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Published in Crop Sci 39:1491-1496 (1999)
© 1999 Crop Science Society of America
677 S. Segoe Rd., Madison, WI 53711 USA
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Crop Science 39:1491-1496 (1999)
© 1999 Crop Science Society of America

CELL BIOLOGY & MOLECULAR GENETICS

Increasing Yield of Spring Oilseed Rape Hybrids through Introgression of Winter Germplasm

D.V. Butruillea, R.P. Guriesb and T.C. Osborna

a Dep. of Agronomy, 1575 Linden Dr., University of Wisconsin, Madison, WI 53706 USA
b Dep. of Forest Ecology and Management, 1630 Linden Dr., University of Wisconsin, Madison, WI 53706 USA

tcosborn{at}facstaff.wisc.edu


   ABSTRACT
TOP
 NOTES
 ABSTRACT
INTRODUCTION
 Materials and methods
 Results and discussion
 REFERENCES
 
Yields of spring oilseed rape (Brassica napus L.) hybrids are generally higher if derived from crosses of more distantly related spring cultivars. Winter cultivars are genetically very distinct from spring cultivars; however, it is not practical to use them directly as parents with spring cultivars for F1 hybrids because of their noncoincident flowering times, and the excessively delayed maturity of the F1. Thus, we tested whether the introgression of winter germplasm into spring-type oilseed rape could further increase the yields of spring hybrids. Nineteen doubled-haploid lines, derived from a cross between a spring canola (`Stellar') and a winter rapeseed (`Major') cultivar, were selected based on early-flowering phenology and having a range of germplasm from Major estimated to be 21 to 74% by screening with 480 molecular marker loci. These lines were test-crossed to two spring cultivars and evaluated for yield in 1994, 1995, and 1996 in Madison or Arlington, WI. The mean yield of the experimental hybrids was higher than the yields of cultivars, inbreds, and spring by spring hybrids in every year. The oil content was similar in all categories of germplasm. A slight delay in maturity may have contributed to the yield advantage of the experimental hybrids; however, there was no significant correlation between flowering time and yield among these hybrids. This study indicates that winter rapeseed may be a valuable source of germplasm for spring hybrid breeding.

Abbreviations: AFLP, amplified fragment length polymorphism • CV, coefficient of variation • DH, doubled haploid • NMR, nuclear magnetic resonance • OP, open-pollinated • RCB, randomized complete block • RFLP, restriction fragment length polymorphism


   INTRODUCTION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
Materials and methods
 Results and discussion
 REFERENCES
 
CANOLA, the low erucic acid and low glucosinolate rapeseed, is a great achievement of modern plant breeding (Busch et al., 1994), and has gained widespread popularity for the high quality of its oil. Since the U.S. Food and Drug Administration granted "Generally Recognized As Safe" status to canola oil in 1985 (Food and Drug Administration, 1995), consumption has increased steadily in the USA. This increase in demand has been met only partially by increases in domestic production (USDA Economic Research Service, 1996); further increases in the area planted and improvement of the yield potential of this crop are likely to prove profitable.

Rapeseed cultivars vary in their low temperature requirement to induce flowering (vernalization). Some are sown in the fall and grown as a winter crop requiring vernalization to flower the next summer, whereas others are grown as a spring-seeded crop and have little or no response to vernalization. Winter cultivars are usually higher yielding than spring cultivars, but they can only be grown profitably in areas where they regularly survive the winter.

The potential for hybrid rapeseed cultivars is well documented (Schuster and Michael, 1976; Sernyk and Stefansson, 1983; Grant and Beversdorf, 1985; Lefort-Buson et al., 1987; Brandle and McVetty, 1989). Several pollination systems have been developed (reviewed in Buzza, 1995), and hybrids are now commercially available. Among spring cultivars, the highest levels of heterosis were detected in crosses between more distantly related parents, such as between the Canadian spring cultivar `Regent' and the Australian spring cultivar `Marnoo' (Brandle and McVetty, 1989; Diers et al., 1995). Phylogenetical analyses based on RFLP markers differentiated the germplasm of oilseed Brassica napus into two main groups: winter cultivars (used throughout most of Europe) and spring cultivars (used mostly in northern Europe and Canada). A third, intermediate category includes East Asian and Australian genotypes, having characteristics of both growth habits (Diers and Osborn, 1994). To date, studies of heterosis in rapeseed have been confined to spring by spring (Sernyk and Stefansson, 1983; Grant and Beversdorf, 1985; Brandle and McVetty, 1990; Engqvist and Becker, 1991) or winter by winter crosses (Lefort-Buson and Datté, 1985a, 1985b; Léon, 1991; Ali et al., 1995). Sernyk and Stefansson (1983) included Japanese winter parents in their study, but the resulting hybrids did not mature before the end of the growing season and were removed from the analysis. Thus, it remains an unanswered question as to whether the use of germplasm from different phenological groups could further enhance heterosis once their growth habit is matched by selecting lines with coincidental flowering date.

The inheritance of growth habit seems to be under oligogenic control (Thurling and Vijendra Das, 1979; Van Deynze and Pauls, 1994). A major gene (VFN1), strongly influenced by vernalization, and few minor genes were mapped in a doubled-haploid (DH) population derived from the cross of a European winter (Major) and a Canadian spring (Stellar) cultivar, with the use of RFLP and AFLP markers (Ferreira et al., 1995; Osborn et al., 1997). Under such genetic control, the introgression of winter germplasm into spring types would appear to be a straightforward breeding problem. To study the potential of winter germplasm introgression to increase the seed yield of spring hybrids, we evaluated hybrids between DH lines having a range of germplasm from a winter parent, Major, and two spring testers. These hybrids were compared with elite open-pollinated spring cultivars and spring by spring hybrids.


   Materials and methods
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 Materials and methods
Results and discussion
 REFERENCES
 
Germplasm
We selected 19 lines from a population of DH lines derived from a cross between Major (a European winter rapeseed) and Stellar (a Canadian spring canola). This population was used to map molecular markers (Ferreira et al., 1994) and traits, such as oil and meal quality (Thormann et al., 1996; Toroser et al., 1995), and growth habit (Ferreira et al., 1995; Osborn et al., 1997). Selection criteria were: (i) maximizing the range of loci with alleles from the winter parent using information at 480 previously scored marker loci (Thormann et al., 1996; Osborn et al., 1997), (ii) early flowering time based on information from a previous trial (Ferreira et al., 1995), and (iii) presence of the spring-type allele at the main locus for flowering time, VFN1 (Ferreira et al., 1995; Osborn et al., 1997). These lines also differed in oil content, percentage erucic acid, and glucosinolates levels (Table 1) .


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  		Table 1 Characteristics of the doubled haploid lines chosen as parents from the Major (winter) x Stellar (spring) population

 
Two spring canola cultivars were chosen as testers: `Westar' (Canadian) and Marnoo (Australian). These cultivars are genetically very distinct and belong to two different spring heterotic groups that give some of the highest yielding hybrids (Diers et al., 1995; Sernyk and Stefansson, 1983; Brandle and McVetty, 1990). `Karat', a European spring canola shown to belong to a third heterotic group (Sernyk and Stefansson, 1983; Brandle and McVetty, 1990), was evaluated as a line per se and in hybrid combination with both testers. Other hybrid checks included the F1 hybrids between the testers, between Major and the two testers, between Stellar and the two testers, and the hybrid cultivar Hyola 401 (from Zeneca seeds). Open-pollinated (OP) checks included Stellar (Major would not flower without vernalization and thus was not included in the trial), `Crusher' (from Svalof-Weibull), `Cyclone' (from King Agro), and Westar (from Agriculture Canada). Hyola 401, Cyclone and Crusher were chosen because of their good performance in the Wisconsin Spring Canola Cultivar Trial for several years (E. Gritton, 1990–1995, personal communication).

Seed Production
For the 1994 yield trial, hybrid seed was produced using at least two plants from each DH line as females and seven plants of each tester as common pollen donors. Selfed seed of at least two plants of each DH line were also produced. Seeds from at least four plants were bulked for the check hybrids. For the 1995 and 1996 yield trials, due to the difficulty in obtaining enough seed for some of the crosses when the DH line was used as a female, we repeated the crosses using the DH lines as males and two plants of each tester as females. The infertility of some DH lines also caused a seed shortage of selfed progeny and poor performance in the 1994 yield trial. For these reasons the DH lines per se were not included in the 1995 and 1996 yield trials, and we included more checks to compare the performance of the F1 hybrids with well-performing commercial varieties and hybrids (Table 2) .


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  		Table 2 Entries included in field trials.{dagger}

 
All the plants from each tester and from Karat were the selfed progeny of one plant from breeder's seed of their respective cultivar. Crosses were done by hand emasculation and controlled pollination in the greenhouse. Commercial seed of Westar, Cyclone, Crusher, and Hyola 401 were used directly as entries in the trial.

Field Trial and Trait Measurements
In 1994 and 1995, the trial was conducted at the West Madison Agricultural Research Station (Madison, WI) on an irrigated field; irrigation was used once each year. In 1996, the trial was conducted at the Arlington Agricultural Research Station (Columbia County, Wisconsin) on a nonirrigated field. The plots were four rows wide, with 0.3 m between rows, and 3.6 m long in 1994 and 4.5 m long in 1995 and 1996. Planting was done during the first 3 d of May each year. The center 3.6 m of the two middle rows was harvested. In 1994, the two outer rows were planted with certified Westar seeds to homogenize the border effect. In 1995 and 1996 the two outer rows consisted of the same genetic material as the two inner rows. Standard field practices were used; trifluralin ({alpha},{alpha},{alpha}-trifluro-2,6-dinitro-N,N-dipropyl-p-toluidine) herbicide (2 L ha-1) was incorporated prior to planting, N fertilizer was broadcast 20 d after planting in 1994 and 1995 or incorporated prior to planting in 1996, and additional manual weeding was performed as needed. The seeding rate was 30 seeds m-1 of row. The experimental design was a randomized complete block design (RCB) with three replications. For each plot, the date when half of the plants had at least one open flower was recorded. In 1996, we recorded plant height in each plot by straightening a few plants randomly chosen from the middle of the plot and then measuring from the ground to the highest point. When seeds started to turn color, an indication of maturity, the plots were hand harvested, dried, and threshed. The seed samples were dried to <5% moisture content and weighed. The oil content of a composite seed sample of the three replicates of each entry was determined for the 1995 trial by Juan Romero, Mycogen Corp., and for the 1996 trial by David Syme, PGS Canada, using NMR analysis with established protocols. The weight of 1000 seed was measured for each plot in 1996. In 1995, the 1000-seed weight was measured from a composite sample of the three replicates of each entry. Data were analyzed using the General Linear Models procedure of the Statistical Analysis System (SAS) software (SAS Institute, 1989).


   Results and discussion
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 Materials and methods
 Results and discussion
REFERENCES
 
The distribution of yields of individual entries in 1994 reveals a large range of values for the DH x tester lines, most of which were higher than those of the best spring by spring hybrids (Fig. 1) . Both entries and blocks were significant effects in 1994 (P < 0.001), with an R2 of 0.80 and a coefficient of variation (CV) of 37%. The mean yield of all entries was 1162 kg ha-1. Average yields of each category were obtained by pooling the estimates of means of individual entries tested in the RCB design (Table 3) . Mid- and high-parent heterosis values were large for all categories of hybrids, but highest for DH x testers (Table 4) . No significant correlation was observed between the percentage of loci with Major alleles and either high- or mid-parent heterosis. However, the yield per se of the hybrids onto Marnoo was correlated with the percentage of loci with Major alleles (r = 0.47, P < 0.01). There also was a significant correlation (P < 0.01) between the performance of hybrids made with Westar and those made with Marnoo within the population of DH lines (0.59 and 0.71 for high- and mid-parent heterosis, respectively), while the correlation between hybrid yields per se, although positive, was not significant.



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  		Fig. 1 Distribution of yield of entries (average of three replications) within each germplasm category in 1994, 1995, and 1996

 

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  		Table 3 Average seed yields in 1994 by category of germplasm

 

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  		Table 4 Average heterosis in 1994 by category of germplasm

 
A mid-June storm with strong winds caused heavy lodging in 1994. The rows of Westar used as borders lodged more than the adjacent experimental hybrids, allowing hybrids to outcompete their borders. Hence, the high heterosis observed that year is not likely to mirror what would be observed in commercial production. The high yield of the test hybrids, compared with the spring by spring hybrids, may be due more to a greater competitiveness or to a better ability to withstand adverse conditions than to their higher yield potential per se. Azaïs et al. (1986) studied the effect of this interplot competition in a winter rapeseed hybrid trial and concluded that intrinsic yielding ability and competitive ability were positively correlated resulting in an upward bias when estimating heterosis. For this reason, in 1995 and 1996, we sowed four rows of each entry and harvested only the central area of the middle two rows. The results from those two years were subjected to a separate and more thorough analysis than the 1994 data.

The DH x testers were also the highest yielding categories in 1995 and 1996, followed by spring hybrids (Tables 5 and 6) . We found nonhomogeneity of error variance between 1995 and 1996 (based on an F-test with the mean square errors), so results from these 2 yr were analyzed separately. Entries and blocks were significant in explaining the observed variation each year (P < 0.05 for block effect in both years; P < 0.01 in 1995 and P < 0.001 in 1996 for entry effect). In all comparisons but one, the mean yields of hybrids from DH lines crossed to each tester were significantly higher than the yields of spring inbreds and spring hybrids in both years (Table 7) . In comparison to 1994, there was less variation in performance among individual entries within each category for those years (Fig. 1).


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  		Table 5 Average yields and other agronomic traits in 1995 by category of germplasm.{dagger}

 

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  		Table 6 Average yields and other agronomic traits in 1996 by category of germplasm.{dagger}

 

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  		Table 7 Pairwise t-test between the average seed yield of different categories of germplasm in 1995 and 1996.{ddagger}

 
Both 1994 and 1995 were unusually warm, with average weekly temperatures greater than 24°C at the time when most lines started to flower; this heat stress may have been the reason for the lower yields in those two years (Morrisson, 1993; Nuttall et al., 1992). In contrast, the growing conditions of 1996 were nearly ideal. The mean increase in seed size accounted for 36.7% of the increase in yield between 1995 and 1996, the remainder being attributed to total number of seeds produced per plot. This value varied greatly within germplasm category, but it is worth noting that it averaged near 25% in both spring hybrids and inbreds, while it averaged 45% in the DH x tester categories. Although more studies are needed, it seems that the greater ability of these hybrids to withstand adverse years stems, in part, from their ability to maintain more seeds to maturity even at the expense of seed size.

Changes in traits other than seed size, such as a delayed maturity or a reduced oil content, could be associated with the higher yields of hybrids with winter germplasm. These traits were measured and summarized for the different categories of germplasm (Tables 5 and 6). The higher yield of the test hybrids was not accompanied by a reduction in oil when compared with the other categories of germplasm. The DH x tester hybrids were taller than their spring counter parts, and their ability to outcompete the borders (Westar) may explain in part why they performed so well in 1994. In the following years, a different plot design was used to decrease this potential bias. Later maturity could partially explain the higher yields of the experimental hybrids since they flowered, on average, 6 d after the spring hybrids, thus benefitting from the longer growing period of southern Wisconsin.

Correlations, based on means across replications, were determined within each category of hybrid and year to better understand the relationship of these auxiliary traits to the yielding ability of the experimental hybrids. Within each year, the performance of the hybrids with Marnoo was positively correlated with the performance of the hybrids with Westar (0.37 and 0.53 in 1995 and 1996, respectively, significant at P < 0.05 in 1996). These correlations were much higher than the correlations between performance of the same hybrid in successive years. There was no indication that maturity (measured as days to 50% flowering) or taller plant height was related to higher yield, since correlation of these traits with yield ranged from -0.28 to 0.33 (days to 50% flowering) and from -0.33 to 0.17 (plant height), which are all values below significance. The oil content was positively correlated with yield in each population and year, varying from 0.21 to 0.37. Although these values were nonsignificant, they indicate that higher yield was not achieved at the expense of reduced oil content. None of the seed quality traits showed a consistent correlation with yield, in agreement with the more detailed studies by Rücker and Röbbelen (1996) and by Rajcan et al. (1997), involving segregating populations of backcrossed and DH lines, respectively.

Correlations between the percentage of winter alleles introgressed in each DH line and hybrid yield ranged from -0.46 to 0.33 and was significant (R = -0.46, P <= 0.05) in the DH x Marnoo population in 1995. Thus, it seems that Major may contribute both favorable and deleterious alleles to spring hybrids. However, the fact that a small set of unselected hybrids containing introgressed winter germplasm could consistently outperform selected cultivars or hybrids indicates that, within the levels of introgression tested, the effect or number of beneficial alleles outweighs that of deleterious alleles. Molecular markers could allow systematic detection and manipulation of these beneficial alleles.

The number of hybrids tested and the size of the plots we used compare favorably with many of the spring rapeseed studies published to date. However, this experiment was conducted with a relatively small set of parents because of the work involved in generating hybrid seeds. Several hybrid seed production systems are now available and the commercial success of some hybrids, such as Hyola 401, provides an incentive to further study the possibility of using winter germplasm in parental lines of spring hybrid canola. If the contribution brought by this germplasm is a higher heterosis or a slightly delayed maturity and better ability to withstand difficult growing conditions (e.g., heat stress at bloom), this germplasm may play an important role in expanding the area where spring canola can be grown economically.Morrison 1993


   ACKNOWLEDGMENTS
 
We thank Juan Romero and David Syme for their assistance with the seed quality analyses. Funding was provided by NRI Competitive Grants Program/USDA (grant nos. 9500894 and 9801827 to T.C.O.), and by a scholarship from CNPq, Government of Brazil, to D.V.B.


   NOTES
TOP
 NOTES
ABSTRACT
 INTRODUCTION
 Materials and methods
 Results and discussion
 REFERENCES
 
Contribution from the Dep. of Agronomy, Univ. of Wisconsin, Madison.

Received for publication June 20, 1998.


   REFERENCES
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 Materials and methods
 Results and discussion
 REFERENCES
 




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