**2. Genetic diversity in some cotton germplasm populations developed by public breeding programs in U.S.**

Although many germplasm populations derived from interspecific crosses or race stocks have been used in cotton breeding, only a small portion of alleles in these germplasm were intro‐ gressed into the released cultivars due to the selection for regional adaption according to a survey by Van Esbroeck and Bowman (1998). Only 0.3% of the 668 released germplasm lines have been introgressed into successful cotton cultivars. However, most of the successful cultivars released between 1972 and 1996 have some exotic alleles in their pedigree according to this survey. For more successful introgression of exotic germplasm into cultivars, genetic variations of germplasm resources with exotic genetic backgrounds have to be explored.

In this section, the efforts on characterizing genetic diversity in the germplasm populations in a few U.S. public breeding programs were reviewed. There are different cotton breeding programs in U.S. that include both the private and public sectors. The germplasm developed in these programs constitutes the primary gene pool for breeding. Pee Dee and the New Mexico Acala germplasms are two historically important public breeding programs as highlighted by Bowman et al. (2006) in a description of U.S. cotton cultivars released between 1970 and 2005. Up to 50% of strength improvement in Upland cotton cultivars during 1980 and 2000 may have been attributed to alleles from Pee Dee and Acala germplasm populations (Bowman and Gutiérrez, 2003).

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

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

**2. Genetic diversity in some cotton germplasm populations developed by**

Although many germplasm populations derived from interspecific crosses or race stocks have been used in cotton breeding, only a small portion of alleles in these germplasm were intro‐ gressed into the released cultivars due to the selection for regional adaption according to a survey by Van Esbroeck and Bowman (1998). Only 0.3% of the 668 released germplasm lines have been introgressed into successful cotton cultivars. However, most of the successful cultivars released between 1972 and 1996 have some exotic alleles in their pedigree according to this survey. For more successful introgression of exotic germplasm into cultivars, genetic variations of germplasm resources with exotic genetic backgrounds have to be explored.

In this section, the efforts on characterizing genetic diversity in the germplasm populations in a few U.S. public breeding programs were reviewed. There are different cotton breeding programs in U.S. that include both the private and public sectors. The germplasm developed in these programs constitutes the primary gene pool for breeding. Pee Dee and the New Mexico Acala germplasms are two historically important public breeding programs as highlighted by Bowman et al. (2006) in a description of U.S. cotton cultivars released between 1970 and 2005. Up to 50% of strength improvement in Upland cotton cultivars during 1980 and 2000 may have

and Rawlings, 1967; Smith and Goyle, 1997).

232 World Cotton Germplasm Resources

traits and fiber quality.

**public breeding programs in U.S.**

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 al., 2011).

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 crosses between 114 day-neutral accessions with Stoneville 474 and Sure-Grow 747 (McCarty et al., 2005). Twelve germplasm lines derived from converted day-neutral race stocks and introgression of wild species were evaluated and significant additive and dominant effects were identified for yield components and different fiber properties (Wu et al., 2010). Hinze et al. (2011) also identified significant variations for agronomic traits and fiber properties within four germplasm populations derived from non-photoperiodic race stocks. A study of genetic distance between four converted day-neutral lines and Delta and Pine 16 showed a wide range of genetic similarity in these germplasm lines, ranging from 0.37 to 0.65 (Zhong et al., 2002).

is a series of fiber properties which determine the spinnability of fibers and the efficiency of the high speed spinners in the modern textile industries. In a typical breeders' analysis, the measurements of fiber quality include micronaire, elongation, fiber strength, fiber length, short fiber content, and fineness. The neppiness traits including fiber neps, seed coat neps, and motes are getting more attention from breeders in recent years because of their severe affects in textile processing during spinning and dying (Jacobsen et al., 2001). While both lint yield and fiber quality are important traits to improve in Upland cotton cultivars, negative associations usually exist between them. For example, the potential of fiber productivity is highly related to fiber length and thickness of cell walls because longer fibers and thicker cell walls resulted from increased cellulose amount in the fibers (Kohel, 1999). However, the increase of fiber productivity by increasing cell wall thickness will be antagonistic with fineness, an important fiber property in fiber spinning. Determination of gene actions and combining ability for different attributes of yield and fiber quality in germplasm populations can be of help in understanding introgression and is useful information for breaking or reducing the negative

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

http://dx.doi.org/10.5772/57606

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Genetic variations of quantitative traits for yield and fiber quality are the main focus in this section which discusses the differences of gene actions and combining ability among germ‐ plasm resources. For variations related to morphological phenotypes or other taxonomic characteristics, readers can refer to the reports by Percy and Kohel (1999) and Lubbers and Chee (2009). A few recent studies of gene actions in genetic populations derived from different types of germplasm resources are summarized in Table 1. The general low additive gene action for lint yield and most fiber properties in these germplasm populations except for the popu‐ lations derived from crosses among tetraploid species suggests non-efficiency of early selection for lint yield in the populations. High additive gene action in yield components and fiber strength suggests early selection efficiency for these traits in these germplasm populations. In five genetic populations developed by diallel crosses among Upland cotton cultivars, as reviewed by Meredith (1984), gene actions for yield traits and fiber properties were generally partial dominant. In order to compare gene action in these five genetic populations, the degree of dominance was estimated in the same way as described by Meredith (1984) as the ratios of dominant component to additive component with values less than 1 indicating partial dominance and values equal or larger than 1 indicating complete or over-dominance. As shown in Table 1, gene action in the introgression populations was either completely dominant or over-dominant for yield traits except for lint percentage among different germplasm resources. For fiber properties, gene actions were generally partially dominant for micronaire and fiber strength while over-dominant for fiber length, short fiber content, and fineness. A reduction of heterosis values from obsolete cultivars to the modern cultivars due to increased additive genes in breeding practice has been observed previously (Campbell et al., 2008). The increase of dominance gene action in the genetic populations derived from wild cotton and interspecific crosses indicates that the adding of non-additive genes by introgression from wild

associations among fiber traits.

cotton may be an effective approach to promote heterosis.

Species Polycross (SP) and JohnCotton (JC) germplasm populations were developed by U.S. breeders since the 1960s and 1970s. SP germplasm population was derived from multiple crosses among twelve cotton cultivars and strains of four tetraploid species: *G. barbadense* L., *G. tomentosum* Nutt., *G. mustelinum* Watt., and *G. darwinii* Watt. JC germplasm population was derived from multiple crosses between Acala 1517 type cultivars and *G. barbadense*. Both of these two germplasm populations underwent multiple generations of random mating and selfing. Significant genetic variations for lint yield and fiber properties have been identified in field evaluation of 260 SP lines (Zeng et al., 2007) and another evaluation of 200 JC lines (Zeng and Meredith, 2009a). A number of germplasm lines were selected and released from these two germplasm populations for desirable combinations between lint yield and fiber properties (Zeng and Meredith, 2009b; Zeng et al., 2010). Genetic similarity between 12 SP and JC lines and 4 Upland cultivars ranged from 0.44 to 0.99 (Zeng and Meredith, 2011).

In a few molecular studies of genetic distance among Upland cultivars (Gutiérrez et al., 2002; Rahman et al., 2002; Zhang et al., 2005b), genetic similarity ranged from 0.78 to 0.94 between pairs of cultivars in these studies. All the germplasm resources described above have larger genetic distance within the populations and from Upland cultivars. In a recent study of 193 Upland cotton cultivars collected from 26 countries using SSR markers, the pair-wise genetic similarity ranged from 0.64 to 0.99 (Fang et al., 2013). Only in this study, the genetic diversity was comparable to the germplasm populations described above. These studies are consistent with the argument that genetic diversity is maintained in the Pee Dee, New Mexico Acala, Day-neutral converted race stocks, SP, and JC germplasm.
