Crop Science 40:990-995 (2000)
© 2000 Crop Science Society of America
CROP BREEDING, GENETICS & CYTOLOGY
Genetic Relationships between Loci Controlling the High Stearic and the High Oleic Acid Traits in Sunflower
Begoña Pérez-Vicha,
Rafael Garcésb and
Jose María Fernández-Martíneza
a Instituto de Agricultura Sostenible (CSIC), Apartado 4084, E-14080 Córdoba, Spain
b Instituto de la Grasa (CSIC), Apartado 1078, E-41080 Sevilla, Spain
cs9femaj{at}uco.es
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ABSTRACT
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Sunflower (Helianthus annuus L.) lines with seed oils containing either high oleic (C18:1) or high stearic (C18:0) acid have been obtained through induced mutagenesis. The combination of the seed oil phenotypes of these lines would result in a novel oil quality of great value for the food industry. The objectives of this study were (i) to determine the genetic relationships between the high C18:0 and the high C18:1 traits, and (ii) to combine both characters. The sunflower lines HAOL-9 (high C18:1), carrying the high C18:1 Ol alleles, and CAS-3 (high C18:0), with the high C18:0 alleles es1 and es2, were reciprocally crossed. The fatty acid content of the F1, F2, BC1F1 to both parents, and F3 seeds was analyzed by gas–liquid chromatography (GLC). The segregation patterns for C18:0 (F2 ratio of 15:1) and C18:1 (F2 ratio of 1:3 or 7:9) in the F2 and BC1F1 to both parents fit those previously reported when each fatty acid was studied independently. However, a distortion was observed in their combined segregation, indicating that neither character was independently inherited. Furthermore, the analysis of the F3 generation revealed that F2 half seeds with the highest C18:0 concentration in a high C18:1 background were still heterozygous for either the C18:0 or the C18:1 levels, suggesting a genetic linkage between the alleles Es2 and Ol. These alleles determine a lower C18:0 and a higher C18:1 content, respectively. No F2 recombination between them was achieved, resulting in the absence of high C18:0/high C18:1 phenotypes. However, some intermediate C18:0/high C18:1 F2 seeds led to stable F3 seeds with this phenotype (130 g kg-1 C18:0/790 g kg-1 C18:1).
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INTRODUCTION
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THE QUALITATIVE MANIPULATION of seed oils involves the modification of their fatty acid composition, which determines the chemical properties responsible for the particular end use of the oil. Several alternatives to produce novel seed oil fatty acid profiles are being used. In particular, mutation breeding followed by combination of the mutant lines has been successfully used during the last 30 yr (Röbbelen, 1990). In flax (Linum usitatissimum L.) (Green, 1986) and soybean [Glycine max (L.) Merrill] (Fehr et al., 1992; Rahman et al., 1998), a further reduction of the linolenic acid (C18:3) concentration was achieved by crossing single mutants. Following a similar approach, Nickell et al. (1991) in soybean, Ntiamoah et al. (1995) in flax, and Ladd and Knowles (1971) in safflower (Carthamus tinctorius L.) combined mutations affecting the concentrations of two different fatty acids. In the case of soybean and flax, mutations for high or low palmitic acid (C16:0) and low linolenic acid were combined. In safflower, genotypes combining high stearic (C18:0) and high oleic (C18:1) were developed.
Sunflower mutants with increased levels of oleic acid (C18:1>750 g kg-1 compared with 200–500 g kg-1 in commonly grown cultivars; Soldatov, 1976) and stearic acid (C18:0>220 g kg-1 compared with 50 g kg-1 in standard sunflower seed oil; Osorio et al., 1995) have been developed. The combination of the seed oil phenotypes of both types of mutants would associate the oxidative stability and heart-healthy properties of the C18:1 with a higher plasticity of the oil because of the C18:0, which would result in a novel oil quality of great value for the food industry (Purdy, 1986; Wardlaw and Snook, 1990; Ascherio and Willett, 1997).
Studies on the genetic control of the high C18:0 and the high C18:1 traits in sunflower have been carried out separately. The inheritance of the high C18:1 trait has been widely studied in crosses between high C18:1 lines derived from the Soldatov mutant and different types of low C18:1 lines. The first studies reported a single partially dominant (Fick, 1984) or dominant (Urie, 1984) gene controlling the high C18:1 character, designated Ol (Fick, 1984). In addition to this major gene, Urie (1985) detected the presence of modifiers as well as an unexplained reversal in the dominance. Miller et al. (1987) found a second gene, Ml, modifying the C18:1 content determined by the Ol gene. High C18:1 levels were only expressed in genotypes Ol_mlml. Fernández-Martínez et al. (1989) identified two additional dominant genes, complementary to Ol, controlling the high C18:1 trait. The genes were named Ol1, Ol2, and Ol3. Phenotypes having a high C18:1 content carried three dominant alleles (genotype Ol1_Ol2_Ol3_), while the two dominant alleles Ol1 and Ol2 with the third gene in recessive homozygosis (genotype Ol1_Ol2_ol3ol3) expressed intermediate C18:1 levels. The authors concluded that some of the Ol alleles could be present in the low C18:1 lines used in the crossing study, which resulted in segregations for one gene [Ol1 or Ol2, F2 ratio of 1:3 (low: high)] or for two complementary genes [Ol1 and Ol2, F2 ratio of 7:9 (low and intermediate: high)], instead of three complementary genes.
The genetic control of the high C18:0 trait in sunflower has been studied in the mutant line CAS-3. Pérez-Vich et al. (1999) reported that this character was determined by partially recessive alleles at two loci. The loci were designated Es1 and Es2, with the Es1 locus having a greater effect on the C18:0 content than the Es2 locus.
The objectives of the present research were (i) to study the genetic relationships between the loci controlling high C18:0 and high C18:1 concentrations in the mutant CAS-3 and in a high C18:1 line and (ii) to combine both traits in a single line.
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Materials and methods
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The lines used in this study were the high C18:0 mutant CAS-3, obtained after treatment of the line RDF-1-532 with ethylmethane sulfonate (Osorio et al., 1995), and the high C18:1 inbred line HAOL-9 (Fernández-Martínez et al., 1993), developed from the inbred line HA-89 after backcrossing with a high C18:1 line (Soldatov, 1976). HA-89, with standard fatty acid composition in its seed oil and near-isogenic to HAOL-9, was used as a check. Half seeds of CAS-3 and HAOL-9 were analyzed for fatty acid composition to ensure that the plants used in the crosses had either high C18:0 or high C18:1 content. Fatty acid methyl esters were obtained as described by Garcés and Mancha (1993) and analyzed on a gas–liquid chromatograph with a 2-m-long column packed with 3% (v/v) SP-2310/2% (v/v) SP-2300 on Chromosorb WAW (Supelco Inc., Bellefonte, PA). The oven, injector, and flame ionization detector were held at 190, 275, and 250°C, respectively.
Plants of CAS-3 and HAOL-9 derived from half seeds were reciprocally crossed under greenhouse conditions [35/15°C (day/night) with 16-h day length] at the Instituto de Agricultura Sostenible of Córdoba (southern Spain) in November 1994. At flowering, each head was covered with a paper bag to avoid contamination with external pollen. Crossing was achieved through the emasculation of florets of the female parent followed by pollination of their stigmas with pollen from the male parent. The fatty acid composition of F1 half seeds from each cross was analyzed by GLC. The t-test for unpaired observations was used to determine significant differences between means of reciprocal F1s. Because the results did not reveal maternal effects, the fatty acid composition of segregating generations was analyzed on single half seeds.
The F1 reciprocal half seeds and half seeds from the parents were germinated in May 1995 and, after 15 d in a growth chamber [25/15°C (day/night), with a 16-h photoperiod], transplanted in a field nursery at the experimental farm of the Instituto de Agricultura Sostenible at Córdoba (sandy loam, deep alluvial, Typic Xerofluvent). The distance between plants within a row was 40 cm and between rows 85 cm. F1 plants were self-pollinated to obtain the F2 seed. Backcrosses to both parents were also made to produce the BC1F1 seed. Reciprocal crosses between the two parents were repeated to obtain reciprocal F1 seeds in the same environment as the F2 and BC1F1 seed. A total of 742 F2 seeds, 192 BC1F1 to HAOL-9 seeds, and 144 BC1F1 to CAS-3 seeds were analyzed by GLC. Eighteen F2 half seeds, representing all the different F2 classes, were selected on the basis of their fatty acid composition and transplanted in the field in the spring of 1996. The F2 plants were self-pollinated to obtain the F3 seed. The study of the F3 generation was performed through the analysis of 60 half seeds from each segregating F2 plant and of about 12 half seeds from each non-segregating F2 plant.
The C18:0 and C18:1 contents of F2, BC1F1, and F3 seeds were assigned to phenotypic classes based on the values found for the parents grown under the same environment and on previous reports studying the genetics of the high C18:1 and the high C18:0 traits (Urie, 1985; Fernández-Martínez et al., 1989; Pérez-Vich et al., 1999). The observed proportions within each phenotypic class were compared to those expected on the basis of appropriate genetic hypotheses. Chi-square goodness of fit was used to compare observed and expected ratios.
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Results and discussion
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The C18:0 content of CAS-3 and the C18:1 levels of HAOL-9 were five-fold and four-fold higher, respectively, than those of HA-89 (Table 1)
. Maternal effects for C18:0 or C18:1 contents were not observed, as indicated by the absence of significant differences between reciprocal F1 half seeds (Table 1). The C18:0 content in reciprocal F1 half seeds (79 g kg-1 and 85 g kg-1, respectively) was significantly different from that of both parents (230 g kg-1 and 45 g kg-1, respectively) (Table 1) and lower than the midparent value (137 g kg-1), indicating a partial dominance of the low over the high C18:0 levels, as reported previously (Pérez-Vich et al., 1999). In contrast, there was only a minor difference between the C18:1 content in reciprocal F1 half seeds (834 g kg-1 and 824 g kg-1, respectively), and in HAOL-9 (858 g kg-1) (Table 1), suggesting a complete dominance of the high over the low C18:1 content. Urie (1985) and Fernández-Martínez et al. (1989) reported similar results in their genetic analysis of the high C18:1 trait.
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Table 1 Fatty acid composition of the seed oil of sunflower lines CAS-3, HAOL-9, their reciprocal F1s, and a control line (HA-89) with standard fatty acid composition. Fatty acids are expressed as mean value ± standard deviation
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Data from the F2 seeds of the cross HAOL-9 x CAS-3 were analyzed for the es1, es2, and the Ol alleles separately to determine if the data were consistent with the previously reported genetic ratios for these loci. The C18:1 F2 content was assigned to two F2 phenotypic classes: low–intermediate (C18:1
700 g kg-1) and high (C18:1>700 g kg-1) (Fig. 1)
. Four of the six F2 populations analyzed satisfactorily fit a 1:3 (low–intermediate: high) ratio, while the other two were adjusted to a 7:9 ratio (Table 2)
. Both genetic ratios have been previously described for F2 segregations from crosses between low and high C18:1 lines (Fick, 1984; Urie, 1985; Fernández-Martínez et al., 1989). The F2 C18:0 content was assigned to two phenotypic classes: low–intermediate (C18:0 200 g kg-1) and high (C18:0>200 g kg-1) (Fig. 1). All the F2 populations analyzed were adjusted to a 15:1 (low–intermediate: high) genetic ratio (Table 2), a modified form of 1:14:1 (low: intermediate: high), which was consistent with previous results (Pérez-Vich et al., 1999).
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Fig. 1 Scatter plot of stearic acid vs. oleic acid in the oil from seeds of the sunflower parental lines HAOL-9 and CAS-3, and their reciprocal F1, F2 and BC1F1 to both parents generations
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Table 2 Frequency distributions and Chi-square analyses for C18:0 and C18:1 in F2 seeds of sunflower crosses between HAOL-9 and CAS-3
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From the above data, three or four independent genes are expected to segregate in the F2 generation of crosses involving HAOL-9 and CAS-3 when both the high stearic and the high oleic acid traits are studied together. Because these characters segregate to 15:1 (high C18:0) and 1:3 or 7:9 (high C18:1) phenotypic classes (Table 2), a 15:45:1:3 (for a 1:3 C18:1 F2 ratio) or a 105:135:7:9 (for a 7:9 C18:1 F2 ratio) F2 ratio was expected in a combined segregation. However, only three phenotypic classes were observed (Fig. 1). The missing group was that containing high C18:0 (>200 g kg-1) levels in a high C18:1 background. As a consequence, the three or four loci hypotheses were rejected in all the F2 families studied after the chi-square analyses (Table 2).
All the BC1F1 to HAOL-9 seeds had a high C18:1 content (Fig. 1). The C18:0 content of these seeds fell into two classes (Fig. 1). The first one, containing one fourth of the seeds, showed C18:0 values below 50 g kg-1. The second one, which included three fourth of the seeds, was characterized by C18:0 levels between 50 and 100 g kg-1 (Table 3)
. In the BC1F1 to CAS-3 generation, the segregation ratio was 3:1 for the high C18:0 and 1:1 or 3:1 for the high C18:1 when the traits were studied separately (Table 3). However, when both characters were studied for their combined segregation there was not a good fit of the observed to the expected genetic ratios (Table 3).
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Table 3 Frequency distributions and Chi-square analyses for C18:0 and C18:1 in BC1F1 seeds of sunflower crosses between HAOL-9 and CAS-3
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An F2 family (F2-32) showing a two-gene segregation for C18:0 and a one-gene segregation for C18:1 (Table 2) was selected because of its simple C18:1 segregation. The progenies of 18 F2 half seeds from this F2 family representing all the F2 classes were evaluated (Fig. 2)
. The F3 families derived from F2 half seeds with low C18:1 levels (<500 g kg-1) and a low C18:0 content (<50 g kg-1) (families F3-1 and F3-2; Fig. 2g) bred true for low values of both fatty acids (Table 4)
. Similarly, the F3 progenies from F2 seeds with low C18:1 values and a C18:0 content above 200 g kg-1 (families F3-3 and F3-4; Fig. 2i) or with high C18:1 (>700 g kg-1) and low C18:0 contents (family F3-5; Fig. 2d) bred true for high C18:0 or high C18:1 values, respectively (Table 4). In those F3 populations where only one of the studied fatty acids was segregating while the other one was at low concentrations (families F3-6; Fig. 2f, and F3-7, F3-8 and F3-9; Fig 2h), a one-gene segregation for the high C18:0 or the high C18:1 traits was found (Table 4). This kind of segregation was also found in those F3 populations segregating for C18:0 in a high C18:1 background (family F3-10; Fig. 2a; Table 4).
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Fig. 2 Scatter plot of stearic acid vs. oleic acid in the oil from the F2 half seeds (open circles) and the F3 seeds (solid circles) coming from the different F2 classes from sunflower crosses between HAOL-9 and CAS-3. (a) F3 seeds from the F2 class high C18:1/intermediate C18:0, (b) F3 seeds from the F2 class high C18:1/intermediate C18:0, (c) F3 seeds from the F2 class high C18:1/intermediate C18:0, (d) F3 seeds from the F2 class high C18:1/low C18:0, (e) F3 seeds from the F2 class high C18:1/intermediate C18:0, (f) F3 seeds from the F2 class high C18:1/low C18:0, (g) F3 seeds from the F2 class low C18:1/low C18:0, (h) F3 seeds from the F2 class low C18:1/intermediate C18:0, (i) F3 seeds from the F2 class low C18:1/high C18:0
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Table 4 Number of seeds having different C18:0 and C18:1 content in the analysis of F3 sunflower populations from the cross HAOL-9 x CAS-3 and Chi-square ( 2) analyses
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F3 seeds of F2 plants derived from F2 half seeds with intermediate C18:0 levels and a high C18:1 concentration showed three different behaviors (Fig. 2b, 2c, 2e). A first group segregated for both fatty acids (Fig. 2b), following a similar pattern to that described for the F2 generation. A second group did not segregate (family F3-13; Fig. 2c), with all the F3 seeds having intermediate C18:0 and high C18:1 values (Table 4). Finally, a third group of F3 seeds, which was derived from the F2 half seeds with the highest F2 C18:0 content (150 g kg-1 to 196 g kg-1) in a high C18:1 background (families F3-14, F3-15, F3-16, F3-17, F3-18; Fig. 2e) was not stable and segregated for both fatty acids. The segregation for C18:1 in this latter group fit a 3:1 (high: low) ratio (Table 4), with the high C18:1 seeds having C18:0 levels from 130 g kg-1 to 190 g kg-1 and the low C18:1 seeds having C18:0 values higher than 200 g kg-1.
According to the genetic systems proposed to explain the inheritance of the high C18:0 trait in CAS-3 (Pérez-Vich et al., 1999) and of the high C18:1 trait in different high C18:1 lines (Fick, 1984; Urie, 1985; Miller et al., 1987; Fernández-Martínez et al., 1989), the genotype of CAS-3 would be es1es1es2es2ol1ol1ol2ol2 or es1es1es2es2ol1ol1Ol2Ol2 and that of HAOL-9 Es1Es1Es2Es2Ol1 Ol1Ol2Ol2. Such genetic configurations were confirmed in the present study by the observed segregation for each trait independently. The existence of two different C18:1 segregations for one or two complementary genes in crosses with CAS-3 (Table 2) suggested the presence of C18:1 dominant alleles in some genotypes of CAS-3, as reported previously for other low C18:1 lines (Fernández-Martínez et al., 1989).
Whereas the study of the segregations of the high C18:0 and the high C18:1 traits independently was consistent with previous reports (Pérez-Vich et al., 1999; Fernández-Martínez et al., 1989), their combined segregation was significantly different from that expected for an independent inheritance. The combination of the high C18:0 mutant CAS-3 and the high C18:1 line HAOL-9 did not produce F2 or BC1F1 to CAS-3 recombinant phenotypes with high C18:0 levels (similar to those found in CAS-3, from 200 g kg-1 to 280 g kg-1) in a high C18:1/low C18:2 background.
Previous studies on the genetic relationships between loci controlling high or low concentrations of different seed oil fatty acids in other oil crops also reported the lack of a recombinant class combining the fatty acid levels found in the parental lines (Ladd and Knowles, 1971; Ntiamoah et al., 1995). This fact was attributed to the existence of modifying genes or maternal effects (Ladd and Knowles, 1971). However, in those studies, F3 seeds with a stable recombinant phenotype belonging to the F2 missing group were recovered from F2 seeds. This was not the case of the present study, where the F2 seeds with the highest C18:0 values found (from 150 g kg-1 to 196 g kg-1) in high C18:1 background did not lead to stable F3 high C18:0/high C18:1 values, segregating to low C18:1 values in the F3 generation, as described above (Fig. 2e). These results suggested that the F2 seeds of this group had a genotype that was always heterozygous, and could not be fixed.
The above data were interpreted as the existence of a genetic linkage between the allele Es2 and one of the Ol alleles (Ol1 or Ol2). The Es2 locus was hypothesized to be involved in the genetic linkage instead of Es1 because the contribution of es2 allele to the C18:0 content is lower than that of the es1 allele (Pérez-Vich et al., 1999). As the alleles Ol1 or Ol2 are complementary and they have the same effect in the control of the high C18:1 trait (Fernández-Martínez et al., 1989), either of them could be involved in the genetic linkage. For the discussion, it has been considered that the allele Ol1 was the one linked to Es2. Such a linkage would modify the combined segregation of the two traits studied, but not their independent segregation, because of a change in the proportion of the parental classes (high C18:0/low C18:1 and low C18:0/high C18:1) in relation to the recombinant classes (low C18:0/low C18:1 and high C18:0/high C18:1). The lethality of the embryos expressing a phenotype double high (high C18:0/high C18:1) was ruled out because it would have modified the independent segregation of the high C18:0 and the high C18:1 traits when they were studied separately. According to the proposed Es2-Ol1 linkage model, the highest F2 C18:0 values in a high oleic acid background would be the result of the expression of a genotype es1es1es2Es2 ol1O11O12O12. This F2 genotype is always heterozygous, and will segregate to low C18:1 values in the F3 generation (Fig. 2e). Thus, the genetic linkage determines the absence of a recombinant F2 genotype es1es1es2es2O11O11O12O12 and the lack of F2 or F3 recombinant phenotypes completely fixed for high values of both fatty acids.
In addition, the linkage Es2-Ol1 would only lead to F3 one-gene segregations for the C18:0 content either in low or high C18:1 backgrounds (Fig 2h, 2a). These segregations would correspond to the segregation of the F2 genotypes Es1es1es2es2ol1ol1Ol2Ol2 (Fig. 2h) and Es1es1Es2Es2Ol1Ol1Ol2Ol2 (Fig. 2a). F2 genotypes such as Es1es1Es2es2Ol1Ol1Ol2Ol2 or Es1es1Es2es2ol1ol1Ol2Ol2 (segregating for two C18:0 genes in high or low C18:0 backgrounds) would not be found (Fig. 2).
An objective of this research was to create a novel sunflower seed oil profile with a high C18:0 in high C18:1 background. This phenotype was not recovered, as has been described. However, F2 seeds with C18:0 values from 80 g kg-1 to 150 g kg-1 and with high C18:1 levels were obtained (Table 4), and some of them led to stable intermediate C18:0/high C18:1 values (130 g kg-1 / 800 g kg-1) in the F3 generation (Fig. 2c). Taking into account the proposed model, these F3 seeds have the genotype es1es1Es2Es2Ol1Ol1Ol2Ol2.
According to the results of the present study, further strategies are needed to obtain higher C18:0 levels in a high C18:1 background. First, the use of biochemical or molecular approaches will give us further information about the nature of the relationships between the high C18:0 and the high C18:1 traits. Second, the analysis of even larger segregating populations will increase the probability of recovering high recombinant values of both C18:0 and C18:1. Finally, the use of other high C18:1 backgrounds, different from that of HAOL-9 might also contribute to overcome the limitations imposed by the Es2-Ol1 linkage detected in this research.
Received for publication June 25, 1999.
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ABSTRACT
INTRODUCTION
