**3. Gene action and combining ability for lint yield and fiber quality in germplasm resources**

Lint yield is determined by bolls per square meter, seeds per boll, and lint per seed (Worley et al., 1974; Worley et al., 1976). Although the yield component, bolls per square meter, is an inherited trait, it is highly depended on plant density, and environmental effects on this component are large (Meredith and Bridge, 1973). Lint yield is determined by seed cotton yield and lint percentage, and a number of within boll yield components contributing to lint yield including seeds per boll, lint per seed, lint weight per unit seed surface area, fibers per seed, and fibers per unit seed surface area (Worley et al., 1976; Coyle and Smith, 1997). Fiber quality 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 associations among fiber traits.

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). 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).

Day-neutral converted race stocks, SP, and JC germplasm.

**germplasm resources**

234 World Cotton Germplasm Resources

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,

**3. Gene action and combining ability for lint yield and fiber quality in**

Lint yield is determined by bolls per square meter, seeds per boll, and lint per seed (Worley et al., 1974; Worley et al., 1976). Although the yield component, bolls per square meter, is an inherited trait, it is highly depended on plant density, and environmental effects on this component are large (Meredith and Bridge, 1973). Lint yield is determined by seed cotton yield and lint percentage, and a number of within boll yield components contributing to lint yield including seeds per boll, lint per seed, lint weight per unit seed surface area, fibers per seed, and fibers per unit seed surface area (Worley et al., 1976; Coyle and Smith, 1997). Fiber quality

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 cotton may be an effective approach to promote heterosis.


**Traits VA † VD (VD/VA)1/2‡ Germplasm type Sources** MIC‡ 0.14\*\* 0.02\* 0.37 Primitive accession derived F2, F3 McCarty et al

0.27\*\* 0.15\*\* 0.75 Upland cotton cultivars and breeding lines Jenkins et al.

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

0.04\*\* 0.60\*\* 3.9 Cultivars and race stock derived F2 Cheatham et al

0.07\*\* 0.33\*\* 2.2 Wild tetraploid species derived F2 Zeng et al (2013)

0.57\*\* 0.12\*\* 0.46 Upland cotton cultivars and breeding lines Jenkins et al.

0.26\*\* 0.31\*\* 1.1 Wild tetraploid species derived F2 Zeng et al (2013) 0.47\*\* 0.01\*\* 0.14 Cultivars and race stock derived F2 Cheatham et al

0.09\*\* 0.18\*\* 1.4 Wild tetraploid species derived F2 Zeng et al (2013) 0.07\*\* 0.24\*\* 1.8 Cultivars and race stock derived F2 Cheatham et al

T1 0.19\*\* 0.15\*\* 0.89 Primitive accession derived F2, F3 McCarty et al

SL1 0.10\*\* 0.18\*\* 1.3 Primitive accession derived F2, F3 McCarty et al

Short fiber 0.06\*\* 0.41\*\* 2.6 Wild tetraploid species derived F2 Zeng et al (2013) Fineness 0.12\*\* 0.39\*\* 1.8 Wild tetraploid species derived F2 Zeng et al (2013)

† VA, additive variance component; VD, dominance variance component; VAE, additive by environment; VDE, dominant by

Determination of general combining ability for lint yield and fiber quality in germplasm lines can identify parents with potential of simultaneous improvement of lint yield and fiber quality. In reality, a parent with good general combining ability (GCA) in fiber quality usually had negative GCA in yield components or vice versa. In review of the previous studies since 1990s, a line with all desirable combinations between lint yield and fiber properties has not been reported. However, a number of studies reported by the U.S. cotton breeders have identified cultivars or germplasm lines with good lint yield and one or a few desirable fiber properties in the half diallel mating design. Green and Culp (1990) detected PD 3249 having positive GCA

1/2 was calculated as described by Meredith (1984): VA was set to 100; the dominant component (VD) was

environment. All components were expressed as the ratio to VP, the phenotypic variance.

**Table 1.** Variance components in different germplasm resources of Upland cotton.

\*, \*\* Significant at < 0.05 and 0.01, respectively.

§MIC, micronaire; T1, fiber strength; SL1, 50% span length.

‡ (VD/VA)

converted to VD/VA × 100.

(2004)

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

237

(2009)

(2003)

(2004)

(2009)

(2003)

(2004)

(2003)


\*, \*\* Significant at < 0.05 and 0.01, respectively.

**Traits VA † VD (VD/VA)1/2‡ Germplasm type Sources** Lint yield 0.06\*\* 0.25\*\* 2.0 Primitive accession derived F2, F3 McCarty et al.

236 World Cotton Germplasm Resources

0.01 0.06\*\* 2.5 Chromosome substitution lines of TM-1 introgressed from *G. barbadense*

Lint % 0.35\*\* 0.11\*\* 0.56 Primitive accession derived F2, F3 McCarty et al.

0.54\*\* 0.10\*\* 0.44 Chromosome substitution lines of TM-1 introgressed from *G. barbadense*

Boll wt 0.18\*\* 0.34\*\* 1.4 Primitive accession derived F2, F3 McCarty et al

0.17\*\* 0.00 0.00 Chromosome substitution lines of TM-1 introgressed from *G. barbadense*

Seed index 0.39\*\* 0.33\*\* 0.92 Wild tetraploid species derived F2 Zeng and Wu

Lint index 0.44\*\* 0.32\*\* 0.85 Wild tetraploid species derived F2 Zeng and Wu

Seeds boll-1

0.08\*\* 0.12\*\* 1.2 Upland cotton cultivars and breeding lines Jenkins et al.

0.38\*\* 0.31\*\* 0.90 Wild tetraploid species derived F2 Zeng and Wu

0.250\*\* 0.28\*\* 1.1 Cultivars and race stock derived F2 Cheatham et al

0.33\*\* 0.22\*\* 0.82 Upland cotton cultivars and breeding lines Jenkins et al.

0.49\*\* 0.32\*\* 0.81 Wild tetraploid species derived F2 Zeng and Wu

0.81\*\* 0.11\* 0.37 Cultivars and race stock derived F2 Cheatham et al

0.27\*\* 0.32\*\* 1.1 Upland cotton cultivars and breeding lines Jenkins et al.

0.35\*\* 0.31\*\* 0.94 Wild tetraploid species derived F2 Zeng and Wu

0.23\*\* 0.29\*\* 1.1 Cultivars and race stock derived F2 Cheatham et al

0.23\*\* 0.36\*\* 1.3 Wild tetraploid species derived F2 Zeng and Wu

(2004)

(2009)

(2012)

(2003)

(2004)

(2009)

(2012)

(2003)

(2004)

(2009)

(2012)

(2003)

(2012)

(2012)

(2012)

Saha et al (2010)

Saha et al (2010)

Saha et al (2010)

† VA, additive variance component; VD, dominance variance component; VAE, additive by environment; VDE, dominant by environment. All components were expressed as the ratio to VP, the phenotypic variance.

‡ (VD/VA) 1/2 was calculated as described by Meredith (1984): VA was set to 100; the dominant component (VD) was converted to VD/VA × 100.

§MIC, micronaire; T1, fiber strength; SL1, 50% span length.

**Table 1.** Variance components in different germplasm resources of Upland cotton.

Determination of general combining ability for lint yield and fiber quality in germplasm lines can identify parents with potential of simultaneous improvement of lint yield and fiber quality. In reality, a parent with good general combining ability (GCA) in fiber quality usually had negative GCA in yield components or vice versa. In review of the previous studies since 1990s, a line with all desirable combinations between lint yield and fiber properties has not been reported. However, a number of studies reported by the U.S. cotton breeders have identified cultivars or germplasm lines with good lint yield and one or a few desirable fiber properties in the half diallel mating design. Green and Culp (1990) detected PD 3249 having positive GCA for lint yield (79 kg ha-1), strength (2.5 kN m kg-1), and 2.5% span length (0.20 mm) in crosses among five cultivars. Coyle and Smith (1997) detected Deltapine 90 having positive GCA for lint percentage (1.1-1.2 %) and strength (2-10 kN m kg-1) in crosses among four cultivars and two germplasm lines. Jenkins et al (2009) reported positive predicted GCA in Acala Ultima, FM 966, and PSC 355 for lint yield (8-84 kg ha-1), strength (3.6-29 kN m kg-1), and uniformity ratio (0.32-0.71%) in crosses among 10 cultivars and one breeding line. Zeng et al. (2011) identified five germplasm lines, SP156, SP224, SP192, SP205, and JC65, having positive GCA for lint yield and favorable GCA for a few fiber properties including strength, elongation, short fiber content, and fineness in crosses among four cultivars and twelve exotic germplasm lines by a North Carolina Design II mating design. These germplasm lines can be used as parents in breeding for simultaneous improvement of lint yield and fiber quality.
