 15/gram of seed coat fragments, and at least 83 % for length uniformity ratio. Currently, high yield U.S. cultivars of Upland cotton still lack sufficient fiber quality to fully meet these industry requirements. On the other hand, increasing lint yield in Upland cotton cultivars is always a top priority to keep profit for cotton growers. Increases of lint yield in U.S. cotton cultivars reached a plateau in the last two decades (Helms, 2000; Meredith, 2000; Gingle et al., 2006). Cotton yield peaked in the U.S. in 1992 and declined of an annual rate of 3.3 % in the next seven years (Helms, 2000). Furthermore, yearly fluctuation of yield increased 4 times from periods of 1960-1979 to 1980-1998 (Gingle et

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al., 2006). It is a great challenge for U.S. cotton breeders to simultaneously improve lint yield and fiber quality because of negative associations between yield traits and fiber quality (Miller and Rawlings, 1967; Smith and Goyle, 1997).

been attributed to alleles from Pee Dee and Acala germplasm populations (Bowman and

Broadening the Genetic Base of Upland Cotton in U.S. Cultivars – Genetic Variation for Lint Yield and Fiber…

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Although there is no a single reference that fully describes the Pee Dee germplasm due to the complex nature of this breeding program, there is a series of publications that reported germplasm lines derived from the program and evaluation of genetic variation in these germplasm resources (Culp and Harrell, 1973; Culp and Harrell, 1979a; Culp and Harrell, 1979b; Bowman and Gutiérrez, 2003; Campbell and Bauer, 2007; Campbell et al. 2009a; Campbell et al., 2009b; Campbell et al., 2011). As described by Culp and Harrell (1973) and Campbell et al. (2011), Pee Dee germplasm was introgressed from triple hybrid strains (*G. arboretum* L. × *G. thurberi* Todaro × *G. hirsutum* L.) since the 1940s and followed by 50 years of intercrosses among progenitor lines and crosses with commercial Upland cotton cultivars. Based on an evaluation of 82 released Pee Dee germplasm lines (Campbell et al., 2011), genetic variation for lint yield and fiber properties has been maintained in this germplasm population. Genetic similarity among these 82 lines ranged from 0.64 to 0.96 estimated by Simple Sequence Repeat (SSR) markers (Campbell et al., 2009b). The maintenance of genetic diversity in the Pee Dee germplasm after so many years of selection in breeding may be due to multiple breeding methods including random mating, backcrossing, and composite crossing in addition to pedigree selection applied during the breeding history (Culp and Harrell, 1973; Campbell et

Acala germplasm populations were developed by the New Mexico State University breeding programs. The history for the development of this germplasm has been described in detail by Smith et al. (1999). The series of Acala 1517 cultivars, commonly planted in the southwestern regions of U.S., were developed in this breeding program. Similar to Pee Dee germplasm, Acala germplasm also has genetic background of triple hybrids (*G. arboretum* L. × *G. thurberi* Todaro × *G. hirsutum* L.) (Smith et al., 1999). These germplasm populations are characterized by high fiber quality and Verticillium wilt tolerance and perform well in semiarid and hot regions in the southwestern of U.S. according to Zhang et al (2005a). Genetic diversity was maintained within Acala germplasm population. Genetic similarity ranged from 0.62 to 0.94 in 30 Acala cultivars estimated by SSR markers in this study. It was also concluded in this study that divergent germplasm introgressed in the Acala breeding program has contributed to the maintenance of genetic diversity in this germplasm and the genetic gain in the Acala cultivars.

Race stocks are another germplasm resource that can be utilized for introgression breeding. There are more than 2,000 primitive accessions in the cotton germplasm collection maintained by ARS at College Station, TX (Percival, 1987). However, utilization of these accessions has been limited due to their photoperiodic sensitivity which requires short days to flower and produce bolls. A group of U.S. public breeders have converted a large number of these accessions into the day-neutral lines by incorporating day-neutral genes in the primitive accessions through backcross breeding (McCarty et al., 1979; McCarty and Jenkins, 1992). Useful genetic variations for lint yield and fiber properties have been identified by evaluation of backcrossed progenies of 14 day-neutral accessions (McCarty et al., 1995) and F2 bulks of

Gutiérrez, 2003).

al., 2011).

It is commonly accepted that the genetic base of Upland cotton cultivars in the U.S. is narrow (Bowman et al., 1996; Van Esbroeck and Bowman, 1998). The narrowed genetic base in commercial cotton cultivars of *G. hirsutum* was caused by using only a small number of introduced wild genotypes during domestication (Van Esbroeck and Bowman, 1998; Gingle et al., 2006), breeding practices for high yield and early maturity (May et al., 1995; May, 1999), the dominance of transgenic cultivars in recent years, and insufficient utilization of exotic germplasm resources in cotton breeding. The narrowed genetic base in Upland cotton cultivars is believed to be the cause for the limited success in breakup of the negative associations between lint yield and fiber quality. Introgression from exotic germplasm resources into cultivars may be the most effective approach to broaden the genetic base of Upland cotton. The word "exotic" was defined in Google Search as "strikingly different", "strikingly unusu‐ al", or "introduced from other region or country". The term exotic germplasm is defined in this chapter as the germplasm without commercial applicability before introgression of the germplasm which includes land races, wild species, and the induced mutation stocks. It is expected that the introgression of novel genes from exotic germplasm into cotton cultivars or breeding lines can increase genetic variation in the introgression populations for agronomic traits and fiber quality.
