The Journal of Heredity 2001:92(6)
© 2001 The American Genetic Association 92:509-511
Brief Communication |
Linkage Between Loci Controlling Nodulation and Testa Variegation in Peanut
From the Department of Plant and Soil Sciences, Oklahoma State University, Stillwater, OK 74078-6028 (Dashiell), Agronomy Department, P.O. Box 110300, University of Florida, Gainesville, FL 32611-0300 (Gallo-Meagher), and Agricultural Research and Education Center, University of Florida, Marianna, FL 32446 (Gorbet).
Address correspondence to Maria Gallo-Meagher at the address above.
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Abstract |
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Linkage of loci controlling nodulation (N1) and testa variegation (V) was studied for cultivated peanut (Arachis hypogaea L.). The lines M4-2 (nonnodulating, variegated; VVn1n1n2n2N3N3) and UF 487A (nodulating, nonvariegated; vvN1N1n2n2N3N3) were used as parents in the crosses M4-2 x UF 487A, M4-2 x (UF 487A x M4-2), and their reciprocals. Individual plants were evaluated for nodulation and testa variegation in the F1, F2, F3, F1BC1, and F2BC1 generations. Data indicate that the N1 and V loci are linked with calculated crossover percentage of 7.1%.
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Introduction |
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Although the inheritance of numerous traits has been studied in peanut (Arachis hypogaea L.), there have been surprisingly few reports of linkage (Knauft and Ozias-Akins 1995). Patel et al. (1936) reported that growth habit and branching type did not segregate independently. They estimated the rate of crossing over between the genes for growth habit and branching to be 20%. Patil, as reported by Hammons (1973), found that the crossover rate between genes for growth habit and pod reticulation was 40.4% and the crossover rate between genes for stem hairiness and pod reticulation was 31.5%. Coffelt and Hammons (1973) observed an association between albino seedlings and small seed size. Murthy et al. (1988) reported that four genes for pod shape were linked, and Knauft et al. (1991) found linkage between loci for orange corolla color and purple testa. Garcia et al. (1996) identified two peanut genes, Mae and Mag, that conditioned resistance to the root-knot nematode Meloidogyne arenaria (Neal) race 1 and were tightly linked with a crossover rate of 18%.
Nonnodulating peanuts have been identified in progeny from certain crosses in Florida (Gorbet and Burton 1979), Georgia (Essomba et al. 1991), and India (Dutta and Reddy 1988; Nigam et al. 1980, 1982). Dutta and Reddy (1988) reported that nodulation was controlled by three independent genes, with nodulation being a product of two genes and inhibited by a third gene when it is dominant and the others are homozygous recessive (n1n1n2n2N3N3; n1n1n2n2N3n3). We confirmed this model for inheritance of nodulation, except that in our study there was a parental influence that was observed when n1n2N3 male gametes fused with n1N2- female gametes or when n1n2n3 male gametes fused with n1N2N3 female gametes (Gallo-Meagher et al. 2001). These unions resulted in plants that had reduced nodulation or nonnodulation instead of the expected normal nodulation.
Three alleles have been identified which control red testa color in peanut (Ashri 1969, 1970; Holbrook and Branch 1989). The dominant allele R1 and the recessive r2 and r3 alleles produce red testa color. Branch and Hammons (1979) found that inheritance of red on white testa variegation in peanut fit a genetic model for incomplete dominance at one locus. The genotypes designated VV, Vv, and vv produced the phenotypes variegated, trace amount of variegation, and no variegation, respectively. They later confirmed earlier reports on the inheritance of testa variegation and the R2 locus and presented evidence for independent assortment between the V and R2 genes (Branch and Hammons 1980).
Observations made during preliminary studies on nonnodulating peanut indicated that nonnodulated plants often had variegated testa. In this study, crosses were made in which there would be segregation for both nonnodulation and testa variegation. The objective was to determine if the gene controlling testa variegation was linked to a gene or genes controlling nodulation.
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Materials and Methods |
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Two peanut (A. hypogaea ssp. hypogaea var. hypogaea) genotypes were used as parents: M4-2 is nonnodulating with a red-white variegated testa (VVn1n1n2n2N3N3), and UF 487A is nodulating with a solid pink testa (vvN1N1n2n2N3N3) (Gallo-Meagher et al. 2001). The crosses M4-2 x UF 487A and its reciprocals were made. F1, F2, F3, F1BC1, and F2BC1 generations were field grown at the University of Florida Agricultural Research and Education Center (Marianna, FL). Recommended agronomic practices were utilized, including inoculation of seed at planting with cowpea-type Rhizobium spp.
All F1, F2, and F1BC1 plants were tagged before digging and 30 plants per plot were tagged in selected F3 plots immediately after digging. Roots of individual plants were visually classified into two groups: nodulated and nonnodulated, as previously described (Gallo-Meagher et al. 2001). Pod samples were hand picked from all plants that were tagged. If a plant was to be progeny tested, 50 or more pods were removed; at least three pods each were collected from other plants. Testa were examined in the laboratory and classified as solid, trace amount of variegation (trace-v), or variegated, as previously described (Branch and Hammons 1979, 1980). These data were then analyzed by chi-square tests for goodness-of-fit to the proposed model.
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Results and Discussion |
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All F1 plants examined were nodulated and produced seed with trace-v testa. These results are consistent with findings of other studies on inheritance of testa variegation (Branch and Hammons 1979, 1980) and nodulation (Dutta and Reddy 1988; Gallo-Meagher et al. 2001; Nigam et al. 1980, 1982) in peanut. The F2 plants segregated into three phenotypic categories for testa classification: solid, trace-v, and variegated. In some F2 plants which produced testa with trace-v, the variegation area on the seed was difficult to detect and could not be seen on all seeds. When 50 or more pods were collected from a plant, testa classification was accurate and was verified with progeny testing. However, some plants that produced seed with trace-v were probably classified as solid, therefore these two categories, trace-v and solid, were combined for analysis of the F2 data. Ten F1 families were scored, with 963 having solid or trace-v testa and 361 having variegated testa. Total (6.88), pooled (3.63, P = .06), and homogeneity (3.23, P = .95) chi-squared values fit a 3:1 ratio of solid plus trace-v:variegated, thus indicating that testa variegation is controlled at a single locus in these crosses. Based on the allele symbols used in previous studies on the inheritance of testa variegation, the solid, trace-v, and variegated phenotypes have the genotypes vv, Vv, and VV, respectively (Branch and Hammons 1979, 1980). Our earlier work showed that UF 487A x M4-2 resulted in a 3:1 F2 ratio (nod:nonnod) and that their respective genotypes are N1N1n2n2N3N3 and n1n1n2n2N3N3 (Gallo-Meagher et al. 2001). This result was confirmed in the present study as the 10 F1 families were scored as 973 being nodulated and 351 being nonnodulated. Total (7.34), pooled (1.61, P = .20), and homogeneity (5.73, P = .77) chi-square values fit the expected 3:1 ratio. Therefore, the genotypes of UF 487A and M4-2 are vvN1N1n2n2N3N3 and VVn1n1n2n2N3N3, respectively.
When the combined F2 data for nodulation and testa variegation were tested by chi-square analyses for goodness-of-fit against the expected ratios, low probability values for pooled and high probabilities for homogeneity chi-squares indicated that the two traits did not segregate independently (Table 1). The F2 segregation for nodulation from the UF 487A x M4-2 cross was controlled at the N1 locus because the F1 generation was heterozygous at the N1 locus and homozygous recessive at the N2 locus (VvN1n1n2n2N3N3). High chi-square values and low probabilities that deviations from expected are due to chance support linkage of testa variegation and nodulation in this cross and indicate that the V and N1 loci are linked.
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The calculated crossover percentages of each F1 population ranged from 6.2% to 8.0%. The mean crossover percentage was weighted according to the number of observations in each population. The weighted mean was 7.1% for UF 487A and M4-2. This value was then used as the best estimate of the crossover percentage when calculating the expected value to be used in the chi-square test. The results obtained from F3 plants, which were in F2 families that were segregating for nodulation and testa variegation, are presented in Table 2. Chi-square analyses of the frequencies of the F2 families (F3 data) indicated no significant differences from the expected frequencies, figuring the indicated crossover percentage. The F3 data thus supported the F2 findings for linkage (Table 1).
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The observed frequencies in the F1BC1 generation were not significantly different from the expected frequencies calculated with the indicated crossover percentage (P = .38). F2 plants are classified as noncrossover, crossover, or two crossover types by comparing the F2 plant's testa variegation with the segregation for nodulation in the F3 (Table 3). When the number of noncrossover, crossover, and two crossover F2 plants were compared with the expected numbers, assuming the appropriate crossover percentage, no significant difference was detected.
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A comparison of testa variegation of F1BC1 plants with segregation for nodulation in the F2BC1 was performed with F1BC1 plants being classified as noncrossover or crossover. There is no two-crossover classification for F1BC1 plants because a crossover can be detected only if it occurs in gametogenesis of the hybrid parent. No significant difference was detected when the number of noncrossover and crossover F1BC1 plants were compared with the expected values (P = .80).
Results from the F2, F3, F1BC1, and F2BC1 generations have shown that the N1 and V loci are linked. The recombination frequency between N1 and V was 7.1% for the parental combination UF 487A and M4-2. This is the strongest linkage thus far reported in peanuts.
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Acknowledgments |
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This work was approved for publication as Journal Series no. R-07617 by the Florida Agricultural Experiment Station.
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Footnotes |
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Corresponding Editor: Sally Mackenzie
Received June 6, 2000
Accepted June 30, 2001
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References |
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Ashri A, 1969. A second locus controlling red testa in peanuts, Arachis hypogaea. Crop Sci 9:515–517.
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Branch WD and Hammons RO, 1979. Inheritance of testa color variegation in peanut. Crop Sci 19:786–788.
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