**Author details**

Linghe Zeng\*

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‐

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

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

tions.

240 World Cotton Germplasm Resources

**5. Conclusions and perspectives**

of linkages between lint yield and fiber quality.

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

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