QTL analysis of stomatal conductance and relationship to lint yield in an interspecific cotton.

Extended periods of high temperature can reduce cotton (Gossypium hirsutum L. and G. barbadense L.) lint yield, even under adequate irrigation. High stomatal conductance may confer some adaptive advantage to genotypes that experience supra-optimum temperatures. The primary objective of this research was to practice divergent selection for stomatal conductance in a segregating population (n = 118 F2.3 progenies) derived from the cross NM24016/TM1. Divergent selection for high and low stomatal conductance was practiced in Maricopa, AZ, in 1996. DNA was isolated from all 118 F2 plants in 1995 and a linkage map produced with 199 random amplified polymorphic (RAPD) and simple sequence repeat (SSR) DNA markers. Genetic analysis of the replicated F3 families in 1996 at Maricopa M. Ulloa, USDA-ARS Crop Genet. & Prod. Res. Unit, Stoneville, MS 38776; R.G. Cantrell, Agron. & Hort. Dep. , New Mexico State Univ., Las Cruces, NM 88003; R.G. Percy, USDA-ARS Pima Cotton Breeding & Genet. Unit, Maricopa Agric. Ctr. , Maricopa, AZ 85239; E. Zeiger, Dep. of Biol., Univ. of California, Los Angeles, CA 90024; and Zhenmin Lu, 1 Huntsman Dr., Boothwyn, PA 19061. Received 3 Sept. 1999. *Corresponding author (rcantrel@nmsu.edu). Abbreviations: QTL, quantitative trait loci; RAPD, random amplified polymorphic DNA; SSR, simple sequence repeat. 11 CANTRELL ET AL.: STOMATAL CONDUCTANCE IN COTTON permitted identification of quantitative trait loci (QTL) influencing stomatal conductance. Replicated experiments of 20 selected F2.4 progeny (10 with high, 10 with low stomatal conductance) were grown in Maricopa and Las Cruces in 1997. The 10 families selected for high stomatal conductance in 1996 averaged 542.6 mmol H2O m s at Maricopa in 1997 and were significantly (P = 0.0001) different from the mean of the low families (472 mmol H2O m $2 s). The two selected groups were not significantly different for stomatal conductance at Las Cruces (P = 0.0631). Lint yield was significantly (P = 0.0027) affected by selection for stomatal conductance in Maricopa. The F3.4 family group with high stomatal conductance produced the highest cotton lint yield averaging 1842 g plot while the family with low stomatal conductance averaged 1655 g plot. Two putative QTLs for stomatal conductance were identified on two cotton linkage groups. Lint yield of cotton can be dramatically reduced by supra-optimal temperatures during peak reproductive development in irrigated conditions in the U.S. Southwest. Recent studies with pima have shown that increases in stomatal conductance have accompanied increases in cotton lint yields (Lu et al., 1994, 1998; Lu and Zeiger, 1994). The level of stomatal conductance at high temperature was positively correlated with stomatal sensitivity to temperature and independent of photosynthesis (Lu et al., 1996). Our working hypothesis was that selection for high yields has imposed indirect selection pressure for elevated stomatal conductance at supra-optimal temperature under irrigated environments. This increased conductance may reduce leaf temperatures and confer tolerance to high temperatures, especially during critical fruiting periods. Genetic modifications of the sensory transduction pathway(s) are required in guard cells for genetically stable changes in stomatal conductance (Zeiger, 1983). This higher level of conductance provides an avoidance mechanism for heat tolerance in hot environments that would not prevail or contribute to higher lint yields under cooler environments, such as the San Joaquin Valley in California (Lu et al., 1998). The adaptive advantage of greater stomatal conductance would be absent at lower leaf temperatures. Genetic studies of stomatal conductance have been restricted to each cultivated species (upland and pima cotton) and not interspecific hybrid populations. Roark and Quisenberry (1977) found stomatal conductance to be under genetic control in upland cotton. Percy et al. (1996) estimated relatively low heritabilities for the trait in pima populations and reported existence of dominance and epistatic interactions controlling the trait. Divergent selection for stomatal conductance in a pima segregating population revealed that high yielding F4 progeny were derived predominantly from F2 plants with high levels of stomatal conductance (Radin et al., 1994). As a result of selection for high lint yields under hot environments in Arizona, the stomatal conductance of some recent pima cultivars has approached that of upland cotton (Radin, 1992). Genes for high levels of stomatal conductance from upland cotton may have contributed to the increases in the trait in advanced pima lines selected for high lint yields in hot Arizona environments (Lu et al., 1994). Introgression of genes between upland and pima cotton has been a long-standing goal of cotton breeders. Traits that have been the target of introgression include heat tolerance from upland sources and fiber quality from pima germplasm. The degree of introgression is hindered by genetic breakdown in segregating interspecific breeding populations (Stephens, 1949). Genetically stable lines that exhibit significant combinations of upland and pima chromatin have been developed (Tatineni et al., 1996; Cantrell and Davis, 1993). These genotypes display a continuous spectrum of morphological traits between the two parental species. The genetic distance of these lines from typical upland TM1 has been determined using molecular and morphological markers. Evaluation of these lines for stomatal conductance under heatstress conditions reveals that lines more genetically similar to TM1 have higher stomatal conductance values than those more similar to pima 3-79 (Cantrell and Zeiger, 1995, personal communication). One of these introgressed lines, denoted as NM24016, exhibits significant introgression from 12 JOURNAL OF COTTON SCIENCE, Volume 4, Issue 1, 2000 Fig. 1. Flowchart of derivation and evaluation of progeny from the population, NM24016 × TM1 cotton. upland and pima (Cantrell et al., 2000). The variability derived from hybridization of NM24016 with upland TM1 would provide a valuable opportunity for associating introgressed chromosome regions with physiological traits, such as stomatal conductance. Most important characteristics of agricultural crops are quantitatively inherited, and stomatal regulation appears to be one such trait. The location and effects of the genes controlling quantitative traits can be determined by DNA marker-based genetic analysis. A region of the genome linked to or associated with DNA markers that affects a quantitative trait is defined as a quantitative trait locus (QTL) (Geldermann, 1975). Shappley (1996) provided the first linkage map of QTLs in an upland cotton cross. Recently, other cotton QTLs have been identified for fiber quality (Shappley et al., 1998; Jiang et al., 1998). To date, no QTLs have been reported for physiological traits in cotton. The objectives of this study were: (i) to determine the effect of divergent selection for stomatal conductance on lint yield in a diverse segregating population derived from the cross NM24016/TM1, (ii) to determine the stomatal conductance and lint yield of selected progenies in two environments representing drastically different temperatures during peak fruiting periods, and (iii) to utilize DNA markers and a linkage map to identify QTLs controlling stomatal conductance. MATERIALS AND METHODS