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CROP BREEDING & GENETICS

Pollen-Mediated Gene Flow in Wheat at the Commercial Scale

^a Dep. of Plant Sciences and Crop Development Centre, Univ. of Saskatchewan, 51 Campus Dr., Saskatoon, SK, Canada, S7N 5A8
^b Grain Research Laboratory, Canadian Grain Commission, 1404-303 Main St., Winnipeg, MB, Canada, R3C 3G8

^* Corresponding author (pierre.hucl{at}usask.ca).

	ABSTRACT

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION REFERENCES

 
Currently, information is lacking on gene flow in common wheat (Triticum aestivum L.) at distances greater than 300 m based on commercial-scale fields. The objective of this research was to measure pollen-mediated gene flow rates from a blue-aleuroned pollinator (T. aestivum cv. ‘Purendo-38’) to neighboring commercial fields of common wheat grown within a 10-km radius of a central pollinator field. In the 2-yr study, 33-ha (2002) and 20-ha (2003) fields of Purendo-38 were sown 200 km east-northeast of Saskatoon, Saskatchewan. Sixty-nine fields in 2002 and 76 fields in 2003 were identified as having overlapping flowering relative to Purendo-38. At maturity, up to 2 m² samples were harvested from each corner of each recipient field. Gene flow was identified by the expression of a light-blue pigment in the aleurone layer of F₁ hybrid seed. In 2002 one case of gene flow was confirmed at 190 m northeast of the pollinator at a rate of 0.01%. In 2003 nine putative hybrid seeds were confirmed to be the result of gene flow between Purendo-38 and the recipient field using gliadin fingerprinting. Consequently, gene flow was confirmed at 0.01% at 500 m northeast, 630 m southeast, and 2.75 km northwest from the pollinator. In commercial production, gene flow in wheat occurs at trace levels ( 0.01%) at distances up to 2.75 km.

Abbreviations: A-PAGE, acidic polyacrylamide gel electrophoresis • CPS, Canada Prairie Spring • CPSR, Canada Prairie Spring Red-Seeded • CPSW, Canada Prairie Spring White-Seeded • CWHWS, Canada Western Hard White Spring • CWRS, Canada Western Red Spring

	INTRODUCTION

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION REFERENCES

 
THE POTENTIAL for pollen-mediated gene flow over long distances is currently undocumented for common wheat (Triticum aestivum L.). A range of genetically engineered genes has been introduced into wheat (Janakiraman et al., 2002), but no commercial releases have occurred to date. The eventual release of transgenic cultivars has left the wheat industry with a number of questions regarding potential gene flow. For example, how far would transgenic wheat have to be isolated from neighboring non–genetically modified fields to eliminate the flow of transgenes and vice versa? Rieger et al. (2002) reported that pollen-mediated flow of herbicide resistance between commercial canola (Brassica napus) fields using 63 large commercial-scale pollen fields (25–100 ha) remained below 1%, but constant, at distances up to 3 km. Stokstad (2002) suggested that pollen from any crop was expected to likely travel in similar ways but that it was difficult to extrapolate from canola to wheat because self-fertilizing crops are less likely to transfer foreign genes. These large-scale commercial field studies have yet to be reported for wheat.

Wheat is predominantly a self-pollinating crop with a gene flow rate of usually less than 1% (Johnson and Schmidt, 1968). Successful gene flow in wheat not only depends on the receptivity of the stigmas, the viability of the pollen, and availability of pollen during the receptive period (Johnson and Schmidt, 1968; Waines and Hegde, 2003), but these factors vary with genotype and the environment (de Vries, 1971, 1972, 1974; Waines and Hegde, 2003). The occurrence of gene flow in wheat and the tendency for some cultivars to possess higher gene flow rates (up to 6%) than others when plants are grown in close proximity is well established (Heyne and Smith, 1967; Martin, 1990; Hucl, 1996; Lawrie et al., 2006). In general, research has indicated that low male fertility is generally associated with cultivars possessing higher gene flow rates (Hucl, 1996; Ingram, 2000; Hucl and Matus-Cádiz, 2001).

Research to date has not focused on measuring gene flow over long distances (> 300 m) in wheat. Generally, gene flow has been studied at distances of less than 48 m and has centered on the production of hybrid wheat, which has little application to most current agronomic production systems. Various studies have reported seed set on target male sterile lines at 5 to 48 m from a known pollen source (Bitzer and Patterson, 1967; Wilson, 1968; Jensen, 1968; Zeven, 1968; de Vries, 1974; Khan et al., 1973; Miller et al., 1975). Jensen (1968) reported that pollen could travel as far as 60 m. Pollen loads generally remained concentrated within 3 to 8 m of their pollen source and decreased with greater distance from the pollinator. Hucl and Matus-Cádiz (2001) have detected an gene flow rate of 0.03 to 0.09% for two male fertile common wheat cultivars at a distance of 27 m over cropped soil using a 5 x 5 m blue-grained pollinator block. Hanson et al. (2005) reported pollen-mediated gene flow rates generally below 0.02% by 42 m in wheat when grown adjacent to a 46-m diameter (0.16-ha) central blue-grained winter wheat pollinator. Matus-Cádiz et al. (2004) reported intraspecific hybridization (0.01%) at 300 m and interspecific hybridization (0.01%) at 20 m using a 50 x 50 m (0.25-ha) blue-grained pollinator block. No gene flow was detected beyond 300 m in this latter study even though sampling occurred up to 2.76 km from the 0.25-ha pollinator block. Studies using commercial-scale donor and recipient fields are needed to measure the level of pollen-mediated gene flow that may exist at the commercial-scale level.

Our research proposed to measure gene flow over long distances (up to 11.8 km) in wheat between pollinator fields (20 or 33 ha) carrying the blue-aleurone trait as a detectable marker (Keppenne and Baenziger, 1990) and 145 neighboring non–blue-grained fields (mean field size = 65 ha). The blue-grained trait is a dominant gene marker that has been applied to studying small-scale gene flow in spring wheat (Hucl and Matus-Cádiz, 2001; Matus-Cádiz et al., 2004; Lawrie et al., 2006). Cross-pollination from blue-seeded to non–blue-seeded wheat is identifiable by the expression of a light-blue pigment in the triploid aleurone layer of F₁ seed. An understanding of the distance and frequency to which pollen-mediated gene flow can occur at the commercial scale will provide information useful for determining if (i) gene flow will be a major or minor contributor to product admixture and (ii) whether a tolerance level of 0% transgenic wheat in nontransgenic wheat grain is realistic. The objective was to measure pollen-mediated gene flow rates from a 20- to 33-ha blue-aleuroned pollinator (T. aestivum cv. ‘Purendo-38’) to neighboring commercial-scale fields of common wheat grown within a 10-km radius of the central pollinator source.

	MATERIALS AND METHODS

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Seed of Purendo-38, a spring-type blue-aleuroned wheat (Abdel-Aal and Hucl, 1999; Matus-Cádiz et al., 2004) was obtained from the Crop Development Center, Saskatoon, SK. In 2001 seed of Purendo-38 was increased by the Crop Development Centre for use in 2002 and 2003. Purendo-38 was seeded as a 33-ha (408 x 797 m in 2002) and 20-ha (262 x 771 m in 2003) field approximately 200 km east-northeast of Saskatoon in an area spanning two rural municipalities. The area was chosen because it is commonly used for spring wheat production. Standard agronomic practices for growing spring wheat were used within the central pollinator and neighboring fields. The soil type for the study site was loamy, Black Chernozem (Udic Boroll). Purendo-38 was seeded on 24 May 2002 and 19 May 2003 at a low rate (100 seeds m^–2), with rows spaced 0.2 m apart. The reduced seeding rate of the pollinator field was used to promote tillering and thereby extend the period of pollen shedding. The central Purendo-38 field sown in 2003 was located 4.8 km south of the pollinator field established in 2002. In 2003 the 2002 Purendo-38 field was seeded to peas (Pisum sativum), and no wheat volunteers were found within the 33-ha area during the flowering period of the 2003 Purendo-38 block. Duration of flowering (days between the first and last observed occurrence of anthesis) was noted for the pollinator field. Presence or absence of flowering was noted for each recipient wheat field during the pollination period of Purendo-38. The average size of recipient fields in both years was 65 ha. Meteorological data were collected within 60 km of the pollinator fields by Environment Canada (Table 1).

In 2003 76 recipient fields were used, with 34 fields at a distance of 0 to 5 km and 42 fields at a distance of greater than 5 to 11.8 km from the pollinator field (Table 3). The NE quadrant contained 20 fields located 0.5 to 9.5 km from the pollinator. The NW quadrant contained 28 fields located 2.7 to 11.8 km from the pollinator. The SE quadrant contained 11 fields located 0.6 to 5.0 km from the pollinator. The SW quadrant contained 17 fields located 3.8 to 10.7 km from the pollinator. Of the 76 fields, 69 fields were seeded with a CWRS cultivar, five fields with the CPSR cultivar AC Taber, and two fields with Canada Western Hard White Spring (CWHWS) wheat cultivar AC Snowbird. The CWRS class was represented by nine cultivars, including AC Barrie (n = 15), McKenzie (n = 9), AC Cadillac (n = 3), CDC Teal (n = 8), AC Elsa (n = 19), AC Superb (n = 3), HR5500 (n = 4), HR5600 (n = 5), and AC Splendor (n = 3). The average number of seeds collected per sample was 32 087 seeds (SD = 6879; SE = 789; range = 18 130–49 936 seeds). The total number of seeds collected per field averaged 12 8347 seeds (SD = 27 518; SE = 3157; range = 72 519–199 744). Approximately 10 million seeds were screened.

Cross-pollination from Purendo-38 to neighboring wheat fields was identified by the expression of a light-blue pigment in the aleurone layer of the F₁ hybrid seed. In both years of the study, all samples were visually screened for putative light-blue seeds (data not presented) using the following procedure. In 2002 seed samples were placed in 2-lb (0.9 kg) mesh bags and soaked overnight in water because grain quality was poor due to weathering and staining. Presoaking enhanced the expression of the light-blue grain color. In 2003 the grain appearance was excellent, and presoaking was not required to enhance grain color. In each year seeds suspected of possessing a light-blue aleurone were visually identified and kept separate from the remaining seed lots. Putative light-blue hybrid seeds were grown out to maturity under controlled growth conditions to verify the 3:1 ratio (blue to nonblue) expected in the F₂ population.

Putative hybrid seeds of interest were pregerminated at 15°C (in darkness) for 7 d in a petri dish (each containing a Whatman No.1 filter paper) before being transferred to potting mix. Seeds were planted (1 cm depth) in 15-cm diameter pots (maximum of seven plants per pot) filled with Terra-Lite Redi-Earth (W.R. Grace and Co. of Canada, Ltd., Ajax, ON). Growth cabinet conditions were set to 23/18°C (day/night) with 18-h light and a photosynthetically active radiation level of 400 µmol m^–2 s^–1. Plants were watered every 2 d and fertilized using Type 100 Nutricote controlled release granular fertilizer (14–14–14) (Plant Products Co., Ltd., Brampton, ON) at a rate of 0.8 kg m^–2. Plants were allowed to self-pollinate, and segregation among F₁-derived F₂ seed was classified as segregating (3 blue:1 nonblue seed ratio) or nonsegregating (all nonblue seeds) for the single-gene, dominant blue-aleurone trait. Gene flow rates were calculated as follows: gene flow (%) = (number of confirmed light-blue seeds observed in s_i at d_j/total number of seeds screened in s_i at d_j) x 100, where s_i is the ith sample and d_j is the jth distance.

In 2003 F₁-derived F₂ seed was harvested from each confirmed hybrid plant, and subsequently, 96 F₂ seeds (48 blue-aleurone and 48 nonblue) were grown out from each hybrid plant to maturity using the growth conditions described above. Each of the established F₂ plants were phenotyped for spike morphology, kernel shape, and seed color. Purendo-38, AC Splendor, and HR5500 are tip-awned while AC Taber is awned. For each of nine hybrid plants, five F₂-derived F₃ seeds from four F₂ plants were advanced for gliadin fingerprinting. Only the putative hybrid seeds detected in 2003 were tested using gliadin fingerprinting as only these nine outcrossing events were at distances (> 300 m) previously unreported in the literature (Matus-Cádiz et al., 2004).

Acidic Polyacrylamide Gel Electrophoresis (A-PAGE)
Gliadin protein fingerprinting using A-PAGE was used to verify that the nine F₁ hybrid seeds detected in 2003 were from an outcrossing event resulting from pollination of recipient fields by Purendo-38. The Variety Identification Monitoring unit of the Grain Research Laboratory, Winnipeg, MB, performed A-PAGE gliadin analysis using the International Standardization Organization Method 8981:1993 (International Standardization Organization, 1993), with minor modifications. Single kernels were individually crushed and extracted overnight in 200 µL of 70% ethanol (v/v). Approximately 100 µL of sample dilution buffer (120% [w/v] sucrose dissolved in 50 mL of 8.5 mM aluminum lactate buffer, pH 3.1) was added to the ethanolic extract and vortexed. Extracts (3 µL) of single seeds were loaded onto 6% A-PAGE gels (1.5 mm x 8 cm x 18 cm) and run at constant current (90 mA) for 65 min in vertical slab electrophoresis units (GE Healthcare, Baie d'Urfé, QC). After completion of the runs, gels were removed and stained overnight in a solution of Coommassie Brilliant Blue R-250, ethanol, and tricholoracetic acid. Breeder seed of AC Splendor, AC Taber, or HR5500 were run on each gel alongside Purendo-38 and 15 to 30 F₃ seeds per hybrid plant to confirm hybridity. Breeder seed of AC Splendor, AC Taber, and HR5500 were run against samples collected from recipient fields no. 1, 22, and 49 to verify that the fields were in fact AC Splendor, HR5500, and AC Taber, respectively. The electrophoretic patterns obtained from each seed were compared with seed of Purendo-38 and AC Splendor, HR5500, and AC Taber. Major banding patterns in the omega (), gamma (), and alpha () gliadin regions, and occasionally the beta (ß) gliadin region, were identified and recorded to confirm hybridity in F₂-derived F₃ seed.

	RESULTS

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In 2002 the estimated duration of flowering was 7 d for Purendo-38 (5% anthesis on 14 July to 95% on 20 July) (Table 1). The estimated overlap in pollination periods between the Purendo-38 pollinator field and recipient fields varied from 1 to 7 d (Table 2). This duration of pollination indicates that some level of nicking was expected between all recipient fields and the pollinator field. The period of overlap in this study may be a conservative estimate as the stigma of male-fertile plants are known to be receptive for a period of 4 to 13 d (de Vries, 1971). Prevailing winds were from the south for 2 d (averaging 21 km h^–1), northwest or north for 2 d (averaging 13 km h^–1), east-northeast for 2 d (averaging 15 km h^–1), and west for 1 d (averaging 15 km h^–1) of pollination (Table 1). Prevailing wind direction is expected to be associated with elevated outcrossing rates. Mean wind speed during pollination was 16 km h^–1 (range = 12–26 km h^–1). Mean temperature, relative humidity, and precipitation values were 22°C (range = 19–27°C), 70% (range = 49–93%), and 4.0 mm (range = 0–20 mm) during pollination, respectively. Gene flow rates of 0 to 0.01% were calculated once putative light-blue hybrid seeds were confirmed to be segregating for the blue-aleurone trait (Table 2). Long-distance gene flow was not detected beyond 190 m from the edge of the pollinator field. A trace level of long-distance gene flow was confirmed in one of four samples harvested from field no. 1. That is, a trace gene flow rate of 0.01% ([1/12360] x 100) was detected in AC Cadillac at 190 m to the north of the pollinator. The detection of this outcrossed seed was from a field located across the highway from the Purendo-38 source field.

In 2003, the estimated duration of flowering was 9 d for Purendo-38 (5% anthesis on 10 July to 95% on 18 July). The estimated overlap in pollination periods between the Purendo-38 pollinator field and recipient fields varied from 5 to 9 d (Table 3). Prevailing winds were from the west or southwest for 4 d (averaging 8 km h^–1), northwest or west-northwest for 3 d (averaging 14 km h^–1), east-northeast for 1 d (averaging 9 km h^–1), and south-southeast for 1 d (averaging 22 km h^–1) of pollination (Table 1). Mean wind speed during pollination was 14 km h^–1 (range = 9–22 km h^–1). Mean daily temperature, relative humidity, and precipitation values were 19°C (range = 16–22°C), 76% (range = 65–88%), and 1 mm (range = 0–7 mm) during pollination, respectively. Gene flow rates of 0 to 0.01% were calculated once putative light-blue hybrids seeds were confirmed to be segregating for the blue-aleurone trait (Table 3). Long-distance gene flow was detected at 0.5 to 2.75 km from the edge of the pollinator block in fields no. 1, 22, and 49. One of four samples harvested from field no. 1 contained four confirmed hybrid seeds. One of four samples harvested from field no. 22 contained one confirmed hybrid seed. Two of four samples harvested from field no. 49 contained three and one confirmed hybrid seeds. That is, a trace gene flow rate of less or equal to 0.01% was detected in AC Splendor at 500 m to the northeast of the pollinator ([4/33 310] x 100 = 0.01%), AC Taber at 630 m to the southeast (ranged from [1/28396] x 100 = 0.004% in the first sample collected from this field to [3/28 396] x 100 = 0.01% in the second sample), and HR5500 at 2.75 km to the northwest ([1/19 218] x 100 = 0.01%). Long-distance pollen-mediated gene flow was not detected beyond 2.75 km of the pollinator source in either year of study.

Of the nine putative hybrids detected in 2003, all conformed with expectations for a cross with Purendo-38 based on morphological data (Table 4). All F₁ hybrid plants were tip-awned, and as expected, only the AC Taber–Purendo-38 F₁ hybrid segregated for awns in the F₂ population. All F₂ populations segregated for seed type as expected based on the recipient field. Of the 96 F₂ seeds advanced from each confirmed hybrid plant, as expected, only the 48 blue-aleurone seeds segregated or bred true for blue grain color, while the 48 nonblue seeds bred true for nonblue grain color (data not shown). Furthermore, all nine putative hybrids conformed with expectations for a cross with Purendo-38 based on omega-, gamma-, beta-, and alpha-gliadin protein fingerprints. Samples collected from recipient fields no. 1, 22, and 49 were verified to be AC Splendor, HR5500, and AC Taber, respectively, when compared with breeder seed reference samples. For field no. 1, all 20 to 25 F₂-derived F₃ seeds analyzed from each of the four putative hybrid seeds showed segregation for gliadin patterns when compared with the patterns of AC Splendor and Purendo-38 (Fig. 1 ). For field no. 22, all 30 F₂-derived F₃ seeds analyzed (from the one putative hybrid seed identified) showed segregation for gliadin patterns when compared with the patterns of HR5500 and Purendo-38 (data not shown). For field no. 49, all 15 to 25 F₂-derived F₃ seeds analyzed from each of the four hybrid seeds identified showed segregation for gliadin patterns when compared with the patterns of AC Taber and Purendo-38 (data not shown).

View larger version (102K): [in this window] [in a new window]   Figure 1. Using A-PAGE, segregation of omega (), gamma (), beta (ß), and alpha () gliadin banding patterns was detected among putative AC Splendor–Purendo-38 F2-derived F3 seeds when compared with the gliadin fingerprints of recipient parent AC Splendor (field no. 1) and pollen donor Purendo-38.

	DISCUSSION

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION REFERENCES

Pollination periods in 2002 were generally hotter and less humid relative to pollination periods in 2003, indicating that the higher gene flow rates observed in 2003 were likely promoted by cooler and more humid conditions. De Vries (1972) reported that the highest concentration of pollen dispersal appeared to be released at a temperature of 16 to 20°C and relative humidity of 70 to 75%. Wheat pollen grains have been reported to be viable for 15 to 20 min, or up to 30 min under optimal conditions (de Vries, 1971). In the present study weather conditions in 2003 fell within the optimum range reported by de Vries (1972). Environmental conditions from year to year are considered random factors or factors that cannot be controlled by the researcher. Consequently, the weather conditions in some years will promote relatively high gene flow rates (e.g., cool, humid, and wet conditions with strong prevailing winds), and conditions in other years will depress gene flow rates.

Pollen dispersal during flowering varies with pollinator field size (de Vries, 1974). Gene flow studies in wheat have generally used small pollinator plots ( 0.25 ha) and, thus, likely have limited application for estimating the amount of gene flow taking place between neighboring commercial fields. Matus-Cádiz et al. (2004) reported trace intraspecific pollen-mediated gene flow (0.01%) at 300 m using a 50 x 50 m (0.25 ha) blue-grained pollinator block. The latter study used the largest pollinator field tested to date; however, its small size relative to using commercial-scale pollinator fields (20–100 ha) may partly explain why gene flow was not detected beyond 300 m even though sampling occurred up to 2.76 km from the 0.25-ha pollinator source.

Our study is unique within the literature in that it represents the only large-scale commercial study conducted to date in wheat. We report long-distance intraspecific pollen-mediated gene flow at trace levels ( 0.01%) beyond 300 m, which remained constant up to 2.75 km from the pollinator. Trace rates of 0.01% can be considered worst-case scenarios if compared with gene flow rates that are averaged across samples within years. That is, in 2002 one hybrid seed was confirmed out of 3 million seeds (gene flow = [1/3 000 000] x 100 = 0.00003%; 300 times lower than 0.01%), while nine hybrid seeds were confirmed out of 10 million seeds in 2003 (gene flow = [9/10 000 000] x 100 = 0.00009%; 100 times lower than 0.01%). In summary, our results suggest that gene flow will be a minor contributor to product admixture ( 0.01%), but a tolerance level of 0% transgenic wheat in nontransgenic wheat grain, as currently demanded by some groups of producers and consumers, is unrealistic. Tolerance levels, likely ranging from 1 to 5%, will have to be established based on the impurities arising from various transgene contributors such as breeder and certified seed purity, gene flow from neighboring fields, occurrence of gene introgression from related or weedy species, crop volunteers, on-farm admixture, and mechanical admixture during grain handling at or beyond the primary elevator.

	ACKNOWLEDGMENTS

 
Appreciation is expressed to field research technicians L. Ehman and W. Schatz (Crop Development Centre, University of Saskatchewan) and their summer students (A. Bathgate, B. Hinz, V. Spilchuk, G. Nicol, A. Whiteside, C. Saganski, K. Gesy, D. Aschim, S. Pion, J. Costley, and S. Barr) for their assistance throughout the research. Thanks are extended to G. Bell (Grain Research Laboratory; Winnipeg, MB) for his technical assistance in A-PAGE gliadin fingerprinting. This research was funded by a grant from the Saskatchewan Agriculture Development Fund. The Canadian Food Inspection Agency provided financial support for the seed multiplication of the pollen donor. The assistance of the producers whose fields were sampled over the course of this study is very much appreciated. Without their cooperation this study would not have been possible.

Received for publication July 8, 2006.

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