© 2004 The American Genetic Association
Brief Communication |
Segregation Distortion for Seed Testa Color in Mungbean (Vigna radiata L. Wilcek)
From University of Queensland School of Land and Food Sciences, St. Lucia, Brisbane 4072, Australia (Lambrides and Godwin); James Cook University, Townsville 4811, Australia (Lawn); and CSIRO Tropical Agriculture, 306 Carmody Rd., St. Lucia, Brisbane 4067, Australia (Imrie)
Address correspondence to C. J. Lambrides at the address above, or e-mail: chris.lambrides{at}uq.edu.au.
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Abstract |
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Genetic segregation experiments with plant species are commonly used for understanding the inheritance of traits. A basic assumption in these experiments is that each gamete developed from megasporogenesis has an equal chance of fusing with a gamete developed from microsporogenesis, and every zygote formed has an equal chance of survival. If gametic and/or zygotic selection occurs whereby certain gametes or zygotic combinations have a reduced chance of survival, progeny distributions are skewed and are said to exhibit segregation distortion. In this study, inheritance data are presented for the trait seed testa color segregating in large populations (more than 200 individuals) derived from closely related mungbean (Vigna radiata L. Wilcek) taxa. Segregation ratios suggested complex inheritance, including dominant and recessive epistasis. However, this genetic model was rejected in favor of a single-gene model based on evidence of segregation distortion provided by molecular marker data. The segregation distortion occurred after each generation of self-pollination from F1 thru F7 resulting in F7 phenotypic frequencies of 151:56 instead of the expected 103.5:103.5. This study highlights the value of molecular markers for understanding the inheritance of a simply inherited trait influenced by segregation distortion.
Classic genetic experiments with plant species are based on the observation and/or measurement of phenotypes in segregating populations designed to be polymorphic for the trait of interest. Generally F1, BC1, F2, F3, recombinant inbred, or doubled haploid populations are used. When studying the inheritance of a trait in these populations, it is assumed that gene frequency is robust and that gametic and zygotic selection are minimal. That is, each gamete developed from megasporogenesis has an equal chance of fusing with a gamete developed from microsporogenesis, and every zygote formed has an equal chance of survival. If gametic and/or zygotic selection occurs whereby certain gametes or zygotic combinations have a reduced chance of survival, progeny distributions are skewed and are said to exhibit segregation distortion. With the use of molecular markers it is now possible to gauge the level of segregation distortion occurring for a population under study.
Segregation distortion has been reported to be a common phenomenon in studies of numerous plant species and genera (e.g., lucerne [Echt et al. 1994], tomato [Paterson et al. 1988], barley [Heun et al. 1991], soybean [Keim et al. 1990], and Lablab purpureus [Konduri et al. 2000]). The extent of distortion can be quite extreme. Most studies that show severe segregation distortion involve populations derived from wide hybridizations, usually interspecific crosses. For example, Paterson et al. (1988) reported that 69% of markers segregated with distortion in an F2 population derived from the interspecific hybrid of Lycopersicon esculentum and Lycopersicon chmielewskii.
In this study we report severe segregation distortion for testa color in mungbean populations derived from more closely related taxa. We show that this distortion could lead to erroneous conclusions about the inheritance of an easily scored trait such as testa color.
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Materials and Methods |
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Genetic Material
This inheritance study was based on genetic material derived from an intersubspecific mungbean cross between cv. Berken and ACC 41, an Australian collection of wild mungbean (Vigna radiata ssp. sublobata), the putative progenitor of cultivated mungbean. The first cross, designated CROSS 1, was made in 1983 by Dr R. J. Lawn, and it provided F2 seed from the self-pollination of a single F1 plant. Two samples of F2 plants, designated CROSS 1-A and CROSS 1-B, were grown in a greenhouse; CROSS 1-A consisted of 140 plants grown and scored for testa traits in 1985 (by Dr R. W. Williams and Dr R. J. Lawn), and CROSS 1-B consisted of 67 plants grown in 1992. The entire F2 population of CROSS 1 was advanced by single seed decent (by the senior author) to form a population of 207 F8 recombinant inbred lines (RILs). The 67 F2 individuals of CROSS 1-B and the 67 RILs derived from them were also used as mapping populations for the construction of an F2 genetic linkage map (hereafter referred to as F2-Map) and a RIL genetic linkage map (hereafter referred to as RIL-Map) (Lambrides et al. 2000).
Scoring of Testa Color
Because the testa layer is derived from tissue laid down by the maternal parent (maternal effects), testa color phenotypes were determined from seeds of the following generation. For example, F2 testa color phenotypes were determined from F2:3 seeds and F7 phenotypes from F7:8 seeds. The Berken cultivar has green (g) testa color and ACC 41 has green-speckled-black (gsb) testa color. Therefore segregates of the various Berken x ACC 41 crosses were scored as g or gsb.
Genetic Linkage Maps
Details of the genetic linkage maps F2-Map and RIL-Map are presented elsewhere (Lambrides et al. 2000). In brief, F2-Map was composed of 115 markers and based on a framework set of restriction fragment length polymorphism (RFLP) markers derived from a more comprehensive mungbean RFLP map (Menancio-Hautea et al. 1992). RIL-Map was composed of 115 markers and based on a framework set of randomly amplified polymorphic DNA (RAPD) markers selected from F2-Map. The mungbean genome is small, only 560 Mb (Arumuganathan and Earle 1991), consequently the framework sets of markers used for F2-Map and RIL-Map were considered to give satisfactory coverage for this study.
Classical Genetic Analysis of Testa Color
Various two-class genetic models were tested using chi-square tests for goodness-of-fit using the basic formula (Mather 1963)
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Homogeneous data were pooled and deviation chi-square values calculated.
Mapping of Testa Color
The trait testa color was mapped in both the F2 and RI populations using the methods described previously (Lambrides et al. 2000). For mapping testa color in the F2, an F2:3 progeny test was used to identify individuals segregating for testa color. In the RI population, testa color was scored as having either the wild or cultivated phenotype, assuming that each individual was homozygous. All scores were analyzed using MapMaker/EXP version 3.0b (Lander et al. 1987; Lincoln et al. 1993) to determine linkage by the random mapping approach.
Three Models of Segregation of the Testa Color Trait
Three models of segregation for the testa color trait were tested. Testa color was assumed to be controlled by a single locus for all models. Model 1 assumed that segregation occurred without distortion after every generation of self-pollination, with the additional assumption that F2 genotypic frequencies were in the ratio 0.25 AA + 0.5 AB + 0.25 BB. Model 2 is the same as model 1, except F2 genotypic frequencies used for the calculations were derived from the observed F2 segregation pattern (see the Table 3 footnotes for an explanation). Model 3 assumed that segregation occurred with distortion after every generation of self-pollination up to the F7.
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Results and Discussion |
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TopAbstractMaterials and MethodsResults and DiscussionConclusionReferences |
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The Inheritance of Testa Color Using a Classic Genetics Approach
Previous studies with ACC 41 suggested that the gsb testa color in this line was controlled by an allele acting with complete dominance over g (Lawn et al. 1988). Therefore the hypothesis that ACC 41 possesses a single dominant allele conferring green-speckled-black testa color was tested. Under this hypothesis, segregation ratios of 3:1 in the F2 and 1:1 in the F7 were expected.
The testa color of all F1 (gsb x g) individuals was (gsb), indicating dominant gene action in the wild parent, ACC 41. Only one of the two F2 populations scored fit the expected 3 gsb:1 g ratio (Table 1). Chi-square tests of heterogeneity indicated that the data from each F2 population were homogeneous and could be pooled. The segregation ratio of the pooled F2 data (207 individuals) deviated significantly from the proposed 3:1 model and the F7 data deviated significantly from a 1:1 model (Table 1). These segregation patterns were unlikely to be associated with statistical sampling error given the large population sizes (more than 200 individuals) studied. Generally the number of green-speckled-black phenotypes was more than expected and the number of green phenotypes fewer than expected. Since a more complex mode of inheritance for testa color in this cross was indicated, several two-gene models were tested.
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The data gave an excellent fit to a two-gene model that included both dominant and recessive epistasis, but did not fit the models that included either complementary (9:7 F2 ratio) or duplicate gene action (15:1 F2 ratio) (data not shown). A model with dominant and recessive epistasis was tested. This model involves gene action at two loci, with dominant alleles at one locus and recessive alleles at an independent locus. For this model, segregation ratios of 13:3 in the F2 and 3:1 in the RI populations would be expected. The pooled F2 data (207 individuals) gave a very good fit to a 13:3 ratio (P = .30–.50), as did the RI data to a 3:1 ratio (P = .50–.75) (Table 1), lending strong support to the epistasis model.
The Inheritance of Testa Color Using Molecular Markers
Although the inheritance study indicated that the segregation of testa color significantly deviated from expectation for the single-gene model, linkage tests with more than 200 molecular markers detected only one region of the genome associated with the testa color trait. Marker analysis also revealed that the testa color trait mapped to a region of linkage group 2 that was previously shown to segregate with distortion (Lambrides et al. 2000). Three markers linked to testa color (pA235, OPAA16a, and OPAI17b) were identified and each segregated with distortion (Table 2).
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This led us to believe that the abnormal segregation of the testa color trait in these crosses may be due to segregation distortion rather than a complex genetic inheritance. To test whether the segregation pattern of testa color was influenced by segregation distortion, expected genotypic frequencies from F3 to F7 were estimated using three models (Table 3). These genotypic frequencies were used to estimate expected F7 phenotypic frequencies (Table 4). The estimated phenotypic frequencies were then compared with the observed frequencies and tested with chi-square analysis (Table 4). This analysis indicated that model 3 was clearly favored (
2 = 1.81, P = .22, ns), that is, segregation distortion had occurred in the region of the genome containing the testa color trait and after each generation of self-pollination.
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Conclusion |
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TopAbstractMaterials and MethodsResults and DiscussionConclusionReferences |
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We conclude that the testa color trait in the populations studied was controlled by a single locus and that the digenic model suggested by the classical analysis was erroneous due to the effects of segregation distortion. The level of segregation distortion observed in the Berken x ACC 41 populations observed here and elsewhere (Lambrides et al. 2000) have not been reported previously for intraspecific crosses of mungbean. These results are particularly relevant to our breeding program, where ACC 41 is used extensively as an excellent source of seed-weathering tolerance and resistance to bruchid beetle (Callosobruchus spp.) attack. To our knowledge this is the first report of abnormal segregation for a seed quality trait in the species V. radiata and the genus Vigna. This study highlights the value of molecular markers for understanding the inheritance of a simply inherited trait influenced by segregation distortion.
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Acknowledgments |
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This research was supported by a grant provided by the Grains Research and Development Corporation. The critical reading and suggestions made by Dr. Greg Rebetzke and Dr. Chunji Liu were greatly appreciated.
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Footnotes |
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Corresponding Editor: Prem Jauhar
Received October 27, 2003
Accepted June 4, 2004
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References |
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