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a Department of Biological Sciences, University of Alberta, Edmonton, Alberta, Canada T6G 2E9
b Crop Development Centre, 51 Campus Drive, University of Saskatchewan, Saskatoon, Saskatchewan Canada S7N 5A8
* Corresponding author (gordon.rowland{at}usask.ca).
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONREFERENCES |
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONREFERENCES |
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The most comprehensive studies on the seed color of flax were performed by Tammes (1922)(1928) and Shaw et al. (1931). According to Tammes (1922) seed color in flax is governed principally by three genes (factors)—G, D, and B'. Factor G was designated as the basic factor while the latter two (D and B') were regarded as modifying factors. The seed color was brown only in the presence of factor G but when present along with factor D, the color was grayish-green, the result of D acting as an inhibitory factor. Factor B' when present along with the basic factor G was not reported to show any inhibitory action on the latter, hence, the seed color remained brown. But when B' was in the presence of the other two factors, it was observed that factor B' neutralized the inhibitory action of D, thereby reverting to the common brown seed color. Tammes (1922) postulated that the seed coat of flax was colored only when the basic factor (G) was present and colorless in its absence (g). Later she explained that colorless or very light colored seed in flax apparently resulted from the visibility of the yellow cotyledons through the transparent seed coat. The presence of pigments in the seed coat resulted in colored seed.
The genetic system proposed by Shaw et al. (1931) differed from that of Tammes (1922)(1928) by including the factor M and in considering factor G as an additional factor. They found that factor G produced gray color in the seed coat while M, in association with D, produced fawn color. In the presence of the three factors (M, D, and G), the fawn color changed to brown. Yellow was found to be the basal seed color. The M locus has also been observed by Afzal Naz (1976). However, Beard and Comstock (1965) in their review of flax genetics and gene symbols, suggested the symbol Scsc for M, where Sc produces a fawn color and sc produces yellow seed color.
Barnes et al. (1960) reported that yellow seed was produced in the presence of homozygous recessive alleles at any of the three loci, while brown seed was produced in the presence of at least one dominant allele at all three loci. They indicated that seed color in flax was determined by two or three pairs of complementary genes. Comstock et al. (1963) and Comstock (1961) confirmed that the locus responsible for yellow seed color in Minerva flax was the g locus. It was found that the d locus was the accepted explanation for yellow seed color and had a pleiotropic effect on pink petals in the then commercially important cultivars such as ‘Viking’ and Bolley Golden (Comstock, 1966; Beard, 1967; Comstock et al., 1969). Clear evidence from the studies of Culbertson and Kommedahl (1956) involving Crystal flax indicated that the yellow seed trait in this cultivar was conditioned by a recessive allele at the b1 locus, which also had a pleiotropic effect on crimped white flowers.
Apart from the three basic loci, g, d, and b1, that have been reported to govern seed color in flax, there are studies that indicate a modifying gene, X (Comstock, 1971; Beard and Comstock, 1965; Shaw et al., 1931). It was found that this gene had an effect on the intensity of the seed color depending on the allelic status at this locus. A study done by Green and Dribnenki (1995) involving the breeding and development of ‘Linola’ (low-linolenic acid solin flax) reported a dominant gene Y, obtained from the Australian plant introduction CPI84495. In another study, Popescu and Marinescu (1996) reported a homozygous dominant gene Y1 in the German flax cultivar Bionda and speculated that this gene could be similar to the gene reported by Green and Dribnenki (1995) in the phenotype it produces. However, both these findings have not assigned any locus to the reported genes (Y or Y1).
A recent study involving crosses between brown and variegated seeds indicated that variegated seed coat was controlled by a single recessive gene (Saeidi and Rowland, 1997). They did not indicate the locus that conditioned the variegated seed coat. The objective of this study was to determine the allelic–gene relationship of the dominant yellow gene, the variegated recessive gene, and various spontaneous and unknown recessive yellow genes in flax.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONREFERENCES |
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On the basis of the dominance relationship, source of seed color and the results from the F2 generation confirmed by the F3 data, the 11 parental lines were categorized into five groups: CDC recessive yellow (YSED2, YSED4, S95407, and S96071), European recessive yellow (G-1186/94), dominant yellow (CPI84495 and YSED18), brown (Vimy, CDC Bethune, and ED47), and variegated (M96006) (Table 1).
All possible cross combinations (52 in total), excluding reciprocals, were made among the parents. For each cross, three flower buds were pollinated. After maturity all the bolls from the crosses and individual parental plants were harvested separately. No crosses were made among the three brown-seeded parents (ED47, Vimy, and CDC Bethune), since the brown seed color has been reported to be controlled by a dominant trait (Barnes et al., 1960) and hence, the expected result would only be brown seed. The entire crossing procedure was done in the growth chambers located in the College of Agriculture Phytotron, University of Saskatchewan, Saskatoon, Canada.
Following the crosses among the parents, test crosses were performed to establish the seed color genes possessed by each of the parents under study. These crosses were between the F1 plants and four USDA-ARS introductions obtained from the Plant Introduction Station, Ames, IA (Table 2). These plant introductions were reported (Culbertson and Kommedahl, 1956; Comstock, 1961; Comstock et al., 1969; Popescu and Marinescu, 1996) to possess the genes conditioning yellow seed color in flax (b1, g, d, and Y1).
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A chi-square goodness-of-fit test was used to test the F2 observed data against appropriate genetic (phenotypic) ratios. These tests were performed by means of Microsoft Excel (ver. 7.0, Microsoft Corp., Redmond, WA) software at a significance level (
) of 0.05. Data for each class of crosses were tested for heterogeneity and then pooled to determine the possibility of combining the data of each cross under the respective class.
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RESULTS AND DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONREFERENCES |
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The F1 plants were observed to be brown seeded in the variegated x brown crosses, confirming that brown seed color was dominant over the variegated seed color. The F2 observed data gave a good fit to a 3: 1 phenotypic ratio of brown to variegated (Table 4). Data pertaining to the test crosses showed a 1 brown: 1 variegated ratio with Crystal, which confirmed that the variegated seed color in the variegated parent is conditioned by an allele at the b1 locus (Table 5). The progeny of the crosses with Bolley Golden and Minerva were brown seeded while with Bionda were yellow seeded.
Thus far, the b1 locus conditioning variegated seed color has not been reported in flax. Saeidi and Rowland (1997) stated that the variegated trait in flax was controlled by a recessive gene, but did not mention the locus involved. However, it was found that the b1 conditions yellow seed color in Crystal flax when homozygous and also has a pleiotropic effect on white crimped flowers (Culbertson and Kommedahl, 1956; Culbertson et al., 1960). With regard to the new b1 allele, which conditions variegated seed color, no pleiotropic action was observed on flower shape or color. In view of the results obtained in this study, it is proposed that the third allele of the b1 gene be designated as ‘b1vg’ for variegated seed color. Data pertaining to the F2 generation also suggested that the variegated seed color was produced only in the presence of homozygous recessive alleles at the b1 locus (b1vgb1vg). Brown seed color was inherited as a simple dominant as observed by Culbertson and Kommedahl (1956).
In the CDC recessive yellow x European recessive yellow crosses the F1 plants were observed to be brown seeded. The F2 observed data gave a good fit to a 9 brown: 7 yellow phenotypic ratio (Table 4). This ratio inferred the presence of two independent recessive genes for yellow seed color in these parents.
The test cross data pertaining to these crosses gave a 1 brown: 1 yellow ratio with Minerva and Bolley Golden, which indicated that the loci involved in imparting yellow seed color in the CDC recessive yellow parents and European recessive yellow parent were g and d, respectively (Table 5). The data also suggested that the b1 locus was not involved in yellow seed color in either of the parents, since no variation for seed color was observed in the crosses between the F1 plants and Crystal. The progeny of the crosses with Bionda were all yellow seeded.
From these results, it could be explained that yellow seed was produced in the presence of homozygous recessive alleles of either of the genes, or both genes (G-dd, ggD-, and ggdd). However, brown seed resulted only when there was a dominant brown allele at both loci (G-D-). These observations clearly confirmed a pair of complimentary genes for brown seed color. This gene action is in agreement with the studies of Shaw et al. (1931) and Barnes et al. (1960). In a study involving 12 yellow seeded cultivars, Barnes et al. (1960) reported that the inheritance pattern for seed color was determined by two or three pairs of complimentary genes (9 brown: 7 yellow ratio or 27 brown: 37 yellow ratio, respectively).
The F1 plants were observed to be brown seeded in the European recessive yellow x brown crosses, which confirmed that brown seed color was dominant over yellow seed color. Chi-square analysis indicated that the segregation pattern of the F2 plants was not significantly different from the 3: 1 brown to yellow ratio expected for a monogenic inheritance model with brown to yellow (Table 4).
The data pertaining to the test crosses only indicated that the b1 locus was not involved in these crosses (Table 5). No successful test crosses were obtained with Bolley Golden to verify whether it was the d locus that conditioned yellow seed color in the European recessive yellow parent. In a similar trend, no successful progeny were obtained with Minerva. The progeny in crosses with Crystal and Bionda were brown- and yellow-seeded, respectively, indicating that it was neither the b1 nor the Y1 locus that was involved in conditioning yellow seed color in this parent. However, the observations made from the class CDC recessive yellow x European recessive yellow suggested that the locus conditioning yellow seed color in this parent was d. A further confirmation of the involvement of the d locus was evident from the observations on the homozyous F3 lines, which were all pink flowered. This observation was in corroboration with that made by Barnes et al. (1960). They had indicated that the genotype dd apart from conditioning yellow seed was also responsible for pink petals in flax.
Data pertaining to the F2 generation also implied that yellow seed was produced only in the presence of homozygous recessive alleles of the d gene (dd), while brown seed was produced in the presence of at least one dominant allele of the d gene (DD, Dd). These results were in agreement with the observations of Comstock (1966), Beard (1967), and Comstock et al. (1969) with regards to the d locus.
In the CDC recessive yellow x variegated crosses, the F1 seeds were observed to be brown seeded. The data pertaining to the F2 generation gave a poor fit to a 9 brown: 3 variegated: 4 yellow phenotypic ratio at the 0.05 P level. The F2 ratio (Table 4) indicated that the difference between the CDC recessive yellow and the variegated parents was due to two recessive genes. The poor fit of the pooled F2 data to the expected ratio was due to a lack of both variegated and yellow seeded plants. The most plausible reason for this deficit of variegated-seeded plants could be the semilethal action of the b1 locus (Tammes, 1928; Shaw et al., 1931; Myers, 1936). Comstock and Ford (1968) also indicated that yellow flaxseed conditioned by the b1 gene showed the least viability when compared with the other yellow seed conditioning loci (g and d). Thus, as b1vg is an allele of the b1 gene, the low rate of viability for the variegated lines could be explained in this manner too. However, another explanation for this deviation could be linkage.
The test cross data pertaining to these crosses showed that there was variation for seed color in crosses with both Crystal and Minerva. These observations indicated that the loci involved in the expression of yellow seed color in the CDC recessive yellow parents and the variegated seed color in the variegated parent were g and b1, respectively (Table 5). The data also suggested that the d locus did not condition seed color in either of the parents, since no variation for seed color was observed in the crosses between the F1 plants and Bolley Golden. The crosses performed with Bionda yielded only yellow-seeded progeny.
Data from the F2 generation suggested that variegated seed was produced by the homozygous recessive alleles of the b1 locus, in the presence of the dominant allele at the g locus, i.e., GGb1vgb1vg and Ggb1vgb1vg. Yellow seed was produced in the presence of homozygous recessive alleles of the g locus regardless of the allelic status of the b1 locus (ggB1B1, ggB1b1vg, and ggb1vgb1vg). These observations also indicated the epistatic nature of the homozygous recessive alleles of both genes (recessive epistasis).
The F1 seeds produced by the dominant yellow x brown crosses were observed to be yellow seeded, confirming that the yellow seed color was dominant over brown seed color. Chi-square analysis indicated that the segregation pattern of the F2 plants was not significantly different from the 3 yellow: 1 brown ratio expected for a monogenic inheritance model (Table 4). The crosses between the F1 plants and Bionda produced only yellow-seeded progeny. However, progeny of the crosses with Minerva, Crystal, and Bolley Golden showed variation for seed color (Table 5).
The F2 data also indicated that yellow seed was produced in the presence of a dominant allele of the Y1 gene. Brown seed (usually the dominant seed color in flax) was produced only in the presence of homozygous recessive allele of the Y1 gene (y1y1). Green and Dribnenki (1995) reported that yellow seed color was inherited as a dominant trait in all the crosses that had involved the introduction, CPI84495. They had assigned this dominant locus as Y. Later, Popescu and Marinescu (1996) studied the inheritance patterns involving the German cultivar Bionda with five brown seeded parents crossed in all possible combinations (including reciprocals) and suggested a single dominant homozygote allele, Y1Y1, for yellow seed color in Bionda. Hence, the results obtained in this study strongly corroborate those of Green and Dribnenki (1995) and Popescu and Marinescu (1996).
Data from the single cross among the dominant yellow parents showed no segregation for seed color both in the F2 and F3 generations (Table 4). This observation confirmed that the allele conditioning yellow seed color in these parents was the same. Most of the test crosses were unsuccessful and the progeny with Bionda were observed to be yellow seeded (Table 5). The F1 seed of crosses among the dominant yellow parents were also crossed to the brown seeded parents and the F2 progeny from these crosses segregated in a 3 yellow seeded: 1 brown seeded ratio. This was the same ratio that occurred when the individual dominant yellow parents were crossed with the brown parents (Table 4), further confirming that Bionda, CPI84495, and YSED18 all carry the same dominant yellow seed gene.
In the dominant yellow x CDC recessive yellow crosses, the F1 plants were observed to be yellow seeded. The F2 data gave a good fit to a 13 yellow: 3 brown phenotypic ratio (Table 4). This ratio indicated the presence of a single dominant gene for yellow seed color in the dominant yellow parents (CPI84495 and YSED18) and a single recessive gene for the same trait in the CDC recessive yellow parents (YSED2, YSED4, S95407, and S96071). The test cross data pertaining to these crosses (Table 5) did not show variation for seed color with Bionda while variation for seed color was observed with Minerva, Crystal, and Bolley Golden. Data from the CDC recessive yellow x brown and CDC recessive yellow x European recessive yellow crosses confirmed that the recessive locus involved in CDC recessive yellow parents was g, as found in Minerva and not b1 or d.
The F2 ratio also implied that the dominant Y1 gene in the dominant yellow parents had an inhibitory effect on the g gene. This suggested that in the presence of the dominant allele of the Y1 gene, the expression of the dominant allele of the g gene would be inhibited, thereby, resulting in yellow seed. However, its recessive allele (y1) does not seem to have any action by itself, hence, in the presence of the g allele, the seed color was brown (y1y1GG, y1y1Gg). Yellow seed was also produced in the double recessive condition, i.e., when both the genes were homozygous recessive (y1y1 gg). So far, there have been no reports of an inhibitory action of the Y1 locus. Green and Dribnenki (1995) and Popescu and Marinescu (1996) reported dominant genes, Y and Y1, in the introduction CPI84495 and in the German cultivar Bionda, respectively. These studies did not discuss the inhibitory nature of these genes, however.
In the single cross involving the dominant yellow parent (CPI84495) and the variegated parent (M96006), F1 seeds were observed to be yellow seeded. The observed F2 data gave a good fit to a 12 yellow: 3 brown: 1 variegated phenotypic ratio (Table 4). Thus, the F2 results confirmed that the difference between the dominant yellow and the variegated parents was due to two genes. Data pertaining to the crosses between the F1 plants and Bionda showed that all the progeny were yellow-seeded while variation for seed color was observed with Minerva, Crystal, and Bolley Golden (Table 5).
The F2 data also suggested that yellow seed was produced only in the presence of a dominant allele of the Y1 gene (dominant epistasis). Brown seed was produced only in the absence of the Y1 gene (y1y1B1B1, y1y1B1b1vg). Variegated seed was produced in the double recessive condition, i.e., homozygous recessive alleles for both the genes under consideration (y1y1b1vgb1vg). Studies involving the epistatic nature of the dominant gene Y1 (dominant epistasis ratio) have not been reported. The dominant action of the Y1 locus observed in this study corroborates the dominant nature of a similar gene reported by Green and Dribnenki (1995) and Popescu and Marinescu (1996).
The F1 seeds of the single cross between the dominant yellow parent (CPI84495) and the European recessive yellow parent (G-1186/94) were yellow. The observed F2 data gave a good fit to a 13 yellow: 3 brown phenotypic ratio (Table 4). This observation indicated that the difference between the dominant yellow and the European recessive yellow parents was due to two genes. From the F2 ratio it could also be inferred that there are two epistatic genes, one dominant and one recessive, governing yellow seed color in these parents. The crosses between the F1 plants and the four test parents were mostly unsuccessful (Table 5).
The F2 ratio showed that yellow seed was produced in the presence of a dominant allele of the Y1 gene (DDY1Y1, DdY1y1, ddY1Y1, ddY1y1). However, yellow seed was also produced in the absence of the Y1 allele but in the presence of homozygous recessive alleles of the d gene (y1y1dd), i.e., in the double homozygous recessive condition. Brown seed was produced in the presence of at least a single dominant allele of the d gene but only in the absence of the Y1 allele (DDy1y1, Ddy1y1).
In the single European recessive yellow x variegated cross, the F1 seeds were observed to be brown. From this observation and the observations pertaining to the CDC recessive yellow x European recessive, CDC recessive yellow x variegated, and variegated x brown crosses, it was confirmed that the recessive gene conditioning yellow seed in the European recessive yellow parent (G-1186/94) was different from the recessive gene governing variegated seed color in the variegated parent (M96006).
The observed F2 data gave a good fit to a 9 brown: 3 variegated: 4 yellow phenotypic ratio (Table 4). Thus, the F2 results indicated that the difference between the European recessive yellow and the variegated parents was due to two independent recessive genes. The test cross data for these crosses indicated that the loci involved in imparting yellow seed color in the European recessive yellow parents and variegated seed color in the variegated parent were d and b1, respectively (Table 5). The data also suggested that the g locus was not involved in conditioning either yellow or variegated seed in either of the parents, since no variation was observed for seed color. The cross with Bionda was not successful.
Data pertaining to the F2 generation also indicated that variegated seed was produced only in the presence of homozygous recessive alleles of the b1 gene and at least one dominant allele of the d gene, i.e., DDb1vgb1vg and Ddb1vgb1vg. Yellow seed was produced in the presence of homozygous recessive alleles at the d locus regardless of the allelic status of the b1 gene (ddB1B1, ddB1b1vg, and ddb1vgb1vg). These observations indicated the epistatic nature of the recessive alleles to the other locus when homozygous (recessive epistasis).
In summary, a comprehensive genetic analysis of the inheritance of seed color in flax indicated that yellow seed color was due to either a recessive allele at any of the two loci g and d or the presence of a dominant allele at the Y1 locus. Variegated seed coat was produced by a second recessive allele of the b1 locus. This new allele is symbolized as b1vg. All four genes (g, b1vg, d, and Y1) were found to be independently inherited except in the case of the d and b1vg genes, where there may be linkage involved. There was no support for the intensifying factor X in this study, as the seed colors observed were consistent. With regard to the four CDC recessive yellow parents, it was found that the yellow seed trait was the result of the g allele, although the yellow seed color in these parents arose from independent populations.
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ACKNOWLEDGMENTS |
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Received for publication October 16, 2002.
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONREFERENCES |
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