Published in Crop Sci. 43:2028-2030 (2003).
© 2003 Crop Science Society of America
677 S. Segoe Rd., Madison, WI 53711 USA
CROP BREEDING, GENETICS & CYTOLOGY
The Pd2 and Lf2 Loci Define Soybean Linkage Group 16
Thomas E. Devine*
Sustainable Agricultural Systems Laboratory, Animal and Natural Resources Institute, Beltsville Agricultural Research Center-West, USDA, ARS, 10300 Baltimore Ave., Beltsville, MD 20705
* Corresponding author (DevineT{at}ba.ars.usda.gov).
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ABSTRACT
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Despite progress made in constructing the molecular map of soybean [Glycine max (L.) Merr.], many genes remain to be located on the classical map. The genetic linkage studies reported here were undertaken to advance the construction of the classical genetic map. Hybridizations were made in the field and the F2 and F3 generations were produced and classified in the greenhouse or field at Beltsville, MD. Data were tested by chi-square for single-factor segregation and linkage. Recombination estimates were made by the maximum likelihood method. The Lf2 locus (controlling leaflet number) segregated independently of the T, Ln, W1, Rj1, F, Y17, Df4, Fr2, and Pc loci. The Pd2 locus (controlling pubescence density) segregated independently of the W1 locus. The chi-square for Lf2 and Pd2 strongly indicated linkage. The recombination frequency between the Lf2 and Pd2 loci was estimated at 12 ± 2.2%. Soybean breeders attempting to couple or decouple the Lf2 and Pd2 alleles in their breeding populations should take cognizance of this linkage association in estimating the population sizes required to achieve their goals.
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INTRODUCTION
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ALTHOUGH CONSIDERABLE PROGRESS is being made in constructing a molecular map of soybean, the location of genes controlling many traits expressed at the organismal or whole plant level remains a challenge to soybean breeders and geneticists. There are 20 chromosome pairs in the soybean genome (Hymowitz and Singh, 1987). Although 21 classical linkage groups (CLG) or linkage fragments have been identified, many of these fragments consist of three or fewer linked loci and many of these fragments have not been tested for possible association with other fragments (Palmer and Kilen, 1987; Palmer and Shoemaker, 1998). Recently, evidence that CLG 6 should be combined with CLG 8 has been reported (Mahama et al., 2002). Cregan et al. (1999) described the efforts to integrate the classical genetic map with the molecular map. They reported that of the estimated more than 250 named genes controlling traits identified without molecular technology, 68 had been associated in 20 classical linkage groups (CLG) or linkage fragments. The synthesis of several molecular maps with classical genes reported by Cregan et al. (1999) associated 14 classical loci into the molecular map. These 14 loci provided an anchor to the molecular map to associate other classical loci that were previously linked to these 14 loci by classical linkage studies. Additional information is needed to provide a more comprehensive classical genetic map of soybean. For this reason a series of genetic linkage studies were undertaken to advance the construction of the classical soybean genetic map. These studies focused on the Lf2 locus, controlling leaflet number.
Although soybean is usually thought of as a grain crop in North America, it was used primarily as a forage crop in the USA until the 1940s (Caldwell, 1973). The recent release of cultivars bred for use as forage has revived interest in the use of soybean as forage (Nayigihugu et al., 2002). The proportion of leaf tissue in the harvested forage often has a significant positive influence on the quality of the forage (Albrecht and Hall, 1995). The lf2 recessive allele conditions a seven-foliolate leaf. Such leaf morphology is of interest for potential in increasing leaf tissue in forage soybean. In addition, the more highly dissected leaf structure in a seven-foliolate leaf may reduce leaf drag during windstorms and thus reduce lodging.
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MATERIALS AND METHODS
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All T lines were obtained from the Soybean Genetic Type Collection maintained by the USDA at the University of Illinois at Urbana, IL. Gene symbols describe phenotypes as defined by Palmer and Kilen (1987). All hybridizations were performed in the field at Beltsville, MD, with parents T41, T255, T264, BV 12, BV 18, and BV 20. BV 12 was derived by selection of a line homozygous for the f allele, the pc allele, and the rj1 allele from progeny of the cross BARC-1 (Devine and O'Neill, 1986) x T141, with BARC-1 serving as the source of both the f allele and the rj1 allele and T141 serving as the source of the pc allele. In the field evaluation of the F2 plants derived from the BV 12 cross, the plants heterozygous for the curly pubescence trait (Pc pc) were not distinguishable from the plants homozygous for the recessive alleles (pc pc). Therefore, for the purposes of this analysis, the Pc pc and pc pc classes were combined and the curly pubescence trait was treated as if it were controlled by a dominant allele rather than a recessive allele. BV 18 was derived by selection of a line homozygous for the fr2 allele from the cross PI 290136 x T263. The line Clark rj1 was obtained from the USDA Soybean Germplasm Collection at the University of Illinois at Urbana. BV 20 was homozygous for both the lf2 allele and the y17 allele and was selected from the F2 progeny of the cross T255, carrying the lf2 allele, and T162 carrying the y17 allele. The y17 allele conditions light yellowish-green leaf phenotype. T264 carries the Pd2 allele conditioning dense pubescence.
The F1 plants were grown in the greenhouse at Beltsville to produce F2 seed. Chi-square was partitioned to test for single factor segregations and linkage. Where single factor segregations fit the expected segregation ratio the method of maximum likelihood was used to estimate recombination (Mather, 1951; Allard, 1956). Where single factor segregations deviated from the expected 3:1 ratio, the product method was used to estimate recombination (Immer and Henderson, 1943). The bisection method was used to solve the maximum likelihood equations (Yakowitz and Szidarovszky, 1989).
F2 Tests
The hybridizations made and genes tested for linkage associations were as follows.
F3 Tests
To test for the linkage of lf2, conditioning seven-foliolate leaflets, and Pd2, conditioning dense pubescence, genetic stock BV 20 was hybridized with T264. The F2 seeds were planted in the field at Beltsville and F3 seeds were hand harvested from individual F2 plants. The F3 seeds were planted in 3-m rows in the field at Beltsville and the F3 plants were scored for dense or normal pubescence, three- or seven-foliolate leaflets, and chlorotic or green leaves. The number of plants per row ranged from 27 to 16. A few rows with less than 16 viable plants were excluded from the analysis. The information obtained and the resolving power of the analysis was enhanced by conducting the test in the coupling phase and advancing to the F3 generation.
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RESULTS AND DISCUSSION
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F2 Data
As indicated in Table 1, no evidence of linkage was detected for the Lf2 locus and the T locus in CLG 1, the Ln locus in CLG 4, the W1 locus in CLG 8, the Rj1 or F loci in CLG 11, or the Y17 locus in CLG 14 (Table 1). Further, no evidence was found for linkage of the Lf2 locus with the Df4, Fr2 or Pc loci, which presently are not assigned to a CLG. No evidence of linkage of the Pd2 locus with the W1 locus was found. The Lf2 locus previously was found to segregate independently of the Rj1 locus in CLG 11 (Devine et al., 1983).
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Table 1. Results of genetic linkage tests and linkage analyses in F2 populations for the Lf2 and Pd2 loci in soybean.
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F3 Data
In the BV 20 x T264 cross, the F2 genotypes were rationalized from the F3 phenotypes (Table 2). The chi-square analysis established that the lf2, pd2, and y17 loci had the expected 3:1 segregations and, therefore, the method of maximum likelihood was appropriate for estimating recombination. Chi-square analysis indicated that the y17 locus was not linked to either the pd2 or lf2 loci. The linkage chi-square for the association of the lf2 locus and the pd2 locus was very high, thus strongly indicating linkage. The recombination frequency between the lf2 and pd2 loci was estimated at 12% ± 2.2%. Since the test was conducted in the coupling phase and the F3 generation was employed to resolve the genotypes of the F2 generation, the precision of the estimate was strengthened. Neither the Pd2 locus nor the Lf2 locus has been associated with any CLG nor associated with any molecular linkage group of Cregan et al. (1999). Soybean breeders attempting to couple or decouple the lf2 and Pd2 alleles in their breeding populations should take cognizance of this linkage association in estimating the population sizes required to achieve their goals.
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Table 2. Results of genetic linkage test and linkage analyses in an F3 population for the Pd2, Lf2, and Y17 loci in the soybean cross BV 20 x T264.
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ACKNOWLEDGMENTS
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Appreciation is expressed to Mr. Michael P. Donovan of Archbishop Spalding High School for his help in collecting and analyzing data on the linkage of the Pd2 and Lf2 loci. Appreciation is also expressed to Kyle Stuart Hamm of Gonzaga College High School for his help in testing for linkage in the T255 x BV-12 cross and in summarizing the data. Appreciation is due to Andrew Bergen of Eleanor Roosevelt High School for help in testing for linkage in the T41 x BV-20 cross, Benjamin Horlacher of Eleanor Roosevelt High School for help in testing for linkage in the BV-20 x T256 cross, and Paul Miller of Gonzaga College High School for help in testing for linkage in the T255 x BV-18 cross.
Received for publication January 8, 2003.
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