Oilseed from Brassica plants is an increasingly important crop. As a source of vegetable oil, it presently ranks behind only soybeans and palm in commercial market volume. The oil is used for many purposes such as salad oil and cooking oil. Upon extraction of the oil, the meal is used as a feed source.
In its original form, Brassica seed, known as rapeseed, was harmful to humans due to its relatively high level of erucic acid in the oil and high level of glucosinolates in the meal. Erucic acid is commonly present in native cultivars in concentrations of 30 to 50 percent by weight based upon the total fatty acid content. Glucosinolates are undesirable in Brassica seeds since they can lead to the production of anti-nutritional breakdown products upon enzymatic cleavage during oil extraction and digestion. The erucic acid problem was overcome when plant scientists identified a germplasm source of low erucic acid rapeseed oil (Stefansson, “The Development of Improved Rapeseed Cultivars.” (Chapter 6) in “High and Low Erucic Acid Rapeseed Oils” edited by John K. G. Kramer, Frank D. Sauer. and Wallace J. Pigden. Academic Press Canada, Toronto (1983)). More recently, plant scientists have focused their efforts on reducing the total glucosinolate content to levels less than 20 μmol/gram of whole seeds at 8.5% moisture. This can be determined by nuclear resonance imaging (NRI) or by high performance liquid chromatography (HPLC) (International Organization for Standardization, reference number ISO 91671:1992).
Particularly attractive to plant scientists were so-called “double-low” varieties: those varieties low in erucic acid in the oil and low in glucosinolates in the solid meal remaining after oil extraction (i.e., an erucic acid content of less than 2 percent by weight based upon the total fatty acid content, and a glucosinolate content of less than 30 μmol/gram of the oil-free meal). These higher quality forms of rape, first developed in Canada, are known as canola.
In addition, plant scientists have attempted to improve the fatty acid profile for rapeseed oil (Robbelen, “Changes and Limitations of Breeding for Improved Polyenic Fatty Acids Content in Rapeseed.” (Chapter 10) in “Biotechnology for the Oils and Fats Industry” edited by Colin Ratledge, Peter Dawson and James Rattray, American Oil Chemists' Society, (1984); Ratledge, Colin, Dawson, Peter and Rattray, James, (1984) Biotechnology for the Oils and Fats Industry. American Oil Chemists' Society, Champaign; 328 pp; Robbelen, and Nitsch. Genetical and Physiological Investigations on Mutants for Polyenic Fatty Acids in Rapeseed, Brassica napus L. Z. Planzenzuchta., 75:93-105, (1975); Rako and McGregor. “Opportunities and Problems in Modification of Levels of Rapeseed C18 Unsaturated Fatty Acids.” J. Am. Oil Chem. Soc. (1973) 50(10):400-403). These references are representative of those attempts.
Currently, both open pollinated varieties and hybrids of Brassica are grown. In developing improved Brassica hybrids, breeders can utilize different pollination control systems, such as self incompatible (SI), cytoplasmic male sterile (CMS) and nuclear male sterile (NMS) Brassica plants as the female parent. In hybrid crop breeding plant breeders exploit the phenomenon of heterosis or hybrid vigor which results in higher crop yields (grain or biomass) from the combination or hybridization of a male and a female line. Using these plants, breeders are attempting to improve the efficiency of seed production and the quality of the F1 hybrids and to reduce the breeding costs. When hybridisation is conducted without using SI, CMS or NMS plants in a two-way cross, it is more difficult to obtain and isolate the desired traits in the progeny (F1 generation) because the parents are capable of undergoing both cross-pollination and self-pollination. If one of the parents is a SI, CMS or NMS plant that is incapable of producing pollen, only cross pollination will occur. By eliminating the pollen of one parental variety in a two-way cross, a plant breeder is assured of obtaining hybrid seed of uniform quality, provided that the parents are of uniform quality and the breeder conducts a single cross.
In one instance, production of F1 hybrids includes crossing a CMS Brassica female parent, with a pollen producing male Brassica parent. To reproduce effectively, however, the male parent of the F1 hybrid must have a fertility restorer gene (Rf gene). The presence of an Rf gene means that the F1 generation will not be completely or partially sterile, so that either self-pollination or cross pollination may occur. Self pollination of the F1 generation is desirable to ensure the F1 plants produce an excellent yield for the grower. Self pollination of the F1 generation is also desirable to ensure that a desired trait is heritable and stable.
One type of Brassica plant which is cytoplasmic male sterile and is used in breeding is Ogura (OGU) cytoplasmic male sterile (Pellan-Delourme, et al., (1987) Male fertility restoration in Brassica napus with radish cytoplasmic male sterility Proc. 7th Int. Rapeseed Conf., Poznan, Poland, 199-203). A fertility restorer for Ogura cytoplasmic male sterile plants has been transferred from Raphanus sativus (radish) to Brassica by Institut National de Recherche Agricole (INRA) in Rennes, France (Pelletier and Primard, (1987) “Molecular, Phenotypic and Genetic Characterization of Mitochondrial Recombinants in Rapeseed.” Proc. 7th Int Rapeseed Conf., Poznau, Poland 113-118). The restorer gene, Rfl originating from radish, is described in WO 92/05251 and in Delourme, et al., (1991) “Radish Cytoplasmic Male Sterility in Rapeseed: Breeding Restorer Lines with a Good Female Fertility.” Proc 8th Int. Rapeseed Conf., Saskatoon, Canada. 1506-1510.
However, when the Ogura Raphanus restorer gene was transferred from radish to Brassica, a large segment of the Raphanus genome was introgressed into Brassica as well. This large Raphanus genomic fragment carried many undesirable traits, as well as the restorer gene. For example, the early restorer germplasm was inadequate in that restorer inbreds and hybrids carrying this large Raphanus fragment had elevated glucosinolate levels and the restorer was associated with a decrease in seed set—the number of ovules per silique (Pellan-Delourme and Renard, (1988) “Cytoplasmic male sterility in rapeseed (Brassica napus L.): Female fertility of restored rapeseed with “Ogura” and cybrids cytoplasms”, Genome 30:234-238; Delourme, et al., (1994), “Identification of RAPD Markers Linked to a Fertility Restorer Gene for the Ogura Radish Cytoplasmic Male Sterility of Rapeseed (Brassica napus L.)”, Theor. Appl. Gener. 88:741-748). In the case of hybrids, the glucosinolate levels were elevated even when the female parent had reduced glucosinolate content. These levels, typically more than 30 μmol/gram of oil-free meal, exceeded the levels of glucosinolates allowable for seed registration by most regulatory authorities in the world. Thus, the early restorer germplasm could be used for research purposes, but not to develop canola-quality commercial hybrid varieties directly.
INRA outlined the difficulties associated with obtaining restorer lines with low glucosinolate levels for Ogura cytoplasmic sterility (Delourme, et al., (1994) “Identification of RAPD Markers Linked to a Fertility Restorer Gene for the Ogura Radish Cytoplasmic Male Sterility of Rapeseed (Brassica napus L.)”, Theor. Appl. Gener. 88:741-748; Delourme, et al., (1995) “Breeding Double Low Restorer Lines in Radish Cytoplasmic Male Sterility of Rapeseed (Brassica Napus L.)”, Proc. 9th Int. Rapeseed Conf., Cambridge, England). INRA indicated that these difficulties were due to the linkage between male fertility restoration and glucosinolate content in its breeding material. INRA suggested that more radish genetic information needed to be eliminated in its restorer lines (Delourme, et al., (1995) “Breeding Double Low Restorer Lines in Radish Cytoplasmic Male Sterility of Rapeseed (Brassica Napus L.)”, Proc. 9th Int. Rapeseed Conf., Cambridge, England). Although improvements were made to restorers during the early years, isozyme studies performed on the restorer lines indicated that large segments of radish genetic information still remained around the restorer gene (Delourme, et al., (1994) “Identification of RAPD Markers Linked to a Fertility Restorer Gene for the Ogura Radish Cytoplasmic Male Sterility of Rapeseed (Brassica napus L.)” Theor. Appl. Gener. 88:741-748).
INRA attempted to develop a restorer having decreased glucosinolate levels. It reported a heterozygous restorer with about 15 μmol per gram (Delourme, et al., (1995) “Breeding Double Low Restorer Lines in Radish Cytoplasmic Male Sterility of Rapeseed (Brassica Napus L.)”, Proc. 9th Int. Rapeseed Conf., Cambridge, England). However, (i) this restorer was heterozygous (Rfrf) not homozygous (RfRf) for the restorer gene, (ii) this restorer was a single hybrid plant rather than an inbred line, (iii) there was only a single data point suggesting that this restorer had a low glucosinolate level rather than multiple data points to support a low glucosinolate level, (iv) there was no data to demonstrate whether the low glucosinolate trait was passed on to the progeny of the restorer, and (v) the restorer was selected and evaluated in a single environment—i.e. the low glucosinolate trait was not demonstrated to be stable in successive generations in field trials. Accordingly, the original Brassica Ogura restorer lines were not suitable for commercial use. For the purposes of this disclosure, this material is referred to as the “original” Brassica restorer lines.
Improved restorer lines were produced by Charne, et al., (1998) WO 98/27806 “Oilseed Brassica Containing an improved fertility restorer gene for Ogura cytoplasmic male sterility.” The improved restorer had a homozygous (fixed) restorer gene (RfRf) for Ogura cytoplasmic male sterility and the oilseeds were low in glucosinolates. Since the restorer was homozygous (RfRf), it could be used to develop restorer inbreds or, as male inbreds, in making single cross hybrid combinations for commercial product development. The glucosinolate levels were below those set out in standards for canola in various countries and breeders could use the improved restorer to produce Brassica inbreds and hybrids having oilseeds with low glucosinolate levels. This was a benefit to farmers, who could then plant Brassica hybrids which, following pollination, yielded oilseeds having low glucosinolate levels. This breeding effort removed approximately two thirds of the original Raphanus fragment. This estimate is based on the loss of 10 of 14 RFLP, AFLP and SCAR markers (WO98/56948 Tulsieram, et al., 1998-12-17). However, the Raphanus fragment in this material is still unnecessarily large. For the purposes of this disclosure, this material is referred to as the “first phase recombinant” Brassica restorer lines or germplasm.
Despite the improvement in the “first phase recombinant” restorer germplasm, it is still associated with deleterious agronomic performance. These deleterious traits may result from genes within this Raphanus fragment unrelated to fertility. Practically, only the restorer gene in the Raphanus fragment is required for the canola CMS pollination system. Therefore, the shorter the Raphanus fragment in a restorer line, the better the restorer line is expected to perform.
The Ogura restorer gene has been isolated and cloned by DNA LandMarks Inc./McGill University (US Patent Application Publication Number 2003/0126646A1, WO 03/006622A2), Mitsubishi (US Patent Application Publication Nubmer 2004/0117868A1) and INRA (WO 2004/039988A1). The gene can be used to transform Brassica plants.
Others have tried to produce restorer lines with a shortened Raphanus fragment. For example, Institut National de la Recherche (INRA) developed a line with a shortened Raphanus fragment by crossing a restorer line, “R211”, which had a deletion of the Pgi-2 allele and crossing it with a double low B. napus line, Drakkar. The progeny plants were irradiated before meiosis with gamma irradiation to induce recombination. This resulted in one progeny plant, “R2000”, in which the Pgi-2 gene from Brassica oleracea was recombined (WO 2005/002324 and Theor. Appl. Genet (2005) 111:736-746). However, the Raphanus fragment in R2000 is larger than that of the first phase recombinant restorer material developed by the Applicant and described above.
Another example, WO 05074671 in the name of Syngenta describes a shortened Raphanus fragment in their BLR1 recombination event. The BLR1 recombination event was produced solely by crossing and selection, followed by screening with molecular markers; no mutagenesis was used. However, the Raphanus fragment can be shortened further.