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Crop Science 41:1444-1449 (2001)
© 2001 Crop Science Society of America

CROP BREEDING, GENETICS & CYTOLOGY

Inheritance of High Oleic Acid Mutations in Winter Oilseed Rape (Brassica napus L.)

A. Schierholt*,a, B. Rückerb and H. C. Beckera

a Institute of Agronomy and Plant Breeding, Univ. of Göttingen, Von Siebold Strasse 8, 37075 Göttingen, Germany
b Bundessortenamt, Osterfelddamm 80, 30627 Hannover, Germany

* Corresponding author (aschier{at}gwdg.de)


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 REFERENCES
 
High oleic (HO) winter oilseed rape (Brassica napus L.) with increased oleic acid content in the seed is of interest for nutritional and industrial purposes. The objectives of the present study were to (i) describe the fatty acid composition in the seed, leaf, and root material of eight HO mutants; (ii) estimate the number of genes controlling the trait; (iii) test whether the mutants are allelic for the mutated loci; and (iv) determine the inheritance of the HO trait. An 8-by-8 diallel of the HO mutants and two crosses between HO mutants and a normal type cultivar with their segregating F2 and BC generations were used. The results suggested that the variation in oleic acid can be explained by two mutation events. One mutated locus (HO1) was expressed mainly in the seeds and all mutants were assumed to be allelic at this locus. A second mutated locus (HO2), which increased the oleic acid content not only in the seed but also in leaves and roots, was identified in one mutant line. Both loci showed mainly additive effects: for HO1 a = 8.0 ± 1.5 and for HO1+HO2 a = 9.25 ± 1.5 (in percent oleic acid in the seed oil). Only small nonsignificant dominance effects and no epistatic or maternal effects were observed. The reduction of oleic acid desaturation in the mutants indicates that the HO1 locus is equivalent to fad2, the microsomal oleic acid desaturase, whereas the locus HO2 affects a different enzyme involved in fatty acid biosynthesis or desaturation.

Abbreviations: C16:1, palmitoleic acid • C18:1, oleic acid • C18:2, linoleic acid • C18:3, linolenic acid • fad, fatty acid desaturase • GCA, general combining ability • SCA, specific combining ability • Var. Cp., variance components


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 REFERENCES
 
VEGETABLE OILS with a high content of oleic acid (high oleic = HO) are of interest for nutritional and industrial purposes. Reduced levels of polyunsaturated fatty acids and the resulting increase in the level of the monounsaturated oleic acid are associated with a higher oxidative stability and reduced oxidation products in the oil without the need for extensive hydrogenation (Scarth and McVetty, 1999). The high oleic oil can be heated to a higher temperature without smoking, so that the cooking time is reduced and less oil is absorbed (Miller et al., 1987). A diet containing a high content of oleic acid can reduce the content of the undesirable low-density lipoprotein cholesterol in blood plasma (Grundy, 1986) and monounsaturated fatty acids more effectively prevent arteriosclerosis than polyunsaturated fatty acids (Chang and Huang, 1998). Genotypes with increased oleic acid content have been reported for several food crops, such as Helianthus annuus L. (Soldatov, 1976), Glycine max (L.) Merr. (Takagi and Rahmann, 1996), Brassica rapa L. (Tanhuanpää et al., 1996), Brassica carinata A. Braun (Velasco et al., 1996), and Brassica napus (Rücker and Röbbelen, 1995).

Rücker and Röbbelen (1995) selected several high oleic (HO) mutants with oleic acid (C18:1) contents of 77 to 80% in the seed oil in an ethyl methanesulphonate mutagenesis population of winter oilseed rape (Brassica napus, cv. Wotan). The objectives of the present study were to (i) describe the fatty acid composition in the seed, leaf, and root material of eight HO mutants; (ii) estimate the number of genes controlling the trait; (iii) test whether the mutants are allelic for the mutated loci; and (iv) determine the inheritance of the HO trait.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS AND DISCUSSION
 REFERENCES
 
Eight HO mutant lines, 19508, 19517/1944, 19517/7507, 19566, 19646, 19661, 19684, and 19782/7531, were selected from an ethyl methanesulphonate (EMS) mutagenesis program in the winter oilseed rape cv. Wotan by Rücker and Röbbelen (1995). Mutant lines 19517/1944 and 19517/7507 were derived from the same M1 plant (19517). All mutants were maintained through selfing and progenies within lines with highest seed C18:1 contents were selected.

An 8-by-8-half-diallel design (Griffing, 1956) was used to investigate the genetic control of C18:1 content in the seed oil. Eight HO mutant lines in the M3 generation were intercrossed. F1 plants together with their parental lines were tested in three environments from 1996 to 1998. In 1996 and 1997, four plants of each genotype were grown and selfed in the greenhouse. In 1998, all genotypes were sown in single row plots in the field at Göttingen, Germany, and four plants per genotype were selfed. Bulked seed samples of all selfed plants and single seeds of greenhouse grown plants (1996) were analyzed for fatty acid composition.

Reciprocal crosses were made between winter oilseed rape cv. Lisabeth (P1) and the two mutant lines 19661 (P2/1) and 19517/7507 (P2/2) in the M5 generation to develop the populations F1, F1r, BC11, BC12, and F2. F1 is the cross P1 x P2 and F1r is the reciprocal P2 x P1, while BC11 is the backcross F1 x P1 and BC12 is F1 x P2. Crosses and the production of seed for field trials were carried out in the field at Göttingen, Germany, in 1998. Parental lines, F1, F1r, BC11, BC12, and F2 generations were grown in field plots at Göttingen (1999) and plants of each generation were selfed. Bulked selfed seed was analyzed for fatty acid composition.

The fatty acid composition was determined for the leaves and roots from the eight HO mutant lines, and of a segregating F2 population of the cross 19517/7507 x Wotan. Leaf and root material for the fatty acid analysis was obtained from 20-d-old plantlets grown in sand culture in the growth chamber (day: 16 h; 25°C; light intensity 90 µmol m-2 s-1; night: 8 h; 20°C). Plant material was freeze dried before analysis. Mutant lines were tested twice and samples consisted of 20 bulked plants per line in each replication. For the determination of leaf and root fatty acid composition of F2 plants, plant material from 20 F3 seedlings from individual F2 plants were bulked and analyzed.

Fatty acid composition of single seeds and bulked seed samples was determined by gas chromatography (Thies, 1971). The fatty acid composition of leaves and roots was determined according to Thies (1971) for seed samples with minor modifications: 100 mg of the freeze dried plant material was transferred into 15 mL polyethylene tubes and carefully ground to powder, 1.5 mL of 0.5% (w/v) sodium methylate in methanol was added, and the solution was vortexed three times during the next 30 min; then 200 mL 5% (w/v) sodium hydrogen sulfate and 200 mL iso-octane were added and the samples were intensively vortexed.

An ANOVA was calculated by PLABSTAT (Utz, 1994) for the estimation of missing values, the calculation of the error mean squares (MS), and mean values of cross and location, which were than integrated into the diallel analysis. Each selfed plant was considered as one replication. The PLABSTAT ANOVA was performed by means of the following model:

Yijk is the observation of Genotype k at Environment j and Replication i; µ is the overall mean; ej is the environmental effect (e) at Environment j (for j = 1,..., E); ri is the effect of replication (for i = 1,..., R); gk is the effect of genotype (for k = number of genotypes); (ge)jk is the interaction effect of Environment j and Genotype k; (er)ij is the interaction effect of Environment j and Replication i; (rg)ijk is the interaction effect of Replication i and Genotype k and (ger)ijk is the interaction effect of Environment j, Replication i, and Genotype k. The factors e, r, and g were considered random.

General and specific combining ability (GCA and SCA) were determined from the analysis of the half diallel, without reciprocal and parent lines utilizing Method 4 of Griffing (1956) and program PZ14 (Utz, 1989) with the error MS extracted from the ANOVA results. The significance of GCA effects was tested following Griffing (1956).

Additive x additive (aa), additive x dominance (ad), and dominance x dominance (dd) interaction effects and their variances were calculated (Kearsey and Pooni, 1996). Scaling tests for the estimation of significant epistatic effects on the basis of generation means and variation (Mather and Jinks, 1982) were calculated as described in Hill et al. (1998). Three scaling tests were performed, based on BC11, BC12, and F2 generations, referred to as A, B, and C tests. Since, for example, the F2 is composed of 1/4 P1, 1/4 P2, and 1/2 F1, its scaling test is calculated as follows: C = 4 F2 - P1 - 2 F1 - P2. Differences from zero were tested by a t-test.

The number of genes affecting seed C18:1 content was estimated by the Index of Castle and Wright (Wright, 1968) following Hill et al. (1998). The Castle Wright Index (k) is calculated assuming that all involved genes are affecting the trait equally in size and direction, that loci controlling the trait are unlinked, and that dominance effects and epistasis are absent (Hill et al., 1998). The number of genes (k) is estimated as

with VEW as within family environmental variance.


   RESULTS AND DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
REFERENCES
 
All eight mutant lines showed a significantly increased C18:1 content in the seeds, significantly decreased linoleic acid (C18:2) content, and a slight but significant decrease in linolenic acid (C18:3) content compared with the wild-type Wotan (P <= 0.01) (Table 1). As reported by Rücker and Röbbelen (1997), the mutant lines have an altered fatty acid composition in the seeds, as well as in leaves and roots, but only leaf and root C18:1 contents of the two mutant lines 19517/7507 and 19782/7531 proved to be significantly different (P <= 0.05) from Wotan. The ANOVA showed significant environmental effects on seed oleic acid (P <= 0.05), linoleic acid (P <= 0.01) and linolenic acid (P <= 0.05) content.


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  		Table 1. Mean fatty acid composition and standard deviation in the seed oil of eight winter oilseed rape HO mutants and cv. Wotan grown in three environments and mean fatty acid composition of leaves and roots of these mutants grown in the growth chamber. C18:1 (oleic acid), C18:2 (linoleic acid), and C18:3 (linolenic acid) are expressed in percentage of total fatty acids.

 
The diallel analyses revealed highly significant GCA but did not detect significant SCA effects. Furthermore, significant effects for environment x GCA interaction and environment x SCA interaction were observed (Table 2). Diallel analysis for the estimation of general and specific combining ability showed that the two mutant lines 19517/7507 and 19782/7531 had highly positive and significant GCA effects on C18:1 (+1.69 and +1.96, respectively) compared with all other mutants (Table 3).


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  		Table 2. Analysis of variance in an 8-by-8 winter oilseed rape HO mutant diallel.

 

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  		Table 3. Parental means and general combining ability (GCA) in an 8-by-8 winter oilseed rape HO mutant diallel.

 
The analysis of the fatty acid composition of at least 50 single F2 seeds of several mutant x mutant crosses revealed differences in the size of standard deviation (s) of C18:1 contents, ranged from 1.19 to 3.02, between cross combinations. F2 seeds which were derived from crosses with the parental line 19517/7507 had a higher standard deviation in C18:1 content (s = 2.33 to 3.02) compared with all other crosses. No F2 seeds with C18:1 contents in the range of Wotan (54–64% C18:1) were found in any of the mutant cross combinations. This lack of segregation in F2 seeds of the mutant x mutant crosses indicates, that all lines have one mutation at the same locus for seed oleic acid content. This mutated locus is referred to as HO1.

No significant difference in seed C18:1 content between the F1 and F1r populations was found (Table 4), and the two populations were pooled (for total F1 = F1t). C18:1 content of the total F1 population was significantly different from the parental lines and nearly fit the midparent value (m). No significant dominance effects for seed C18:1 content were found in the crosses Lisabeth x 19661 and Lisabeth x 19517/7507. There were mainly additive effects influencing seed C18:1 contents (Table 5).


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  		Table 4. Generation means and variances of seed C18:1 contents in two populations (a) Lisabeth x 19661 and (b) Lisabeth x 19517/7507.

 

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  		Table 5. Means and variances of additive (a) and dominance (d) effects and their interaction effects (aa, ad, dd) of seed oleic acid contents in the winter oilseed rape populations.

 
The segregation pattern in F2, BC11, and BC12 differs in the two crosses. In the F2 population of the cross Lisabeth x 19661, seed C18:1 content segregated into three overlapping phenotypical classes: normal, intermediate, and high oleic acid types (Fig. 1a). While normal and intermediate classes cannot be separated clearly, a group of plants with high seed C18:1 contents can be distinguished (Fig. 1a). The C18:1 contents in seeds of BC11 plants segregated into low and intermediate classes and for the BC12 population in intermediate and high. This segregation pattern indicates a monogenic inheritance by a single major mutated gene controlling the seed C18:1 content. However, it was not possible to detect classes of normal, intermediate, and high C18:1 types in the F2, BC11, and BC12 populations of the cross Lisabeth x 19517/7507, even though the parental lines and F1t segregated clearly with significant differences between parental lines and F1t. Less than 25% of the F2 and less than 50% of BC12 populations fell within the range of values observed for P2/2 (19517/7507) (Fig. 1b). This indicates that seed C18:1 in P2/2 (19517/7507) is not monogenically inherited. It can be assumed from these segregation studies, that seed C18:1 contents are controlled by one major locus in 19661, while more than one locus is influencing this trait in 19517/7507.



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  		Fig. 1. Segregation pattern of seed C18:1 contents in the P1-, P2-, F1-, F2-, BC11-, and BC12-generation in the winter oilseed rape populations (a) Lisabeth (P1) x 19661 (P2/1) and (b) Lisabeth (P1) x 19517/7507 (P2/2). Seed C18:1 contents are expressed in percentage of total fatty acids.

 
In a further investigation of the inheritance of C18:1 content in mutant 19517/7507, the seed, leaf, and root fatty acid composition of a segregating F2 population of the cross 19517/7507 x Wotan was analyzed. Here, a distinct 15:1 (142:9) segregation ratio ({chi}2 = 0.84) for seed versus leaf C18:1 content (Fig. 2a) as well as for seed versus root C18:1 content (Fig. 2b) was revealed. These data establish a clear digenic inheritance for leaf and root C18:1 contents and at the same time for seed C18:1 contents in 19517/7507. This second mutated locus is designated HO2. We concluded that no significant epistatic effects influence the increase of seed C18:1 contents in the mutants 19661 and 19517/7507, since no interaction of additive and dominance effects in seed C18:1 contents was found (Table 5) and scaling tests A, B, and C were nonsignificant.



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  		Fig. 2. Correlation of (a) winter oilseed rape seed and leaf C18:1 contents and (b) seed and root C18:1 contents in F2(3) plants of a cross Wotan (P1) x 19517/7507 (P2). C18:1 contents are expressed in percentage of total fatty acids.

 
The number of genes (k) affecting the seed C18:1 content of 19661 and 19517/7507 was estimated from their P1, P2, F1t, and F2 generation means and variances. The results suggest that one gene (k = 1.29 with a standard deviation of 0.31) in mutant line 19661 and two genes in mutant line 19517/7507 (k = 1.85 with a standard deviation of 0.45) are controlling seed C18:1 contents (Table 6). Since 19661, 19508, 19517/1944, 19566, 19646, and 19684 are allelic in the HO1 locus, it can be concluded that these six mutants show a monogenic inheritance of the HO trait.


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  		Table 6. Estimation of minimum number of genes (k) affecting seed oleic acid contents in the winter oilseed mutant lines 19661 and 19517/7507, calculated from generation means of the populations Lisabeth x 19661 and Lisabeth x 19517/7507 with VEW as within family environmental variance.

 
The effect of HO2 on seed C18:1 contents appears to be smaller than the effect of the HO1 locus. The effect of the HO2 locus can be approximated from the seed C18:1 contents of the mutants 19517/7507 and 19517/1944 (Table 3). Both lines were selected from the same M1 plant, but only 19517/7507 possessed the positive allele of the HO2 locus. The mutants show a difference of 1.6% in seed C18:1 contents, which could be considered as the effect of the HO2 locus on seed C18:1 contents. The interaction between the two loci, HO1 and HO2, can be described as duplicate epistasis of HO1 over HO2 (Kearsey and Pooni, 1996). This relationship is demonstrated in the segregating F2 population 19517/7507 x Wotan (Fig. 2a), where high leaf oleic acid content is only present in F2 plants with high seed oleic acid content.

Since these HO mutants produce a seed oil with an increased seed C18:1 content in combination with reduced C18:2 and unchanged C18:3 contents, this indicates, that this mutation might affect the oleic acid desaturation. Since only the seed fatty acid composition is significantly changed, the mutation likely is affecting a seed microsomal delta 12 desaturase (fad2). Lemieux et al. (1990) reported that a fad2 mutant of Arabidopsis produced chloroplast linoleate by a plastid located desaturase and consequently leaf tissue fatty acid composition was not affected to a significant extent. This would explain the increased seed C18:1 contents and the nearly unchanged fatty acid composition of the leaves. Similar effects have been observed in fad2 mutants of Brassica rapa (Tanhuanpää et al., 1996, Tanhuanpää et al., 1998) and Helianthus annuus (Hongtrakul et al., 1998).

The mutants 19517/7507 and 19782/7531 produce not only an increased seed C18:1 content, but also a significant increase in C18:1 levels in leaf and root. Because HO2 only marginally increased the C18:1 content of the seed oil but significantly increased leaf and root C18:1 contents it is unlikely that the second mutation also affected fad2. The increase of C18:1 in the leaves indicates, that fad6, the plastidial oleic acid desaturase, might be affected by HO2. Browse et al. (1989) have reported similar observations for a fad6 mutant in Arabidopsis, but found that the increase in leaf C18:1 was accompanied by an increase of leaf C16:1 content, resulting from the nonspecific desaturation activity of fad6 (C16:1/C18:1 desaturase). This was not observed in 19517/7507 and 19782/7531, where C16:1 content remained unchanged compared with the wild type. In addition, 19517/7507 and 19782/7531 have increased leaf and root C18:1 contents, which is also not in accordance with a mutated plastidial oleic acid desaturase where mainly increased leaf C18:1 contents would be expected. Thus it is possible that the second mutation is not affecting an oleic acid desaturase but another fatty acid biosynthesis enzyme.

In the mutation program of Rücker and Röbbelen (1995), seed of about 20 000 M1 plants were screened for their fatty acid composition. Interestingly, all of the selected and analyzed mutants were mutated at the same locus (HO1), putatively fad2. Differences in the size of GCA effects and in the F2 seed variation for C18:1 contents in mutant x mutant crosses of HO1 mutated lines imply that there exist different alleles for the HO1 locus. If these six mutants had been mutated in exactly the same way, there should not have been any differences in the expression of the mutation and consequently in the C18:1 content of the seed oil.

The amphidiploid Brassica napus contains both the A (B. rapa) and C (B. oleracea) genomes. Scheffler et al. (1997) have localized four copies of the fad2 gene, two in the A and two in the C genome. It is thus surprising, that all tested mutants are affected in the same gene copies of fad2. One interpretation could be the existence of hot spots, genomic regions with a higher probability for mutations. Baranczewski et al. (1997) have demonstrated for Vicia faba L. that chromatid aberrations, induced by an alkylating agent, are clustered in segments (hot spots) and are not randomly distributed along the chromosomes. Other hypotheses could be the existence of only one main functional fad2 locus, or of additional functional fad2 loci, which when mutated, would not be detected in the heterozygous condition.

Maximum seed oleic acid content of 83%, in the F2 population Lisabeth x 19661 (Fig. 1a), could be surpassed in a breeding program, where by crossing these HO mutants with other HO lines, lines with 86% C18:1 in the seed oil could be selected (Schierholt and Becker, 2000). With regard to the development of HO winter oilseed rape cultivars, it can be concluded that mutant lines 19508, 19517/1944, 19566, 19661, 19646, and 19684 with a monogenic and mainly seed specific inheritance of increased oleic acid content are well suited for a HO breeding program since no obvious negative effects could be observed resulting from a changed fatty acid composition of the seed oil.


   ACKNOWLEDGMENTS
 
The authors thank Prof. Dr. Drs. h.c. G. Röbbelen and Norddeutsche Pflanzenzucht (NPZ) for providing the original mutants, Fachagentur Nachwachsende Rohstoffe (FNR) and Gemeinschaft zur Förderung der privaten deutschen Pflanzenzüchtung (GFP) for financial support, and Petra Rakete for excellent technical assistance.

Received for publication October 9, 2000.


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 REFERENCES
 




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