inheritance of fatty acid composition in Brassica juncea
Derek A. Potts and Daryl R. Males
Saskatchewan Wheat Pool, Research and Development, 201-407 Downey Road, Saskatoon, Saskatchewan, Canada, S7N 4L8, derek.potts@swp.com, daryl.males@swp.com
ABSTRACT
Low erucic acid Brassica juncea developed in the 1980’s is more unsaturated than canola-quality B. napus and B. rapa. This decreases the value of the oil due to the lower stability of polyunsaturated fatty acids. Breeding lines with a fatty acid profile similar to B. napus have been selected. Plants with the improved fatty acid profile have higher oleic acid and lower linoleic acid content than previous zero erucic acid lines. The inheritance of the modified fatty acid profile has been studied in F1-derived doubled haploid populations. When crossed with zero erucic lines, the modified fatty acid profile is inherited as a single gene. A high correlation was found between analyses of greenhouse-grown DH1 plants and field-grown DH2 nursery rows. In a cross between a modified fatty acid profile line and a high erucic line, it appears that the gene controlling the modification segregates independently of the two genes controlling the low erucic characteristic. The development of B. juncea with a fatty acid profile similar to B. napus means that the last major scientific hurdle in the development of canola-quality B. juncea has been overcome.
KEYWORDS: oleic, linoleic, erucic
INTRODUCTION
Low erucic acid Brassica juncea (L.) Czern. was first reported by Kirk and Oram (1981). However, low erucic B. juncea genotypes were more unsaturated (higher in linoleic acid and lower in oleic) than low erucic acid rapeseed cultivars of B. napus and B. rapa. The greater degree of unsaturation is undesirable due to the lower stability of the oil. To be accepted as canola, it is necessary for B. juncea to have a fatty acid profile that is essentially the same as that of the current canola species.
PROCEDURE
Modified fatty acid lines (MFA) with higher oleic acid and lower linoleic acid than normal low erucic acid lines (NFA) of B. juncea were identified. Crosses were made between MFA and NFA lines and F1-derived doubled-haploid (DH) populations were produced. One cross between a MFA line and a high erucic (HE) line was also studied. Fatty acid analyses were carried out on self-pollinated seed produced from first generation (DH1) plants grown in a greenhouse. For one population, both MFA and NFA second-generation (DH2) plants were grown in a field nursery. The sample size for DH1 analyses was approximately 10 seeds per plant, whereas about 200 seeds per sample were used for the field-grown DH2 lines. For F1 generation analyses, 10 seeds were analyzed individually.
RESULTS AND DISCUSSION
Average values for the major fatty acids of the parents used in this study are given in Table 1. Five MFA parents and four NFA parents were used. The major difference between NFA and MFA profiles is the ratio of oleic (c-18:1) to linoleic (c-18:2) fatty acids. On average, the MFA parents were 15.1% higher in oleic acid and 14.7% lower in linoleic than the NFA parents. In addition, the linolenic value is slightly lower for the MFA parents.
Table 1. Average fatty acid profile of modified fatty acid (MFA), normal fatty acid (NFA) and high erucic parents used to create DH populations
|
|
c-18:1 (%) |
c-18:2 (%) |
c-18:3 (%) |
c-22:1 (%) |
|
MFA Parents |
60.7 |
18.3 |
11.4 |
0.1 |
|
NFA Parents |
45.6 |
33.0 |
12.3 |
0.1 |
|
High Erucic Parent |
10.1 |
20.1 |
12.7 |
43.5 |
The percent frequency distributions of oleic acid content in four DH1 populations are shown in Fig. 1. All populations show a bi-modal distribution, indicating the effect of a single major gene. A value of 50% oleic acid was used to classify DH1 individuals as MFA or NFA.
Figure 1. Percent frequency distribution for oleic acid content in four DH1 populations. Number of DH plants analyzed: XJ95-046 = 102, XJ95-048 = 240, XJ95-057 = 429, XJ95-073 = 198.
The distinction between MFA and NFA types is generally more clearly defined when field-grown DH2 lines are analyzed (Fig. 2). Although the distribution peaks were not as distinct when greenhouse-grown DH1 plants were analyzed for population XJ95-048 (Fig. 1), the use of 50% oleic acid as a cut-off to categorize plants was quite effective. Out of 205 DH2 lines grown in the field, there was only one discrepancy between DH1 and DH2 classification (Table 2). This indicates high heritability for the trait.
Figure 2. Percent frequency
distribution of oleic acid for DH2 generation of population
XJ95-048 (N = 205)
Table 2. Classification of population XJ95-048 DH1 plants and DH2 lines as MFA or NFA.
|
|
DH1 |
|
|
|
|
MFA (c-18:1 >50%) |
NFA (c-18:1 < 50%) |
Total |
|
DH2 MFA |
113 |
1 |
114 |
|
DH2 NFA |
0 |
91 |
91 |
|
Total |
113 |
92 |
205 |
Using 50% oleic acid as a cut-off to distinguish MFA and NFA types, a single gene hypothesis was tested in four DH1 populations and one DH2 population (Table 3). The single gene hypothesis was accepted in all cases, although populations XJ95-048 and XJ95-057 had c2 values close to the critical value with a = 0.05. The moderate departures from the expected 1:1 ratio may be due to uncontrolled selection during the DH process. Only a small fraction of microspores survive to become mature plants and there may be selection during the DH process that favours one fatty acid type or the other. However, in the XJ95-048 population there is an excess of MFA types while in the XJ95-057 population there is an excess of NFA types, therefore, any selection which may take place during the tissue culture process is not consistent with respect to this fatty acid profile.
When working with DH lines, no information is gathered concerning dominance. Single F1 seeds from three crosses were analyzed and gave mixed results (Table 3). In cross XJ95-046 all 10 F1 seeds were classed as NFA, while the F1 seeds from cross XJ95-073 were all MFA. Cross XJ95-057 gave mixed results, with two MFA and eight NFA seeds.
A cross between a MFA parent and a high erucic parent produced DH1 plants with MFA, NFA and HE fatty acid profiles (Fig. 3). Erucic acid content in B. juncea is controlled by two major genes with additive effects (Kirk and Hurlstone, 1983). Therefore, in a cross between a zero erucic MFA parent and a HE line, it is expected that 25% of the DH progeny will be HE, 25% will be zero erucic and 50% of the zero erucic plants will be MFA (Table 4). Although the population size was limited, the results were as expected. With a larger population it may be possible to clearly identify all of the intermediate types that have a single gene for high erucic acid content, with and without the allele conditioning a high oleic/linoleic ratio.
Table 3. Frequency of F1, DH1 and DH2 plants or lines from selected crosses with modified fatty acid or normal fatty acid profiles. c2 tests are for a single gene hypothesis (1:1 ratio).
|
Population |
Generation |
N |
MFA (c-18:1> 50%) |
NFA (c-18:1< 50%) |
c2 (P) |
|
XJ95-046 |
DH1 |
102 |
54 |
48 |
0.25 (0.62) |
|
XJ95-048 |
DH1 |
240 |
134 |
106 |
3.04 (0.08) |
|
XJ95-048 |
DH2 |
205 |
114 |
91 |
2.36 (0.12) |
|
XJ95-057 |
DH1 |
429 |
195 |
234 |
3.37 (0.07) |
|
XJ95-073 |
DH1 |
198 |
111 |
87 |
2.67 (0.10) |
|
XJ95-046 |
F1 |
10 |
0 |
10 |
|
|
XJ95-057 |
F1 |
10 |
2 |
8 |
|
|
XJ95-073 |
F1 |
10 |
10 |
0 |
|
Figure 3. Percent frequency distributions for oleic and erucic acid content in DH1 population XJ97-023 (N = 78).
Table 4. Chi-square tests for hypotheses of two gene control for erucic acid content and single gene control of oleic acid content in population XJ97-023 (N = 78).
|
Type |
Expected |
Observed |
c2 (P) |
|
Erucic < 1.0% |
19.5 |
21 |
0.10 (0.75) |
|
Erucic > 35% |
19.5 |
15 |
1.64 (0.20) |
|
Oleic > 50%, erucic < 1% |
9.75 |
12 |
0.63 (0.43) |
CONCLUSION
The increased oleic - decreased linoleic acid trait in B. juncea is controlled by a single gene and is highly heritable. The dominance of the gene appears to be variable, making it difficult to identify heterozygotes in a backcrossing program. The development of B. juncea with a fatty acid profile similar to B. napus means that the last major scientific hurdle to the development of canola-quality B. juncea has been overcome.
REFERENCES
Kirk, J.T.O. and Hurlstone, C.J. 1983. Variation and inheritance of erucic acid content in Brassica juncea. Z. Pflanzenzüchtg 90: 331-338.
Kirk, J.T.O. and Oram, R.N. 1981. Isolation of erucic acid free lines of Brassica juncea: Indian mustard now a potential oilseed crop in Australia. J. Aust. Inst. Agric. Sci. 47: 51-52.