**4. Genetic associations among yield traits and fiber properties in different breeding programs**

Negative associations between lint yield and fiber quality is the main obstacle for simultaneous genetic improvement of yield and fiber quality. Genotypic correlations between lint yield and fiber properties in different genetic populations developed by some old and modern breeding programs are summarized in Table 2. In general, unfavorable correlations were identified for lint yield vs. strength, lint yield vs. span length, and lint yield vs. fineness. The highly unfav‐ orable association between lint yield and fineness identified in studies by Miller and Rowlings (1967) and Zeng et al. (2007) are expected since the thickness of fiber walls relates to the total amount of cellulose in fiber production. Since fineness is an important property in textile industry, the breakup of this unfavorable association would be critical in future breeding. The genetic improvement of lint yield may be achieved by selection of within-boll yield compo‐ nents and a balance between lint weight per unit seed surface area and fibers per unit seed surface area. In contrast, the correlations of lint yield with elongation and short fiber content were generally favorable or not significant in these germplasm resources which indicated the possibility for simultaneous improvement of these traits.

Although general negative association between lint yield and fiber strength and between lint yield and span lengths have been reported, low negative correlations were identified in studies of Percy et al. (2006), Zeng et al. (2007), and Hinze et al. (2011) compared with studies in the old breeding programs. The low correlations in these genetic populations indicated the possible recombinants with breakup of the negative associations. The genetic populations described by Percy et al. (2006) and Zeng et al. (2007) were introgressed populations from interspecific crosses. It is possible that the novel genes introgressed from wild germplasm increased recombination among parental chromosomes and therefore increased the chance to breakup negative associations.

between two traits appears unfavorable. However, it doesn't necessarily mean an unfavorable linkage of genes controlling the traits, but simply resulted from confounding effects from other traits. The relationships among maturity, short fiber content, and fineness in our previous studies can be an example to demonstrate how a relation between two traits is superimposed by relations among other traits. In a study of 200 germplasm lines from JohnCotton (JC) germplasm (Zeng and Meredith, 2009a), short fiber content was negatively correlated with fineness (r=-0.41). However, this relation was superimposed by their relations with maturity ratio, r=-0.77 for maturity ratio vs. short fiber content and r=0.81 for maturity ratio vs. fineness. This suggests that when fibers were more mature, short fiber content became less and fibers

† Values of standard variation were shown in parenthesis when the significance probability was not reported.

**Table 2.** Genetic associations between lint yield and fiber properties in the old and modern breeding programs.

**Lint yield vs. fiber properties r values Germplasm types Sources**

6 generations mixed intermating of

2 generations random mating of

Introgressed recombinant inbred

A mixed intermating population

A mixed intermating population

21 F2 populations Hinze et al. (2011)

(Species Polycross, SP)

(John Cotton, JC)

Miller and Rowlings

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239

Meredith and Bridge

Percy et al. (2006)

Zeng et al. (2007)

Zeng and Meredith

(2009a)

(1967)

(1971)

Empire 10 × TH131-5

ST7A × PD165

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

population



0.09 0.06 -0.08




\*, \*\*, \*\*\* Significant at *P* < 0.05, *P* < 0.01, *P* < 0.001, respectively.

Strength Elongation Fineness

Strength 50% span length 2.5 % span length Elongation

Strength 50% span length 2.5% span length

Strength Elongation 50% span length 2.5% span length Short fiber content

Strength Elongation 50% span length 2.5% span length Short fiber content

Fineness

Strength UHM length Elongation Uniformity

An elaborate explanation of interrelationships among fiber traits based on phenotypes has been difficult in the previous studies. The limited success in the past was mainly due to the confounding relationships of some fiber traits with other traits. Sometimes a correlation


\*, \*\*, \*\*\* Significant at *P* < 0.05, *P* < 0.01, *P* < 0.001, respectively.

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

**4. Genetic associations among yield traits and fiber properties in different**

Negative associations between lint yield and fiber quality is the main obstacle for simultaneous genetic improvement of yield and fiber quality. Genotypic correlations between lint yield and fiber properties in different genetic populations developed by some old and modern breeding programs are summarized in Table 2. In general, unfavorable correlations were identified for lint yield vs. strength, lint yield vs. span length, and lint yield vs. fineness. The highly unfav‐ orable association between lint yield and fineness identified in studies by Miller and Rowlings (1967) and Zeng et al. (2007) are expected since the thickness of fiber walls relates to the total amount of cellulose in fiber production. Since fineness is an important property in textile industry, the breakup of this unfavorable association would be critical in future breeding. The genetic improvement of lint yield may be achieved by selection of within-boll yield compo‐ nents and a balance between lint weight per unit seed surface area and fibers per unit seed surface area. In contrast, the correlations of lint yield with elongation and short fiber content were generally favorable or not significant in these germplasm resources which indicated the

Although general negative association between lint yield and fiber strength and between lint yield and span lengths have been reported, low negative correlations were identified in studies of Percy et al. (2006), Zeng et al. (2007), and Hinze et al. (2011) compared with studies in the old breeding programs. The low correlations in these genetic populations indicated the possible recombinants with breakup of the negative associations. The genetic populations described by Percy et al. (2006) and Zeng et al. (2007) were introgressed populations from interspecific crosses. It is possible that the novel genes introgressed from wild germplasm increased recombination among parental chromosomes and therefore increased the chance to

An elaborate explanation of interrelationships among fiber traits based on phenotypes has been difficult in the previous studies. The limited success in the past was mainly due to the confounding relationships of some fiber traits with other traits. Sometimes a correlation

in breeding for simultaneous improvement of lint yield and fiber quality.

possibility for simultaneous improvement of these traits.

**breeding programs**

238 World Cotton Germplasm Resources

breakup negative associations.

† Values of standard variation were shown in parenthesis when the significance probability was not reported.

**Table 2.** Genetic associations between lint yield and fiber properties in the old and modern breeding programs.

between two traits appears unfavorable. However, it doesn't necessarily mean an unfavorable linkage of genes controlling the traits, but simply resulted from confounding effects from other traits. The relationships among maturity, short fiber content, and fineness in our previous studies can be an example to demonstrate how a relation between two traits is superimposed by relations among other traits. In a study of 200 germplasm lines from JohnCotton (JC) germplasm (Zeng and Meredith, 2009a), short fiber content was negatively correlated with fineness (r=-0.41). However, this relation was superimposed by their relations with maturity ratio, r=-0.77 for maturity ratio vs. short fiber content and r=0.81 for maturity ratio vs. fineness. This suggests that when fibers were more mature, short fiber content became less and fibers became coarser. In another study of 45 F2 populations derived from crosses between five cultivars and nine SP and JC germplasm lines (Zeng et al., 2013), nearly zero additive corre‐ lation was observed between short fiber content and fineness which confirmed a lack of genetic mechanism underlying the phenotypic relationships between the two traits in these popula‐ tions.

this approach for introgression breeding is the reduction of likelihood for undesirable associations by increasing recombination in the substituted alien chromosomes or chromo‐ some segments (Saha et al., 2013). (D) Application of appropriate breeding methods such as random mating to maintain genetic diversity in germplasm populations or facilitate intro‐ gression of novel genes from wild germplasm resources to Upland cotton. Random mating may improve genetic variation in cotton germplasm populations and increase occurrence of recombinants and further improve opportunities to break up unfavorable associations between lint yield and fiber quality. A recent germplasm population, RMBUP-C4, was developed from crosses between three cultivars and 18 chromosome substitution lines followed by 4 cycles of random mating to introgress *G. barbadense* alleles into Upland cotton germplasm (Jenkins et al., 2013). This germplasm was released and available for cotton

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

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[1] Bowman, D.T., O.L. May, and D.S. Calhoun. 1996. Genetic base of upland cotton cul‐

[2] Bowman, D.T., and O.A. Gutiérrez. 2003. Sources of fiber strength in the U.S. upland

[3] Bowman, D.T., O.A.Gutierrez, R.G. Percy, D.S. Calhoun, O.L. May. 2006. Pedigrees of upland cotton and pima cotton cultivars released between 1970 and 2005. Bull.

[4] Campbell, B.T., and P.J. Bauer. 2007. Genetic variation for yield and fiber quality re‐ sponse to supplemental irrigation within the Pee Dee upland cotton germplasm col‐

[5] Campbell, B.T., D.T. Bowman, and D.B. Weaver. 2008. Heterotic effects in topcrosses

[6] Campbell, B.T., O.L. May, and D.C. Jones. 2009a. Registration of PD 99035 germ‐

tivars released between 1970 and 1990. Crop Sci. 36:577-581.

1017. Miss. Agric. and For. Exp. Stn., Mississippi, MS.

of modern and obsolete cotton cultivars. Crop Sci. 48:593-600.

plasm line of cotton. Journal of Plant Registrations 3:73-76.

lection. Crop Sci. 47:591-599.

cotton crop from 1980 to 2000. The Journal of Cotton Sci. 7:164-169.

breeders worldwide in introgression breeding.

Address all correspondence to: linghe.zeng@ars.usda.gov

USDA-ARS, Crop Genetics Research Unit, Stoneville, USA

**Author details**

Linghe Zeng\*

**References**

#### **5. Conclusions and perspectives**

Germplasm populations with novel genes from exotic resources heve been developed from different breeding programs. Molecular marker data showed genetic variation within these germplasm populations which indicated their usefulness in breeding for continuing genetic improvement of lint yield and fiber quality. Although useful genetic variations in different attributes related to lint yield and fiber quality exist in different germplasm populations, limited success has been reported in the identification of parents with desirable combination between lint yield and fiber properties. It is a challenge for U.S. cotton breeders to further broaden the genetic base of Upland cotton in the future to assist in a more successful breakup of linkages between lint yield and fiber quality.

There are a few approaches that should be considered in order to improve utilization of exotic germplasm resources and their introgression into Upland cotton cultivars for simultaneous genetic improvement of lint yield and fiber quality. (A) Genome-wide characterization of genetic diversity in different germplasm resources. There are a total of 5164 accessions of *G. hirsutum*, 1337 accessions of *G. barbadense*, and 25 accessions of *G. tomentosum*, *G. mustelinum*, and *G. darwinii* as primary gene pool, 1952 accessions for A, B, D, and F genome species as secondary gene pool, and 82 accessions for C, G, K, and E genome species as tertiary gene pool available at the National Plant Germplasm System of USDA-ARS (GRIN, 2013). These accessions have served as primary germplasm resources in cotton breeding worldwide, but most of these resources have yet to be utilized for genetic improvement of cotton cultivars. A genome-wide characterization of genetic diversity in these germplasm will undoubtedly improve their utilization in breeding. An establishment of a core set of these germplasm accessions according to the molecular characterization will definitely help their further utilization. (B) Elaboration of the complex interrelationships between yield traits and fiber properties. Because yield traits and fiber properties are often interrelated, the explanation of their interrelationships based on phenotypes would be difficult. Identification of molecular markers closely associated with these traits and determination of their genome location can help elaborate these interrelationships at the genome level and improve our understanding of the mechanisms underlying unfavorable associations between lint yield and fiber properties. (C) Use of alien chromosome substitution lines in introgression breeding. One of the major obstacles hindering utilization of exotic germplasm is the linkage between beneficial genes and unfavorable genes from the wild un-adapted germplasm during introgression. A group of U.S. scientists have developed an approach with a set of chromosome substitution lines to introgress beneficial genes from primary gene pools of *Gossypium* tetraploid species into Upland cotton (Stelly et al., 2005; Saha et al., 2011; Saha et al., 2013). The major advantage of this approach for introgression breeding is the reduction of likelihood for undesirable associations by increasing recombination in the substituted alien chromosomes or chromo‐ some segments (Saha et al., 2013). (D) Application of appropriate breeding methods such as random mating to maintain genetic diversity in germplasm populations or facilitate intro‐ gression of novel genes from wild germplasm resources to Upland cotton. Random mating may improve genetic variation in cotton germplasm populations and increase occurrence of recombinants and further improve opportunities to break up unfavorable associations between lint yield and fiber quality. A recent germplasm population, RMBUP-C4, was developed from crosses between three cultivars and 18 chromosome substitution lines followed by 4 cycles of random mating to introgress *G. barbadense* alleles into Upland cotton germplasm (Jenkins et al., 2013). This germplasm was released and available for cotton breeders worldwide in introgression breeding.
