ABSTRACT

A prerequisite for any successful hybrid breeding program is the existence of genetically diverse gene pools. Because of the narrow genetic base of Brassica napus and the intensive quality breeding in the past the genetic diversity is limited. As a long time perspective for hybrid breeding the utilisation of artificially resynthesized rapeseed could be of interest. Resynthesized lines show inferior grain yield, compared to current breeding material. Nevertheless, regarding their genetic diversity they offer a base for hybrid breeding when combined with high-yielding material from conventional breeding programs. Hybrids resulting from crosses between resynthesized lines and the spring rapeseed cultivar “Korall” and their parents were investigated under field conditions for two years at two locations in Sweden and Denmark. Twelve resynthesized lines were used, which had very different origin but were not preselected for combining ability. Grain yield and heterosis in the F₁ were estimated. The hybrids showed in general higher values than the parents and the average mid parent heterosis was about 30 %. These results demonstrate the potential of resynthesized rapeseed for future hybrid breeding programs.

Introduction

Oilseed rape as the most important oleiferous Brassica crop has a narrow genetic base because of its recent origin and the absence of wild Brassica napus. The likely origin of amphidiploid Brassica napus (AACC) is spontaneous natural hybridisation between Brassica rapa (AA) and Brassica oleracea (CC). Thus the current breeding material is probably derived from one or very few interspecific hybrid plants that occurred some centuries ago. Intensive quality breeding during the last decades contributed additional to narrow down the genetic base of oilseed rape. One strategy to widen the genetic variability of Brassica napus is the utilisation of rapeseed artificially resynthesized from its parental species. These resynthesized lines exhibit inferior agronomic performance comparing to current breeding material. Therefore they have been used so far only as a source of single genes transferred to breeding material.
In a long term perspective the establishment of a new gene pool of artificially resynthesized rape might be useful for hybrid breeding. In summer oilseed rape high parent heterosis for seed yield averaged 30%, with a range of 20 to 50% (Sernyk and Stefansson 1983, Brandle and McVetty 1990) A requirement for hybrid breeding is the availability of genetically diverse genpools.
Most of the resynthesized lines are different from commonly used breeding material in their RFLP pattern (Becker et al. 1995). When investigating in how far the genetic distance estimated with molecular markers could serve to predict heterosis, Becker and Enqvist (1995) found significant correlation between the genetic distance between the parents and the mid parent heterosis for leaf dry matter in hybrid rapeseed plants.
Our investigations were designed to study in cross combinations with the spring rapeseed cultivar “Korall” the genetic value of 12 different resynthesized lines.

Materials and Methods

Materials

The material consisted of 12 resynthesized lines and their F1-hybrids with the Swedish spring rapeseed cultivar “Korall”. The resynthesized genotypes were derived from a wide range of interspecific crosses between Brassica oleracea and Brassica rapa (Table 1).
A monogenic recessive male sterile mutant was used to create the testcrosses (Takagi 1970). The source of sterility was crossed with “Korall” and under selection for 00-quality and male sterility two times backcrossed to “Korall”. The line “Korall-ms” segregates 1:1 in fertile and sterile plants. Male sterile plants were identified during early flowering and were used as mother for the testcrosses in isolated greenhouse cabins.

Methods

Hybrids and parents were investigated in field trails during two years (1995, 1996) at two locations (Svalöv, Sweden and Dyngby, Denmark). The trials had two replications in two separate blocks for hybrids and resynthesized lines. As standards the varieties “Maskot”, “Sponsor” and “Korall” were used.

Table 1: Survey of resynthesized rapeseed

*FUB: Freie Universität Berlin; SW: Svalöf Weibull; DP: DFL-Trifolium/Dansk Planteförädling;
SLU: Swed. University of Agric. Sciences; GAU: Georg-August-Universität Göttingen

Results

Comparing the grain yield of hybrids and resynthesized lines an average mid-parent heterosis of 32% with a range of 17 to 52% was estimated (Table 2). High-parent heterosis was on average 9,5% with a range of 0 to 21%. The highest yielding testcrosses were those with a high yielding resynthesized line as pollen donor (Fig. 1).

Table 2: Average performance of resynthesized lines (R) and hybrids (H), mid-parent performance (P), absolute and relative mid parent Heterosis (Het_mp), absolute and relative higher parent Heterosis (Het_hp).
(Means over two years, two locations and two repetitions))

Figure 1: Yield of the hybrids and their parents.

Discussion

The agronomic performance of resynthesized lines is obviously inferior to current breeding material. Comparing seed yield of resynthesized rapeseed genotypes with standard varieties the resynthesized lines attain merely 60 % of the varieties. These results correspond with findings of Engqvist and Becker (1994), who reported an average yield decrease of 30% for resynthesized forms in comparison with varieties. In testcrosses with adapted material such low yielding parental genotypes cause a reduced mid-parent performance and consequently high mid-parent heterosis values up to 50%.

With regard to the higher yielding parent, the hybrids display a yield increase with a range of 15 to 21% considering the best crosses.

In hybrid winter oilseed rape Knaak and Ecke (1995) reported that mid-parent heterosis for seed yield averaged 17%. The combined lines were selected according to their genetic distances estimated with RFLP analysis. The correlation between heterosis of hybrids and the genetic distances of their parental lines was r = 0,72.

Considering our results, some of the F₁-hybrids exhibit only moderate increase in seed yield compared with the higher yielding parent (Table 2), although resynthesized rapeseed lines have large genetic distances to all cultivars (Becker et al. 1995, Diers et al. 1996). This observation is explainable with the poor agronomic performance of some resyn-lines, according to late flowering time or insufficient resistance against fungal diseases. For that reason, a good combining ability for seed yield might be masked by disadvantages in other traits. Becker and Engqvist (1995) observed significant correlation between leaf dry matter of six week old hybrid plants and genetic distance between the parents.

Conclusions

In 1995 and 1996 twelve F₁-hybrids between the spring rapeseed “Korall” and resynthesized rapeseed lines were investigated in field trails at two locations. Mid-parent heterosis was calculated to 32%. In comparison with the higher yielding parent the hybrids revealed an average heteosis of 9,5%. Some of the crosses exceeded the yield of their superior parent up to 21%. In consideration of the small number of investigated resyn genotypes, our results indicate the potential that resynthesized oilseed rape might offer to hybrid breeding.

Acknowledgements

This work was financially supported by the Swedish council for forest and agricultural research.

We thank Svalöf Weibull AB for providing the line „Korall-ms“ and performing field trails in Svalöv and Pajbjergfonden for performing the field trails in Dyngby.

References

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2. Becker, H. C, Engqvist, G. M. and Karlsson, B. (1995): Comparison of rapeseed cultivars and resynthesized lines based on allozyme and RFLP markers. Theor. Appl. Genetics 91: 62-67.

3. Brandle, J. E. and McVetty, P. B. E. (1990): Geographical diversity, parental selection and heterosis in oilseed rape. Canadian Journal of Plant Science 70: 935-940.

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