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CROP BREEDING, GENETICS & CYTOLOGY

Phenotypic Effects of Introgressing Chinese Winter and Resynthesized Brassica napus L. Germplasm into Hybrid Spring Canola

^a Dep. of Botany, Bessey Hall, Iowa State University, Ames IA 50011
^b Dep. of Agronomy, 1575 Linden Dr., Univ. of Wisconsin-Madison, Madison, WI 53706
^c Bayer CropScience, 407 Downey Road, Saskatoon, SK S7N4L8, Canada

^* Corresponding author (tcosborn{at}facstaff.wisc.edu)

	ABSTRACT

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION REFERENCES

 
The seed yields of hybrid spring canola (Brassica napus L.) could be improved by introducing favorable alleles from unadapted germplasm. Two sources of unadapted germplasm, a Chinese winter cultivar, Hua-dbl2, and a resynthesized B. napus line derived from a cross between B. rapa cv. Reward and rapid cycling B. oleracea L., were crossed to the male parent of a hybrid combination. From both crosses, a segregating population of doubled haploid (DH) lines was created by microspore culture. DH lines were evaluated per se (2 yr in Wisconsin, USA) and in testcrosses to the female parent of the hybrid combination (2 yr in Wisconsin, USA, and 1 or 2 yr in Saskatchewan, Canada). Many of the testcrosses had significantly higher seed yields than the starting hybrid combination in both the Wisconsin and Saskatchewan environments. Some of the testcross lines had significantly higher seed yields than all of the commercial hybrid checks included in the trial. These results indicate that the seed yield of spring canola hybrids may be improved through introgression of novel alleles residing within these unadapted germplasms. The populations developed for this study can be used to map loci for which alleles from the unadapted parent increase seed yield of spring canola hybrids.

Abbreviations: DH, doubled haploid • OP, open-pollinated

	INTRODUCTION

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION REFERENCES

 
WITHIN WINTER and spring types of canola, genetic complementation between germplasm from distinct geographic regions and pedigrees has helped facilitate the rapid development of hybrid canola cultivars. Lefort-Buson et al. (1987) described significant heterosis for seed yield between winter germplasms of European and Asian origins, despite several generations of adaptive selection within the Asian lines for the European environment. Moderate to high levels of heterosis were reported between spring types of Canadian and European or Canadian and Asian origins (Brandle and McVetty, 1990). As with maize (Zea mays L., Moll et al., 1962, 1965), these initial studies found good, serendipitous heterotic combinations between B. napus accessions from different geographic regions probably because of genetic differences among accessions from the different regions. Indeed, the genetic relationship among accessions within winter and spring germplasms as determined by molecular marker data (Diers and Osborn, 1994; Diers et al., 1996) generally corresponded to the geographic origin of the accessions. However, genetic distances between pairs of accessions did not always predict the highest yield of F₁ hybrids for spring germplasm (Diers et al., 1996; Riaz et al., 2001), similar to results reported for maize (Lee et al., 1989; Melchinger et al., 1990).

Although general estimates of genetic diversity may not be useful predictors of heterosis, molecular markers have been useful for identifying sources of novel alleles that could further expand genetic diversity and lead to higher yielding hybrid cultivars. Molecular marker data indicated that the greatest genetic diversity in natural B. napus germplasm resides between winter and spring types (Diers and Osborn, 1994; Becker et al., 1995). Butruille et al. (1999a) evaluated the effect of introgressing alleles from a French winter cultivar (Major) into two spring hybrid combinations and found that hybrids with winter alleles had significantly higher seed yields than the spring inbreds or spring hybrids checks. Butruille et al. (1999b) also found that alleles from a German winter cultivar (Ceres) on linkage groups N3 and N14 improved seed yield of spring canola hybrids. Quijada et al. (2004) evaluated two large segregating populations of DH lines containing introgression from French winter cultivars and found significant yield enhancement for many hybrids made from the DH lines.

Other unadapted germplasms also may contain alleles which could improve seed yield of spring canola hybrids. Molecular diversity studies have shown that Asian winter germplasm is distinct from European winter germplasm (Diers and Osborn, 1994; Becker et al., 1995). Although Japanese winter germplasm appears in the pedigrees of some Australian spring cultivars (L. Sernyk, personal communication), winter germplasm from Asia has not been widely used in spring hybrid breeding.

The diploid progenitor species of B. napus, B. rapa, and B. oleracea (U, 1935), also represent largely unexplored sources of diversity, because B. napus contains only a subset of the molecular diversity existing with the extant diploid progenitors (Olsson, 1960; Song and Osborn, 1992). The incorporation of resynthesized B. napus into winter type hybrids was shown to improve seed yield (Kräling, 1987); and B. rapa has been used in breeding of some Chinese winter lines (L. Sernyk, personal communication); however, the potential benefits of this introgression into spring hybrids have not been investigated.

In this study, we evaluated the phenotypic effects of introgressing alleles from two diverse, unadapted germplasm sources, a Chinese winter type accession and a resynthesized B. napus line, into a commercial spring hybrid combination. We evaluated two large segregating populations (of DH lines as line per se and in testcrosses). These populations were compared with the starting hybrid combination and to open pollinated (OP) and hybrid canola cultivars.

	MATERIALS AND METHODS

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION REFERENCES

 
Germplasm
The hybrid combination P124 x P1804 was selected as a Canadian x European heterotic combination in which to evaluate the effects of introducing Chinese winter and resynthesized B. napus germplasms. According to results reported here and by Quijada et al. (2004), this hybrid shows a mid-parent heterosis of 11%. The female, P124, segregates for male-sterility due to a barnase transgene (Mariani et al., 1990), and was a parent of commercial hybrids developed by Bayer Crop-Science. P1804 contains the barstar transgene (Mariani et al., 1992) which restores male-fertility (Rf), and sister lines of P1804 have been used as male parents in commercial hybrid cultivars. The transgenic constructs containing the male-sterility gene, barnase, and male-fertility restorer gene, barstar, also contained a selectable marker gene conferring glufosinate herbicide resistance (Mariani et al., 1990, 1992).

Two lines representing unadapted germplasm pools were chosen for introgression into the commercial hybrid combination, a Chinese cultivar Hua-dbl2 and a resynthesized B. napus line, TO1147. Hua-dbl2, an OP winter cultivar with a facultative vernalization requirement for flowering, was kindly provided by Dr. Jinling Meng, Huazhong Agricultural University, China. Hua-dbl2 is a partial canola quality (low erucic acid and moderate glucosinolate levels) selection from ‘Zhongyou 821’, a widely grown cultivar in China (Yuanhui and Xiufang, 1989). A single plant from the cultivar Hua-dbl2, designated RV289, was used as a female parent in the cross to P1804. The resynthesized B. napus parent, TO1147, was created by crossing a B. rapa plant (as the female), grown from a yellow seed of cultivar Reward, to an inbred B. oleracea plant, TO1000. ‘Reward’ is a spring canola quality cultivar with white rust [caused by Albugo candida (Pers.) Kunze] resistance (Scarth et al., 1992). TO1000, is a S₅ inbred line derived from the rapid cycling B. oleracea stock CrGC3-3 developed by the Crucifer Genetics Cooperative, Madison, WI (http://www.fastplants.org; verified 9 July 2004). Progeny from this interspecific cross was obtained by embryo rescue of an F₁ seedling, TO1147, that was subsequently chromosome doubled with colchicine, as described by Song et al. (1993). TO1147 was used as a female parent in the cross to P1804.

Microspores from one (TO1147 x P1804) or two (RV289 x P1804) F₁ plants were used to develop two DH populations, SYN and HUA, respectively. Microspores, collected from F₁ plants, were cultured in media (as described by Chuong and Beversdorf, 1985) which also contained glufosinate to select herbicide resistant haploid plantlets containing Rf. Haploid plantlets were immersed in colchicine (0.34%, v/v) for 1.5 to 2 h to induce chromosome doubling. About 1000 treated haploid plants of each cross were transplanted and covered with Delnet bags (DelStar Technologies, Austin, TX) to allow self-pollination. About 150 DH lines with high seed set were selected in each population.

Seed Production
Each DH line from both HUA and SYN populations was used as a male parent for testcross seed production in field plots consisting of a 2-m male row and a 2-m female row (P124) on either side of the male row. Male-sterile plants of P124 were selected by herbicide application before flowering. The plots were covered with pollination bags before flowering, and twenty to thirty leaf-cutter bees (Megachile rotundata F.) were introduced into each cage as pollination vectors (Soroka et al., 2001). Testcross seed (T x HUA and T x SYN) was harvested from P124 plants and self-pollinated seed was harvested from each DH line. Seed for the HUA and SYN populations was produced in Outlook, Saskatchewan, Canada (51°N 30'W) during the summer of 1998 and in Mount Gambien, South Australia (37°S 45'E) during the summer of 1999-2000, respectively.

Field Trial and Trait Assessment
Testcrosses and DH lines, along with their respective parental lines P124, P1804, and RV289, and reference hybrids T x P1804 and T x RV289, were evaluated in multi-environment field trials using a randomized complete block (RCB) design with two replications. Because TO1147 was self-incompatible, a sister line named TO1141, was used (per se and as a hybrid, T x TO1141) as references in the SYN trials. The HUA testcross population was planted 2 yr in two locations: 1999 and 2000 in Arlington, WI (WI1999 and WI2000) and Saskatoon, SK (SK1999 and SK2000). The SYN testcross population was planted 2 yr at one or two locations: 2000 in Arlington, WI (WI2000) and Saskatoon, SK (SK 2000); and in 2001 in Arlington, WI (WI2001). Two replications of the DH populations were evaluated in the same environments as their respective testcross populations in WI field trials in separate RCB designs.

In Wisconsin, plots were seven rows wide (0.15 m between rows and 4.9 m long) and were planted with a seed density of 168 seeds m^–2 in a Plano silt loam soil. Plots were separated by 0.61 m to minimize competition between plots. In Saskatchewan, plots were five rows wide, with 0.19 m between rows, 6 m long and planted with a seed density of 185 seeds m^–2 in a Dark brown clay loam Chernozemic soil. Also in the Saskatchewan field trials, a winter B. napus cultivar was planted between plots to standardize competition between plots. Standard field management practices for growing canola were used; seed was treated with the fungicide benomyl {[methyl 1-(butlycarbamoyl)-2-benzimidazolecarbamate}, and the insecticide imidacloprid {1-[(6-chloro-3-pyridinyl)methyl]-N-nitro-2-imidazolldinimine} to control flea beetles (Phyllotreta cruciferae); trifluralin ([,,-trifluro-2,6-dinitro-N,N-dipropyl-p-toluidine]) herbicide (1.2 L ha^–1) and 150 kg ha^–1 N (applied as urea) were incorporated into the soil before planting, and additional manual weeding was performed as needed. In WI2000 and WI2001, additional benomyl was sprayed (1.12 kg ha^–1) shortly after flowering to deter white mold infection caused by Sclerotinia sclerotiorum (Lib.) de Bary. Both hybrid and OP cultivars were included as checks in the field trials. The OP checks were ‘Crusher’ (Svalof-Weibull, Ontario, CA), ‘Phoenix’ (Bayer CropScience, Saskatoon, SK), ‘Ebony’, ‘LG3295’, ‘LG3222’ (Limagrain, ON), ‘45A02’, and ‘45A51’ (Pioneer Hybrid International, Grand Forks, ND). The hybrid checks were ‘Hyola 401’ and ‘Hyola 420’ (Interstate Seed Company, West Fargo, ND), and InVigor 2373 (Bayer CropScience, Saskatoon, SK).

Several traits were measured for each plot including days to flowering, plant height, lodging, seed yield, test weight, and seed weight. Days to flowering were measured when 10% of the plants within a plot had open flowers. Plant height was recorded by measuring the highest point of at least 5 straightened plants in the middle of the plot. Lodging was estimated on a scale of 1 to 5, where 1 = completely upright and 5 = completely prostrate. When seeds began to turn color, individual plots were swathed and left to dry for more than 7 d in the field before combining. In Wisconsin, seed from each plot was combined with a small plot combine (Hege Equipment, Inc., Colwich, KS), manually cleaned in the field using a sieve with 3.2-mm holes, dried to <50 g kg^–1 moisture content in drying ovens at 39°C for more than 14 d, and cleaned again with a Clipper 440 seed cleaner (Seedburo, Chicago, IL). Seed yield was measured as the total seed weight per plot. Test weight was measured as the weight of seeds in 100 cm³. Five hundred seeds of each plot were counted and weighed to determine seed weight. In Saskatchewan, both seed yield and test weights were measured during harvest of the field plots. Moisture content, estimated by near infrared absorbance, was used to adjust the seed yield of each plot to 80 g kg^–1 moisture. Seed weight was not recorded at the Canadian locations. The oil content of seed samples from each plot in the Wisconsin field trials in 1999 was determined by David Syme, Bayer CropScience Saskatoon, Canada, by NMR analysis following conventional protocols.

In Wisconsin, the severity of a bacterial disease caused by Pseudomonas syringae pv. maculicola (McCulloch) Young et al. was recorded each year about 40 d after planting in both DH populations with a scale of 1 to 10, where 1 = 0 to 10% of the plants with chlorosis, 4 = 31 to 40% of the plants with chlorosis, 7 = 61 to 70% of the plants with chlorosis, and 10 = 90 to 100% of the plants with chlorosis. The testcross populations showed no disease presumably because the P124 tester conferred a moderate level of resistance.

Statistical Analysis
Data for each of the four populations (HUA DH, HUA testcross, SYN DH, and SYN testcross) were analyzed separately by mixed model analysis of variance (SAS Institute, 2000), where replication effects were considered random, and environment, genotype, and genotype x environment interaction (G x E) effects were considered fixed. For most traits including seed yield, significant nonhomogeneity of error was found among environments by both Bartlet's ² and Levene's test (Steel et al., 1997). Consequently, the "repeated" statement was used to specify environmental variance groups in the complete mixed model (Littell et al., 1996). In a separate analysis for each environment, nearest neighbor analysis was investigated as a means to improve the accuracy of plot seed yield estimates by adjusting the seed yield of each plot with residuals from its neighboring plots. However, this provided no, or only marginal, improvement to each model, and thus these adjustments are not reported in this study. Phenotypic correlations were based on least square means for each genotype and were calculated by the CORR procedure in SAS (SAS Institute, 2000).

	RESULTS AND DISCUSSION

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION REFERENCES

 
For both the DH and testcross populations, genotype was a significant source of variation for all traits and genotype performance was somewhat dependent on the test environment. Genotype x environment interaction was significant for all traits in most populations (Table 1), indicating that data for each environment should be analyzed separately. Although G x E was significant for seed yield in all populations, phenotypic correlations between environments were highly significant (P < 0.01) ranging between r = 0.22 and r = 0.55, except for the correlations between WI2000 and SK2001 in the T x SYN population (r = 0.07; P = 0.42) and between SK1999 and WI2000 in the TxHUA population (r = 0.12; P = 0.18). Supporting these significant correlations, we observed some genotypes which had relatively high seed yields in multiple environments. For example, testcross T x JU101, was ranked 6th, 6th, 7th, and 28th, for seed yield in WI1999, WI2000, SK1999, and SK2000, respectively.

View larger version (37K): [in this window] [in a new window]   Fig. 1. Frequency distributions of least-square means for seed yield of doubled haploid (DH) lines from two populations (HUA and SYN) grown in Wisconsin and their testcross progenies (T x HUA and T x SYN) grown in Saskatoon and WI during 1999, 2000, and 2001. The arrows indicate the positions of the tester (T), parents of the DH lines (C = RV289, S = TO1147, and P = 1804), the starting hybrid (T x P), the open-pollinated (OP) and hybrid (H) cultivars, and the overall mean (M) of each population. Horizontal lines below each histogram indicate the LSD at the 0.05 probability level. The left side of the bar is positioned at the seed yield of P in the distributions of DH lines. The left side of the bars is positioned at the seed yield of T x P in the distributions of testcrosses. No data were recorded for P during WI1999.

The plant height of lines in populations derived from the resynthesized B. napus was often significantly correlated with seed yield (P < 0.001) suggesting that plant height may be an important determinant for seed yield. The B. oleracea parent of the SYN population was small in stature, averaging only 40 cm in height at WI2001. Not surprisingly, there were large, significant differences in plant height among genotype in both the SYN testcross and DH populations (Tables 2 and 3). Similar positive correlations between plant height and seed yield in other studies using resynthesized B. napus lines as parents have been reported previously (Kräling, 1987); however, part of the observed relationship between seed yield and plant height for both DH populations in WI2000 may have been due to a bacterial disease which reduced plant height and also had a significant negative correlation with seed yield.

Oil content, measured only in the HUA testcross population in WI 1999, was positively and significantly correlated with seed yield (r = 0.32). This result is similar to those reported by Butruille et al. (1999a) and by Quijada et al. (2004) for other spring hybrids containing winter germplasm, and they suggest that increases in seed yield from winter germplasm introgression are not achieved at the expense of reduced oil content.

The introgression of a Chinese winter line and a resynthesized B. napus into a spring canola hybrid produced a wide range of variation for seed yield and other traits. Although many of the testcrosses were equal to or lower yielding than the starting hybrid, some testcrosses in both segregating populations were significantly superior to the starting hybrid and some lines were superior to all included checks. These results suggest that at some loci, the unadapted parents have alleles that are superior to those contained in P1804 when combined with the tester alleles. These loci could be identified through development of genetic maps for the DH populations and QTL analysis of the hybrid populations. Such an approach would also allow comparison of loci from different populations to detexrmine if different germplasm sources contribute unique favorable alleles.

	ACKNOWLEDGMENTS

 
We thank the researchers at Bayer CropScience for their assistance, particularly Tom Shuler and Stewart Brandt for seed production and field trials, Debbie Doell for microspore culture and plantlet development and David Syme for oil analysis. J.A.U. was funded in part by a UW Pioneer Plant Breeding Fellowship 1999-2000 and P.A.Q was funded by a scholarship from CDCH-UCV, Government of Venezuela. Additional funding was provided by the North Central Biotechnology Initiative and USDA-NRI grant no. 98-353006-6286 to T.C.O.

	NOTES

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION REFERENCES

 
Part of a dissertation submitted by J.A. Udall in fulfillment of the requirements for a Ph.D. at the Univ. of Wisconsin-Madison.

Received for publication November 10, 2003.

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