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Genetic Analyses of Agronomic Traits Controlled by Wheat Chromosome 3A M. M. Shah, P. S. Baenziger,* Y. Yen, K. S. Gill, B. Moreno-Sevilla, and K. Haliloglu ABSTRACT tween a chromosome substitution line having the chromosome of interest and the parent cultivar having the homologue for the chromosome of interest (Law, 1966; Yen and Baenziger, 1992). Trait comparisons can be made and the genes controlling these traits can further be identified and mapped by conventional biometrical approaches or by linkage studies with molecular markers (Lander and Botstein, 1989; Paterson et al., 1990; Joppa et al., 1997). Law and coworkers (1966, 1967, and 1976) and Snape et al. (1985) were able to identify segregating loci and their gene action for important agronomic characters on wheat chromosomes 7B, 5A, and 5D using RICL populations developed between a wheat cultivar (often ‘Chinese Spring’) and the chromosome substitution lines for the respective chromosomes. However, these studies were limited because they generally were conducted in only a single or few environments and did not estimate genotype 3 environment interactions, which are known to affect measurements of quantitative traits such as grain yield in wheat (Carver et al., 1987; Sharma et al., 1987; Mahal et al., 1988; Silis et al., 1989). The mapping of genes controlling agronomic traits on specific chromosomes in the above studies was possible because monogenically inherited phenotypic markers were available. An adequate number of these phenotypic markers often are not found in conventional breeding populations involving two parental cultivars. Hence, little is known concerning the number and nature of the genes responsible for important agronomic characters on specific chromosomes of wheat. The knowledge of major gene(s) (one or few genes having large effects on the expression of a trait) vs. minor genes (many genes having small and equal effects on a trait) and their relationship on a chromosome is important to develop efficient breeding strategies. The objectives of this study were to determine if previously identified important agronomic traits on chromosome 3A were controlled by major or minor gene(s) and if those genes were pleiotropic, linked, or independent. Previous studies with chromosome substitution lines between hard red winter wheat (Triticum aestivum L.) cultivars Cheyenne (CNN) and Wichita (WI) identified genes on chromosome 3A of WI which affect grain yield, yield components, grain volume weight, plant height, and anthesis date. This study was conducted to determine if the trait variation caused by chromosome 3A could be explained by major or minor gene segregation and if these genes are pleiotropic, linked, or independent on the chromosome. A population of recombinant inbred chromosome lines for chromosome 3A (RICLs-3A), developed between CNN and a chromosome substitution line CNN(WI3A), was evaluated in multi-location field trials in 3 yr. Our results indicate significant differences (P # 0.05) between parental lines and among RICLs for grain yield, 1000-kernel weight, plant height, and anthesis date, but not for kernel number per spike, spike number per square meter, and grain volume weight. A 1:1 genetic ratio for anthesis date suggested the presence of a single segregating locus controlling the trait. None of the other agronomic traits could be separated into unequivocal groups and hence, major genes were not detected. This indicates that the traits were controlled either by several genes or few genes with enough environmental influence, or both, to obscure their effects. Significant correlations and possible crossover products between anthesis date, plant height, and 1000-kernel weight suggest that these traits were controlled either by linked gene(s) or by pleiotropic genes with additional genes affecting one of the traits. P revious research with reciprocal chromosome substitution lines between bread wheat cultivars Cheyenne (CNN) and Wichita (WI) determined that WI chromosome 3A contained gene(s) that significantly increased grain yield and 1000-kernel weight, decreased plant height, and delayed anthesis when substituted in CNN (Berke et al., 1992a). In the reciprocal substitution line, WI(CNN3A), CNN chromosome 3A was identified to be the location of gene(s) that significantly decreased grain yield and spike number per square meter, but it had no significant effect on anthesis date and plant height (Berke et al., 1992a). Reciprocal chromosome substitution lines can only be used to determine the effects of a whole chromosome. To detect the number and relative location of genes on a specific chromosome, chromosomal effects need to be partitioned into those of chromosome segmental effects (Paterson et al., 1990; Yen et al., 1992; Joppa et al., 1997). A classical approach to identify chromosomal segments is the development and evaluation of recombinant inbred chromosome lines (RICLs) be- MATERIALS AND METHODS Development of 3A Recombinant Inbred Chromosome Line Population A population of 50 RICLs-3A was developed by R. Morris and Y. Yen following the method described by Law (1966), after crossing the hard red winter wheat cultivar CNN with the chromosome substitution line CNN(WI3A). In the F1, meaningful recombination only occurs between WI3A and CNN3A chromosomes, as all the other chromosomes are from CNN parent. The F1 was crossed as a male to CNN-monosomic M.M. Shah, Dep. of Agronomy, Iowa State Univ. of Science and Technology, Ames, IA 50011; P.S. Baenziger, K.S. Gill, and K. Haliloglu, Dep. Agronomy, Univ. of Nebraska, Lincoln, NE 68583; Y. Yen, Dep. of Biology/Microbiology, South Dakota State Univ., Brookings, SD 57006; B. Moreno-Sevilla, HybriTech Seed International, Boise, ID 83713. Nebraska Agric. Res. Div., J. Series No. 12258. Received 5 June 1998. *Corresponding author (pbaenziger1@ unl.edu). Abbreviations: ANOVA, analysis of variance; CNN, ‘Cheyenne’; RICLs-3A, recombinant inbred chromosome lines for chromosome 3A; WI, ‘Wichita’. Published in Crop Sci. 39:1016–1021 (1999). 1016 SHAH ET AL.: GENETIC ANALYSIS OF AGRONOMIC TRAITS IN WHEAT 3A. Monosomic plants (2N 2 1 5 41) from the resulting progeny were selected by cytological examination of root-tip cells. The hemizygous chromosome in these plants should either be identical to the parental chromosome (therefore non-recombinant) or will have resulted from a crossover event (therefore recombinant). The male parent (F1 in this case) transmits a euploid gamete (1N 5 21) and the female monosomic plant produces aneuploid (1N 2 1 5 20) or euploid (1N 5 21) gametes. The recombinant monosomic progeny resulting from the fertilization of a male euploid gamete with a female aneuploid gamete were selected. The disomic progeny (2N 5 42) resulting from the fertilization of a male euploid gamete with a female euploid gamete were discarded. Upon selfing these monosomic plants and selecting the disomic progeny, again by cytological examination of root-tip cells, 50 homozygous RICLs-3Awere developed in the CNN background. The seed of each individual line was increased in the greenhouse and the lines were then grown in the field trials. Each RICL should differ from another unless no crossover occurred or the identical crossover occurred. The possibility of univalent shifts occurring during RICL development was not tested, but considered low with only two generations of aneuploidy. The common background, except for the substituted chromosome, should have promoted chromosome pairing (Person, 1956). Previous work of developing the CNN monosomic series had indicated few univalent shifts (R. Morris, 1998, personal communication). Field Trials In 1994, 50 RICLs-3A, CNN, and CNN(WI3A) were grown at Lincoln, NE (Sharpsburg silty clay loam, a fine, montmorillonitic, mesic Typic Argiudoll) and North Platte, NE (Holdrege silt loam, a fine-silty, mixed, mesic Aridic Argiustoll). In 1995 and 1996, the 50 RICLs-3A, CNN, and CNN(WI3A) were grown at Lincoln, Mead (Sharpsburg silty clay loam, a fine, montmorillonitic, mesic Typic Argiudoll), North Platte, and Sidney, NE (Holdrege silt loam, a fine-silty, mixed, mesic Aridic Argiustoll). The parent cultivar, CNN, and the substitution line, CNN(W13A), were included in this study to determine if the previously observed chromosome 3A effects could be observed in these environments. The trial at North Platte (1996) was lost due to hail and high winds and no data were collected. Trials at Lincoln (1996) and Sidney (1995) were also damaged severely by adverse weather (heavy rain and hail) and data from these trials were excluded from analyses for grain yield, its component traits, and grain volume weight. However, data from these trials were included for plant height and anthesis date. A randomized complete block design with two replications in 1994 and 1995, or three replications in 1996 was used. Each experimental unit consisted a plot of four 2.4-m rows with 0.3 m between rows. Seeding rate was 68 kg ha21 at Lincoln and Mead, and 50 kg ha21 at North Platte and Sidney. All trials were planted on fallowed ground the previous year and recommended production practices were used. All trials were kept disease free with two applications of the fungicide Tilt (1-[[2-(2,4-dichlorophenyl)-4-propyl-1, 3-dioxlan-2-yl] methyl-1H-1, 2,4 triozole; Novartis, Greensboro, NC). Except where noted, data for grain yield, kernel number per spike, 1000-kernel weight, spikes per square meter, and plant height was collected from all replications at Lincoln, Mead, North Platte, and Sidney. Grain volume weight was measured from Lincoln, Mead, and Sidney, whereas anthesis date, which required daily measurements, was measured only at Lincoln and Mead. Grain yield was measured in Mg ha21 by harvesting the middle two rows of each plot at Lincoln and 1017 Mead (for pure seed) and by harvesting the entire plot with a small plot combine harvester at North Platte and Sidney. Ten spikes were harvested from the outer two rows at Lincoln and Mead, or from the four rows at North Platte and Sidney, and threshed to determine kernel number per spike and 1000kernel weight. Spikes per square meter were estimated by dividing plot grain yield by the multiplication product of kernel number per spike and kernel weight. Plant height (measured from the ground to the tip of the spike, awns excluded) was the visual average of a random sample of spikes from the middle two rows of each plot. Grain volume weight was measured in kg hL21 with Seedburo volumetric scale (Seedburro Equipment Co., Chicago, IL). Anthesis date (days after 1 May) was visually estimated as the date when 50% of the spikes in a plot had extruded anthers. Data Analyses A combined analysis of variance (ANOVA) was conducted across environments by the SAS procedure GLM and the RANDOM statement (SAS Inst., 1990) to test significant (P # 0.05) differences between parents, parents and RICLs-3A, and among RICLs-3A for all traits. Environments, replications, and RICLs were considered as random effects while CNN and CNN(WI3A)) were considered as fixed effects. The genotype 3 environment interaction (G 3 E) or the partition of the G 3 E was used to test genotypic effects or their partitions in the ANOVA. The data were analyzed for normality with PROC UNIVARIATE and the NORMAL statement (SAS Inst., 1990) on the genotypic means across environments. The frequency histograms for each trait were obtained by using the range of genotypic means separated by 0.5 standard error units. Traits that showed a bimodal distribution were divided into two distinct phenotypic classes and were analyzed for deviation from the expected Mendelian segregation ratio (1:1) by a chi-square test. A 1:1 ratio indicated single-locus control. A protected LSD was calculated for each significant trait from the combined ANOVA with synthesized G 3 E error term used to identify transgressive segregates for each trait. Correlation analysis for the 50 RICLs with genotypic means across environments was conducted between pairs of traits exhibiting significant variation to determine if the gene(s) controlling two agronomic traits were coincident or independent. The procedure CORR (SAS Inst., 1990) was used for this analysis. A nonsignificant correlation between two traits was interpreted as the genes were segregating independently or environmental effects masked the effects of these genes. A significant (P # 0.05) correlation was considered an indication of the presence of linkage or pleiotropy. Scatter diagrams were plotted between statistically correlated traits with genotypic means used to determine if distinctive groups of the RICLs were observed in relation to the parental values, i.e., if two parental classes determined the association. Scatter diagrams also were used to determine if the putative recombinant products or additional independent genes existed. Two traits were considered as linked or having additional independent gene(s), and not pleiotropic, if in the scatter diagram putative recombinant lines (lines exhibiting the phenotype of one parent for the first trait and the phenotype of the other parent for the second trait) could be identified. RESULTS AND DISCUSSION The F-tests from the ANOVA across environments detected significant (P # 0.05) differences between CNN and CNN(WI3A) and among the RICLs for grain 1018 CROP SCIENCE, VOL. 39, JULY–AUGUST 1999 Table 1. Overall means and mean squares (MS) from the analysis of variance of seven traits for Cheyenne (CNN), chromosome substitution line CNN(W13A), and 50 Cheyenne-Wichita recombinant inbred chromosome 3A lines (RICLs-3A), combined across a variable number of Nebraska environments. Grain yield Source df MS ha21)2 Environments (E) Genotypes (G) CNN vs. CNN(W13A) Parents vs. RICLs-3A RICLs-3A G3E CNN vs. CNN(W13A) 3 E Parents vs. RICLs-3A 3 E RICLs-3A 3 E Pooled Error Mean CV% 6 51 1 1 49 306 6 6 294 459 (Mg 141.9 0.233** 0.799** 0.489 0.22* 0.144** 0.03 0.18 0.15** 0.113 2.68 13.5 Kernel no. spike21 1000-Kernel weight Spike no. m 22 MS MS MS 2 no. 4503.6 10.2 1.2 0.5 10.5 9.8 4.8 8.8 9.9 8.8 28.5 10.4 2 g 3259.3 12.5** 42.1** 3.2 12.1** 7.9** 3.8 9.4 8.0* 6.3 29.9 8.3 Grain volume weight Plant height Anthesis date (d after May 1) df df df no. 1 394 944 4 957 5 7 291 5 008 4 938** 3 501 4 246 4 981** 3 307 312 18.4 MS hL21)2 2 5 51 1 1 49 250 5 5 240 376 (kg 139.8 0.7 1.3 1.9 0.7 0.8 1.0 1.1 0.8 0.8 62.2 1.4 MS 2 8 51 1 1 49 408 8 8 392 610 cm 11 250 94** 177* 4 94** 39 48 9 39 36 102 5.9 3 51 1 1 49 153 3 3 147 306 MS d2 8656.4 27.6** 56.1** 8.8 27.4** 2.3* 0.5 3.6 2.3 1.8 37.8 3.5 *, ** Significant at the 0.05 and 0.01 levels of probability, respectively. yield, kernel weight, plant height, and anthesis date, but not for kernel number per spike, spikes per square meter, and grain volume weight (Table 1). The substitution line CNN(WI3A) had significantly higher grain yield and kernel weight, was earlier for anthesis, and was shorter than CNN (Table 2). These results are in agreement with previous reports of chromosome substitution line analyses involving chromosome 3A (Berke et al., 1992a; Yen et al., 1997) with the exception of grain volume weight, which was significant in Berke et al. (1992a), but was non-significant in Yen et al. (1997). Significant differences were identified among the RICLs for those traits where the parents were significantly different, which indicated that the parental genes segregated among the RICLs. As expected, the RICL-3A population mean was not significantly different from the mid-parental value for any trait, indicating primarily additive gene action (Yen et al., 1997). Genotype 3 environment interactions for the two parents were non-significant (Table 1) for all the traits indicating that they responded similarly across environments. The G 3 E interactions among RICLs were significant for some traits (Table 1), but they appeared to be due to changes in magnitude and not reversal in order. Therefore we present RICL means averaged across environments. As determined by the normality test, all traits statistically fit the normal distribution except anthesis date (P # 0.001), whose distribution was bimodal (Fig. 1 a). When grouped by the parental values, a Chi-square test (x2 5 2.88, P 5 0.09) indicated 1:1 segregation for a single locus controlling anthesis date. Previous research with substitution or aneuploid lines (Zemetra et al., 1986; Miura and Worland, 1994), has shown that chromosome 3A contained gene(s) controlling earliness as determined by heading date; however, none of the previous research was designed to determine the number of loci affecting the trait. A unimodal normal distribution was observed for grain yield, kernel number per spike, kernel weight, spike number per square meter, grain volume weight, and plant height (Fig. 1), so it was not possible to ascertain as how many genes controlled these traits. The traits are either genetically complex or highly environmentally sensitive, or both. We believe the observed segregation patterns as influenced by chromosome 3A were due to genes at multiple loci, each having a small effect that were environmentally sensitive. The RICLs represented a distribution of genotypes, that along with environmental influences, could give rise to continuous variation. No statistically significant transgressive segregation was observed for any of the traits under investigation, which would suggest if multiple loci controlled these traits, the genes were in coupling phase. Significant correlations were observed between anthesis date and plant height (r 5 0.59**), between anthesis date and kernel weight (r 5 20.56**), and between plant height and kernel weight (r 5 20.32**). These correlations may indicate that the genes on chromosome 3A controlling these traits may be linked or are pleiotropic. Because the number of environments that anthesis Table 2. Mean agronomic performance for Cheyenne (CNN), chromosome substitution line CNN(W13A), and 50 Cheyenne-Wichita recombinant inbred chromosome 3A lines (RICLs-3A), combined across a variable number of Nebraska environments. Seven environments Entry Grain yield ha21 CNN CNN(W13A) RICLs-3A Range LSD0.05† Mg 2.63 2.94 2.69 2.44–2.97 0.26 Kernel no. Spike21 no. 28.5 28.3 28.4 26.7–30.3 ns‡ 1000-kernel weight g 28.4 30.8 30.0 28.3–32.7 2.0 † LSD is for the comparison of individual line mean values. ‡ Nonsignificant (P . 0.05) genotypic variation. Six environments Spike no. m22 no. 331 326 312 276–363 ns‡ Grain volume weight hL21 kg 62.2 62.7 62.2 61.6–62.8 ns‡ Nine environments Four environments Plant height Anthesis date cm 104 99 102 97–107 4 d post May 1 40.3 36.8 37.8 35.1–41.1 1.3 SHAH ET AL.: GENETIC ANALYSIS OF AGRONOMIC TRAITS IN WHEAT 1019 Fig. 1. Frequency distribution for anthesis date (a), plant height (b) , grain yield (c), kernel number per spike (d), 1000-kernel weight (e), spike number per square meter (f), and grain volume weight (g) of 50 recombinant inbred chromosome lines-3A population derived from CNN and CNN(WI3A) parents of hard red winter wheat. The phenotype of the CNN and CNN(WI3A) parents are shown by arrows. The values indicated on the X-axis are the lower limits of each group. date (4), plant height (9), and kernel weight (7) were measured in are different, it is not possible to exclude the possibility that the correlations may be due to an environmental covariate. However, our experience is that early lines tend to be early everywhere in Nebraska, as are tall lines like CNN. None of the traits for which the RICLs exhibited significant variation (anthesis date, plant height, and kernel weight) were correlated with grain yield, indicating that the contributions of genes controlling these traits may be too small or modified by the environment to detect any correlation with grain yield. It was also possible that grain yield may be determined by independent genes on chromosome 3A. Law (1967) described the difficulties in identifying genes controlling complex characters such as grain yield and suggested the genes affecting yield could be better determined by identifying the genes affecting the components of yield. The scatter diagram (Fig. 2a) for anthesis date and plant height indicates that most of the early flowering lines were also short statured [similar to CNN(WI3A)], while most of the late flowering lines were also taller (similar to CNN). However, two lines had plant heights similar to CNN(WI3A) and shorter than CNN, but were as late flowering as CNN (Fig. 2a). Either a crossover had occurred between the genes controlling anthesis date and plant height or additional gene controls one of the traits. This pattern was also observed in the scatter diagram between anthesis date and kernel weight (Fig. 2b). For example, while most of the early lines had heavier kernel weights which is characteristic of CNN(WI3A), most of the late lines had lower kernel weights which is characteristic of CNN. Three lines were late flowering (similar to CNN) but had significantly heavier kernel weight [similar to CNN(WI3A)]. The lower correlation between plant height and kernel weight (Fig. 2c) showed little relationship between these traits. This study demonstrates the power of single-chromosome recombinant inbred lines as a means of locating and identifying loci controlling quantitative agronomic traits and also as a means of chromosome engineering. The field evaluation of RICLs-3A provided the strong evidence that previously identified genes for important agronomic traits on chromosome 3A were also present and segregating in the RICLs population, despite having large G 3 E interactions for some traits. Information from this population is more relevant than interspecific hybrid populations to plant breeders because of the past commercial importance and the significant genetic 1020 CROP SCIENCE, VOL. 39, JULY–AUGUST 1999 Fig. 2. Scatter diagram showing the relationship between mean plant height and anthesis date (a), 1000-kernel weight and anthesis date (b), and 1000-kernel weight and plant height (c) for 50 recombinant inbred chromosome lines-3A derived from CNN and CNN(WI3A) parents of hard red winter wheat. The CNN and CNN(WI3A) phenotypes are shown by arrows, and the degree of relationship is indicated by the correlation coefficient. contribution of CNN and WI to many current winter wheat cultivars (Berke et al., 1992b). Cheyenne has a coefficient of parentage of 0.33 with Wichita, 0.42 with ‘Colt’, 0.81 with ‘Lancer’, 0.16 with ‘Brule’, 0.43 with ‘Siouxland’, and 0.41 with ‘Centurk’. Wichita has a coefficient of parentage of 0.29 with Colt, 0.33 with Lancer, 0.13 with Brule, and 0.25 with Siouxland (Cox et al., 1985). In addition, Brule is the main parent of the cultivars ‘Arapahoe’, ‘Vista’, and ‘Niobrara’ which are widely grown in Nebraska and South Dakota. Hence, many genes from CNN and WI are likely present in currently grown cultivars in the Great Plains, and thus of interest to plant breeders today for wheat cultivar development. In conclusion, anthesis date was found to be simply inherited by detecting a single locus segregating in the RICLs-3A. Other traits did not fit simple monogenic ratios. They were either controlled by genes at multiple loci, influenced by the environment, or both, so that RICLs had a continuous distribution. It is interesting to note that the loci may be dispersed sufficiently far apart so that they appear to segregate independently and when coupled with environmental influences, produce a continuous distribution. Anthesis date, plant height, and kernel weight appeared to be controlled either by linked gene(s), or by pleiotropic genes with additional genes affecting one of the traits. The loci can further be mapped on the chromosome by the use of molecular marker techniques. REFERENCES Berke, T.G., P.S. Baenziger, and R. Morris. 1992a. Chromosomal location of wheat quantitative trait loci affecting agronomic performance of seven traits, using reciprocal chromosome substitutions. Crop Sci. 32:621–627. Berke, T.G., P.S. Baenziger, and R. Morris. 1992b. Chromosomal location of wheat quantitative trait loci affecting stability of six traits, using reciprocal chromosome substitutions. Crop Sci. 32: 628–633. Carver, B.F., E.L. Smith, and H.O. England, Jr. 1987. Regression and cluster analysis of environmental response of hybrid and pure line winter wheat cultivars. Crop Sci. 27:659–664. Cox, T.S., J.P. Murphy, and D.M. Rodgers. 1985. Coefficients of parentages for 400 winter wheat cultivars. Kansas State University, Agronomy Dep. Rep., Manhattan, KS. Joppa, L.R., D. Changheng, G.E. Hart, and G.A. Hareland. 1997. Mapping gene(s) for grain protein in tetraploid wheat (Triticum turgidum L.) using a population of recombinant inbred chromosome lines. Crop Sci. 37:1586–1589. Lander, E.S., and D. Botstein. 1989. Mapping Mendelian factors underlying quantitative traits using RFLP linkage maps. Genetics 121:185–199. Law, C.N. 1966. The location of genetic factors affecting a quantitative character in wheat. Genetics 53:478–498. Law, C.N. 1967. The location of genetic factors controlling a number of quantitative characters in wheat. Genetics 56:445–461. Law, C.N., A.J. Worland, and B. Giorgi. 1976. The genetic control of ear-emergence time by chromosome 5A and 5D in wheat. Heredity 36:49–58. Mahal, G.S., K.S. Gill, and G.S. Bhullar. 1988. Stability parameters and performance of inter-regional crosses in durum wheat (Triticum turgidum Desf.). Theor. Appl. Genet. 76:438–442. Muira, H., and A.J. Worland. 1994. Genetic control of vernalization, day-length response, and earliness per se by homoeologous group 3 chromosomes in wheat. Plant Breeding 113:160–169. BAKER ET AL.: GENETIC ANALYSIS OF FROGEYE LEAF SPOT RESISTANCE IN SOYBEAN Paterson, A.H., J.W. Deverna, B. Lanini, and S.D. Tanksley. 1990. Fine mapping of quantitative trait loci using selected overlapping recombinant chromosomes in an interspecific cross of tomato. Genetics 124:735–742. Person, C. 1956. Some aspects of monosomic wheat breeding. Can. J. Bot. 34:60–70. SAS Inst. 1990. Version 6, SAS/STAT user’s guide. Vol. 1 and 2. Cary, NC. Sharma, R.C., E.L. Smith, and R.W. McNew. 1987. Stability of harvest index and grain yield in winter wheat. Crop Sci. 27:107–108. Silis, D.Y., A.G. Kanevskaya, T.V. Shmakova, and E.N. Mironov. 1989. Effect of ecological factors on genetic control of quantitative characters in winter wheat plant height. Sov. Genet. 24:1297–1303. 1021 Snape, J.W., C.N. Law, B.B. Parker, and A.J. Worland. 1985. Genetical analysis of chromosome 5A of wheat and its influence on important agronomic characters. Theor. Appl. Genet 71:518–526 Yen, Y., and P.S. Baenziger. 1992. A better way to construct recombinant chromosome lines and their controls. Genome 35:827–830. Yen, Y., P.S. Baenziger, R. Bruns, J. Reeder, B. Moreno-Sevilla, and N. Budak. 1997. Agronomic performance of hybrids between cultivars and chromosome substitution lines. Crop Sci. 37:396–399. Zemetra, R.S., R. Morris, and J.W. Schmidt. 1986. Gene location for heading date using reciprocal chromosome substitutions in winter wheat. Crop Sci. 26:531–533. Genetic Analysis of Frogeye Leaf Spot Resistance in PI54610 and Peking Soybean William A. Baker, David B. Weaver,* Jiansheng Qiu, and Patrick F. Pace ABSTRACT Frogeye leaf spot (FLS) of soybean [Glycine max (L.) Merr.] is caused by the fungus Cercospora sojina Hara. The fungus is ubiquitous, but only problematic in hot humid soybean-producing regions such as Brazil, China, Nigeria, and the southern USA. Significant yield losses (10–50%) are commonly associated with FLS epidemics. The quantification of unique alleles for resistance within the southern germplasm pool is an essential step toward developing a more usable set of differential genotypes and thereby clarifying the race situation within the C. sojina–soybean interaction. Our objective was to determine the inheritance of resistance to FLS in PI54610 and Peking and their allelic relationship to Rcs3. ‘Lee’ soybean was used as a susceptible parent for crosses and control in all experiments. Parents and F2 seedlings were inoculated with a C. sojina spore suspension in the greenhouse or field and then rated for disease development 14 to 21 d later. On the basis of segregation ratios (3:1 resistant/susceptible in Peking 3 Lee and PI54610 3 Lee, and 15:1 in ‘Davis’ 3 Peking and PI54610 3 Peking), we found resistance in Peking was determined by a single dominant gene nonallelic to Rcs3. We also found, based on nonsegregation of resistance within the Davis 3 PI54610 population, that PI54610 has the same gene as in Davis (Rcs3). Resistance in Peking should be considered unique for the purpose of race differentiation and as a commercial source of resistance to FLS should Rcs3 fail. F rogeye leaf spot of soybean is caused by the phytopathogenic fungus C. sojina Hara (Hara, 1915). Cercospora sojina infects leaves, stems, and seeds of soybean (Sinclair and Backman, 1989). The pathogen has a worldwide distribution, with significant yield losses reported in China (Ma, 1994), Brazil (Yorinori, 1992), Nepal (Manandhar and Sinclair, 1982), Nigeria (Akem et al., 1992), and the USA (Laviolette et al., 1970; Hartwig, 1990). The severity of FLS infection is highly dependent on the environmental conditions (Akem and Dashiell, 1994) and disease development is favored in warm humid environments. William A. Baker and David B. Weaver, Dep. of Agronomy and Soils, Auburn, AL 36849; Jiansheng Qiu, Dep. of Plant Pathology, Auburn, AL 36849; and Patrick F. Pace, DEKALB Genetics Corporation, 3100 Sycamore Road, DeKalb, IL 60115. Contribution from the Dep. of Agronomy and Soils, Auburn Univ., Auburn, AL 36849. Received 3 Aug. 1998. *Corresponding author (dweaver@acesag. auburn.edu). Published in Crop Sci. 39:1021–1025 (1999). The fungus was first described in Japan (Hara, 1915). The first report of FLS in the USA occurred in 1924 (Melchers, 1925) with significant yield reductions in the Midwest attributed to the pathogen by the late 1940s (Athow and Probst, 1952). Genetic resistance is the most stable, economical, and environmentally sound strategy for control of C. sojina (Sinclair and Backman, 1989); however, the appearance of new highly virulent races such as race 2 in the late 1950s (Athow et al., 1962), races 3 and 4 in the mid 1960s (Ross, 1968), and race 5 in the late 1970s (Phillips and Boerma, 1981, 1989) has demonstrated the need for the identification of alternative sources of genetic resistance to this pathogen. Studies involving yield loss assessments have been performed in Brazil, Nigeria, and the USA. Yield reduction due to FLS has been attributed to reduced photosynthetic capacity and premature defoliation of infected leaves (Akem and Dashiell, 1994), causing a reduction in seed size rather than seed number (Horn et al., 1975; Dashiell and Akem, 1991). A 3-yr study in Indiana with the susceptible ‘Clark’ reported yield losses of up to 21% due to infection by C. sojina (Laviolette et al., 1970). A study in Brazil, involving six cultivars, showed inoculated plots yielded 14% less than noninoculated plots (Yorinori, 1992). Studies in Nigeria, designed to elucidate the interaction between fungicides and natural inoculum levels on three different cultivars, reported a yield loss of 66% in a susceptible genotype (Dashiell and Akem, 1991). Comparison of yields between lines resistant and susceptible to FLS in the USDA Uniform Tests indicated losses ranged from 10% at Quincy, FL (Hartwig and Edwards, 1989) to 30% at Tallassee, AL (Hartwig, 1990). A comparison of near isogenic lines (NILs) resistant or susceptible to FLS was performed in multiple locations across three growing seasons. A significant yield loss of up to 31% was found between the resistant and susceptible NILs (Mian et al., 1998). Twenty-two races of C. sojina occur in Brazil (Yorinori, 1992). Some of the recently identified races are virulent against cultivars that previously had shown resistance (Yorinori, 1992). A report from China identified eleven races of C. sojina (Hu et al., 1995). At least Abbreviations: FLS, frogeye leaf spot; NIL, near isogenic line; PDA, potato dextrose agar.