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a Department of Plant Sciences and Crop Development Centre, University of Saskatchewan, 51 Campus Dr., Saskatoon, SK, Canada S7N 5A8
b Ecological Technology Center, Monsanto Co., 800 North Lindbergh Blvd, St. Louis, MO 63141, USA
* Corresponding author (hucl{at}sask.usask.ca).
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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40 m from the pollinator. The results suggest that gene flow occurrence in spring wheat is relatively low but that a tolerance level of 0% transgenic wheat in nontransgenic wheat grain is unrealistic. |
INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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The occurrence of intra- and interspecific gene flow in wheat and the tendency for some cultivars to possess higher gene flow rates than others is well established. Wheat is a predominantly self-pollinating crop with a gene-flow rate of usually less than 1% (Johnson and Schmidt, 1968). Heyne and Smith (1967) reported that the extent of gene flow in commercial wheat cultivars ranged from 0 to 4% in close proximity. Higher gene flow rates including 0.1 to 5.6% (Martin, 1990) and 0.3 to 6.1% (Hucl, 1996) have been reported in wheat with plants grown in close proximity. A number of studies have observed off-types in wheat cultivars attributable to gene flow between wheat genotypes of the same species (Appleyard et al., 1979; Porter et al., 1980; Griffin, 1987; Takahasi and Isii, 1988). Interspecific gene flow rates have been studied to a limited extent. Zorun-Ko et al. (1996) reported interspecific gene flow rates of 0.04 to 0.30% from common hexaploid (T. aestivum; 2n = 6x = 42 chromosomes; BBAADD) to durum wheat (T. turgidum; 2n = 4x = 28 chromosomes; BBAA), with plants grown in close proximity. 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). Attempts have been made to associate increased gene flow with specific plant traits such as plant height (Martin, 1990; Briggs et al., 1999) and spike characteristics (de Vries, 1971; Hucl, 1996), but no single trait has been useful in predicting whether cultivars will possess gene flow rates >1%.
Gene flow in wheat 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. We have detected an intraspecific 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 blue-grained pollinator (Hucl and Matus-Cádiz, 2001). Sampling did not extend beyond 33 m and, consequently, we speculate that low levels of gene flow may occur at distances beyond 33 m with heavier pollen loading. To date, gene flow in wheat has been studied in small-sized pollinator blocks (e.g., 5 by 5 m) located within close proximity of recipient plants.
Larger-scale field studies are needed to measure the level of pollen-mediated gene flow that may exist in larger-sized pollinator blocks located at varying distances from recipient plants. Transgenic plants with a marker gene, such as herbicide resistance, have commonly been used for investigating pollen dissemination in controlled experimental fields trials (Ingram, 2000). Our research proposes to measure gene flow over long distances in wheat in a 50- by 50-m pollinator block, with the blue-aleurone trait as a detectable marker (Keppenne and Baenziger, 1990). The blue-grained trait is a dominant gene marker that has been applied to studying small-scale gene flow (Hucl and Matus-Cádiz, 2001). Cross-pollination from blue-seeded to non-blue-seeded wheat is identified 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 will provide information useful for (i) risk assessments considering gene-escape from transgenic to nontransgenic wheat, (ii) developing guidelines for maintaining line purity in breeding programs and pedigreed seed production, and (iii) developing strategies for managing gene flow between commercial fields. The objectives of this research were to measure gene flow rates from a blue grained pollinator Purendo-38 to (i) red-grained spring wheat cv. CDC Teal over distances of 0.2 to 160 m, (ii) amber durum wheat cv. AC Navigator over distances of 0.2 to 260 m, and (ii) CDC Teal and AC Navigator over distances of 180 to 2760 m.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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Wheat genotypes carrying the blue aleurone trait were not documented until the artificial introgression of genes from Agropyron species (Zeven, 1991). Therefore, the blue-aleurone trait was not expected to be present in the genetic background of CDC Teal or AC Navigator. Seed lots of CDC Teal and AC Navigator used in 2001 were screened for the presence of blue-aleurone seed. Blue-aleuroned seed was not observed during the screening of 50 000 seeds of each cultivar. In 1990, a bulk population of spring wheat segregating for male-sterility and blue-aleurone, introgressed from an unknown accession of Agropyron elongatum (Host) Beauv., was obtained from Dr. D. Falk (Dep. of Crop Sci., University of Guelph, Guelph, ON). This bulk population was increased in bulk at Saskatoon, SK in 1990 and 1991. In 1992, 2000 blue seeds were hand-picked and space-planted. Approximately 600 individual spikes were harvested based on reduced shattering. Of the 600 spikes, seed from 180 spikes were homogeneous for the blue-grained trait. In 1993, seed from the 180 spikes were grown in head-rows and 53 rows were determined to be homozygous for the blue-grained trait. Purendo-38 was selected from the most promising 10 homozygous lines tested over three years based on its grain color, quality, and agronomic traits. The stability of the blue-aleurone trait in Purendo-38 was verified by evaluating grain samples from 200 head-rows. Purendo-38 has been stable for the blue-aleurone trait over six generation of testing.
Intraspecific Gene Flow
In 2000 and 2001, 400 x 400-m field trials were sown in separate fields at the Kernen Crop Research Farm (KCRF), University of Saskatchewan, Saskatoon, SK. The experiment consisted of a central blue-grained pollinator block (50 by 50 m) surrounded by CDC Teal to a distance of 175 m in all directions (Fig. 1)
. Recipient rows were spaced 0.2 m apart. The pollinator block was seeded at a low rate (100 seeds m–2) over two seeding dates in alternating strips. Strips consisted of eight rows, 50 m long and 0.2 m apart. Pollinator blocks were sown on 1 May (1st planting date) and 8 May (2nd planting date) in 2000 and 24 April (1st planting date) and 2 May (2nd planting date) in 2001. The reduced seeding rate and dual seeding dates of the pollinator block were used to promote "nicking" between the two genotypes. CDC Teal was sown on 3 May 2000 and 2 May 2001 at a rate of 250 seeds m–2 on fallow land. Seeds were treated with the systemic fungicide Vitavax Single Solution (Uniroyal Chemical Ltd., Elmira, ON; active ingredient carbathiin, 5-dihydro-2-methyl-N-phenyl-1,4,oxathiin-3-carboximide) at the recommended rate. Fertilizer was drilled in with the seed at a rate of 7 kg ha–1 of N and 29 kg ha–1 of P. The soil type was a Dark Brown Chernozem (Typic boroll) Sutherland clay, clay-loam at the KCRF. Plant height, days to heading (date of 50% spike emergence), and duration of flowering (days between the first and last observed occurrence of anthesis) were collected. Data for the pollinator block were averaged over values collected at five random positions within each seeding date. Data for CDC Teal were averaged over values collected at eight positions (0.2, 25, 50, 75, 100, 125, 150, and 175 m from the pollinator) in each of four directions (N, E, S, and W). Meteorological data were collected within 3 km of the trial site by Environment Canada.
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Cross-pollination events from Purendo-38 to recipient plants of CDC Teal were identified by the expression of a light-blue pigment in the aleurone layer of F1 seed. Seeds suspected of possessing a light-blue aleurone were visually identified and kept separate from the remaining seed lot. Putative light blue seeds were sown in the greenhouse. Resulting plants were grown to maturity to confirm visual identifications as putative light blue seeds can be confused with discolored seeds because of disease or weathering, and immature seeds. Seeds were surface sterilized for 8 min in a solution of 2.5% (v/v) sodium hypochlorite and 0.1% (v/v) Tween 20, rinsed for 5 min with water, rinsed once with 70% (v/v) ethanol, and subsequently air-dried. Seeds were germinated at 15°C (in darkness) for 10 d in a Petri-dish (each containing a Whatman No.1 filter paper) before transferring to soil. Pregerminated seeds were planted (2.5-cm depth) in 15-cm-diameter pots (one plant per pot). Pots were filled with Terra-Lite Redi-Earth (W.R. Grace and Co. of Canada Ltd. Ajax, ON). Greenhouse conditions were set to 24/18°C (day/night) with 18 h light and a photosynthetically active radiation level of 250 µmol m–2 s–1. Plants were watered every 4 d and fertilized with 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. Segregation among F1–derived F2 seed was classified as segregating (3 blue: 1 non-blue seed ratio) or nonsegregating (all non-blue seeds) for the blue-aleurone trait.
There were 512 putative light blue seeds identified in 2000 and 213 seeds identified in 2001. The percentage of putative light blue seeds that developed to produce established plants for verification purposes was 64% in 2000 and 65% in 2001. None of the established plants produced only blue-aleuroned seed, indicating that seed admixture was not a source of contamination within this study. The number of established plants confirmed to be segregating for the blue-aleurone trait was 303 plants (59%) in 2000 and 127 plants (60%) in 2001. Unconfirmed gene flow rates were calculated as follows: gene flow (%) = (total number of putative light blue seeds observed in si at dj/8000 seeds of si at dj) x 100 where si is the ith sample and dj is the jth distance. Confirmed gene flow rates were calculated as follows: gene flow (%) = (total number of confirmed light blue seeds observed in si at dj/8000 seeds of si at dj) x 100 where total number of confirmed light blue seeds is defined as the total number of putative light blue seeds verified to produce F1 plants segregating for the blue-aleurone trait. Confirmed gene flow rates were expected to be lower than unconfirmed rates as less than 100% of putative light blue seeds germinated to produce established plants for verification purposes. This difference may be explained, in part, by the fact that diseased, weathered, and immature seeds are unlikely to germinate and grow to produce plants. Of the putative light blue seeds failing to produce an established plant, the proportion of these putative seeds that would have shown segregation or nonsegregation for the blue-aleuroned trait is unknown.
Interspecific Gene Flow
The same experimental design as described above was modified as follows for the interspecific gene flow experiment. In 2000 and 2001, 650- by 650-m field trials were sown in separate fields at the KCRF, University of Saskatchewan approximately 2 km from the intraspecific gene flow experiments. The experiment consisted of a central Purendo-38 pollinator block (50 by 50 m) surrounded by AC Navigator to a distance of 275 m in all directions (Fig. 1). The pollinator block was sown as described above. AC Navigator was seeded on 5 May 2000 and 4 May 2001 at a rate of 250 seeds m–2 on fallow land. Plant height, days to heading, and duration of flowering were calculated for AC Navigator by collecting data at 10 positions (0.2, 25, 50, 75, 100, 125, 150, 175, 200, and 225 m from the pollinator) in each of four directions (N, E, S, and W). At maturity, 0.5- by 4-m strips of AC Navigator were sickled and bagged at 0.2, 1, 5, 10, 20, 40, 60, 80, 100, 120, 140, 160, 180, 200, 220, 240, and 260 m along eight transects (N, E, S, W, NE, SE, SW, NW) radiating out from the 50- by 50-m square pollinator block. Strips were harvested perpendicular to the sides and corners of the pollinator block. Spike samples were threshed using a rubber-belted de-awner. The wind-speed on the de-awner was turned off to retain shriveled seed, and consequently, chaff within samples was removed manually. Samples were sorted before threshing on the basis of transect and distance. Samples harvested within each transect were threshed in descending order starting with the sample most distant from the pollinator source. A 2- by 2-m quadrant sample of barley was threshed to clean the combine between samples from different transects.
Cross-pollination from Purendo-38 to recipient plants of AC Navigator were identified by the expression of a light-blue pigment in the aleurone layer of shriveled F1 hybrid seed. Shriveled seeds suspected of possessing a light-blue aleurone were visually identified and confirmed by greenhouse grown-outs. The number of putative light blue seeds identified was 893 seeds in 2000 and 127 seeds in 2001. The percentage of putative light blue seeds that developed to produce established plants was 21% in 2000 and 41% in 2001. The number of established plants confirmed to be segregating for the blue-aleurone trait was 107 plants (12%) in 2000 and 52 plants (41%) in 2001.
Long-Distance Intra- and Interspecific Gene Flow
All CDC Teal and AC Navigator fields seeded at the KCRF, University of Saskatchewan were screened for light blue seeds. Breeder-derived seed of CDC Teal and certified seed of AC Navigator were used as seed sources. A total of 38 and 39 random samples were collected in 2000 and 2001, respectively. At each sampling site, a 0.5- by 4-m strip was sickled, bagged, dried, and subsequently threshed. Seeds suspected of possessing a light-blue aleurone were visually identified and confirmed using greenhouse grown outs. In 2000, the number of putative light blue seeds identified was 14 seeds (CDC Teal) and five seeds (AC Navigator). The percentage of putative light blue seeds that developed to produce established plants was 57% (CDC Teal) and 0% (AC Navigator). One established plant was confirmed to be segregating for the dominant gene marker. In 2001, seven putative light blue seeds were identified with none of them developing to produce established plants. Unconfirmed gene flow rates were calculated as follows: gene flow (%) = (total number of putative light blue seeds observed in si at dj/total number of seeds harvested of si at dj) x 100. Confirmed gene flow rates were calculated as follows: gene flow (%) = (total number of confirmed light blue seeds observed in si at dj/total number of seeds harvested of si at dj) x 100.
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RESULTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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Confirmed gene flow rates were higher in 2000 compared with 2001 (Table 2). Unconfirmed gene flow rates are presented in Table 2 to indicate the furthest distance from the pollinator at which putative light blue seeds were detected. In 2000, maximum confirmed gene flow rates for CDC Teal were equal to, or less than, 0.44%. Percent gene flow declined rapidly as distance increased and leveled off at trace levels (
0.01%) beyond 60 m. Percent gene flow (averaged over eight directions) was 0.20% at 0.2 m, 0.17% at 1 m, and decreased to 0.003% by 100 m. Percent gene flow at 0.2 m ranged from a low of 0.08 (east of the pollinator) and 0.09% (SE) to a high of 0.30% (N), 0.34% (W), and 0.41% (NW). No gene flow was detected at or beyond 120 m. Approximately 78% and 99% of cumulative gene flow occurred within one and 40 m of the pollinator, respectively. Average gene flow (0.2–100 m) within directions was determined to identify the primary directions in which gene flow occurred. Out-crossing (averaged over nine distances) was highest to the NW of the pollinator (0.12%) followed by the W (0.11%) and N (0.09%) directions. Gene flow (averaged over nine distances) was detected at trace levels (0.04%) to the NE, E, SE, S, and SW of the pollinator. Gene flow levels equal to or greater than 0.01% were detected at 0.2 m (SW), 20 m (W, NE, and SE), 40 m (E and S), and 100 m (N and NW) from the pollinator. Approximately 76% of cumulative gene flow occurred to the N, W, and NW of the pollinator. Prevailing wind direction was associated with elevated gene flow rates to the N, W, and NW of the pollinator during 4 of 9 d (44%) of pollination (Tables 1 and 2).
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0.01%) at and beyond 60 m. Percent gene flow (averaged over eight directions) was 0.08% at 0.2 m, 0.06% at 1 m, and decreased to 0.002% by 80 m. Percent gene flow at 0.2 m ranged from a low of 0.01% (S and SE) to a high of 0.13% (N), 0.24% (E), and 0.11% (W). No gene flow was detected at or beyond 100 m. Approximately 79 and 99% of cumulative gene flow occurred within 5 and 40 m of the pollinator, respectively. Gene flow rates (averaged over nine distances) were highest to the east of the pollinator (0.07%). Gene flow was detected at trace levels (0.04%) in the remaining seven directions. Gene flow levels equal to or greater than 0.01% were detected at 1 m (S), 5 m (N, NE, and SW), 10 m (W and NW), 40 m (SE), and 80 m (E) from the pollinator. Approximately 83% of cumulative gene flow occurred to the N, E, W, and NW of the pollinator. Prevailing wind direction was associated with elevated gene flow rates to the east of the experiment for only 2 of 9 d (22%) of pollination (Tables 1 and 2).
Interspecific Gene Flow
The 11-d pollination period in 2000 was generally longer in duration, cooler, more humid, and wetter relative to the 7-d pollination period in 2001 (Table 1). In 2000, average heading date for AC Navigator was 9 July and for Purendo-38 was 10 July (1st planting date) and 12 July (2nd planting date). The estimated duration of flowering was 12 d for AC Navigator (5% anthesis on 11 July to 95% on 22 July) and 17 d for Purendo-38 (5% anthesis on 12 July to 95% anthesis on 28 July). The estimated over-lap in pollination periods between AC Navigator and Purendo-38 was about 11 d (12–22 July) (Table 1). Daily mean wind speed during 11 d of pollination was 15 km h–1 (range = 5–26 km h–1). Prevailing winds were from the E, SE, and ESE during six days (averaging 14 km h–1) and from the W, WNW, or N during the remaining 5 d (averaging 16 km h–1) of pollination. Daily mean temperature, relative humidity, and precipitation values were 17°C (range = 12–24°C), 74% (59–89%), and 0.8 mm (0–4.5 mm) during pollination, respectively.
In 2001, average heading date for AC Navigator was 4 July and for Purendo-38 was 2 July (1st date) and 4 July (2nd date). The estimated duration of flowering was 10 d for AC Navigator (5% anthesis on 7 July to 95% on 16 July) and 9 d for Purendo-38 (5% anthesis on 5 July to 95% on 13 July). The estimated over-lap in pollination periods between AC Navigator and Purendo-38 was about 7 d (7–13 July) (Table 1). Prevailing winds were from the SSE for 2 d (averaging 19 km h–1), the W for 1 d (14 km h–1), and the N, NE, or ENE for the remaining 4 d (averaging 12 km h–1) of pollination. During the seven days of pollination, mean wind speed was 14 km h–1 (range = 11–20 km h–1), mean temperature was 21°C (range = 19–23°C), relative humidity was 66% (range = 56–81%), and mean precipitation was 0.04 mm (range = 0–0.3 mm). Plant heights for Purendo-38 (98 cm; SE ± 2) and AC Navigator (76 cm; SE ± 1) were similar over years. The pollination periods observed in 2000 and 2001 were expected to ensure a good level of nicking between recipient and pollinator plants.
Generally, confirmed gene flow rates observed in 2000 were higher than in 2001 (Table 3). In 2000, maximum gene flow rates for AC Navigator were equal to or less than 0.19% (Table 3). Percent gene flow declined rapidly as distance increased and leveled off at trace levels (0.05%) beyond 20 m. Percent gene flow (averaged over directions) was 0.08% at 0.2 m and decreased to 0.01% by 20 m. Percent gene flow at 0.2 m ranged from a low of 0.01% (SE) to a high of 0.10% (NW), 0.11% (N and S), and 0.14% (W). No gene flow was detected at or beyond 40 m. Approximately 71 and 94% of cumulative gene flow occurred within 1 m and 10 m of the pollinator, respectively. Gene flow rates (averaged over five distances) were highest to the north of the pollinator (0.08%) followed by the west (0.05%), and northwest (0.05%). Gene flow was detected at trace levels (0.04%) to the NE, E, SE, S, and SW of the pollinator. Gene flow levels of equal to or greater than 0.01% were detected at 0.2 m (NE and SW), 5 m (E), 10 m (S, W, and NW), and 20 m (N and SE) from the pollinator. Approximately 64% of cumulative gene flow occurred to the N, W, and NW of the pollinator. Wind direction was associated with elevated gene flow rates to the N, W, and NW of the pollinator during 6 of 11 d (55%) of pollination (Tables 1 and 3).
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0.01%) by 20 m. Percent gene flow (averaged over directions) was 0.05% at 0.2 m and decreased to 0.002% by 20 m. Percent gene flow at 0.2 m ranged from a low of zero (E) to a high of 0.08% (W) to 0.10% (N). No gene flow was detected beyond 40 m. Approximately 54 and 98% of cumulative gene flow occurred within 0.2 and 5 m of the pollinator, respectively. Gene flow rates (averaged over five distances) were highest to the north of the pollinator (0.04%) followed by the seven remaining directions (0.02%). Gene flow levels of 0.01% were detected at 0.2 m (S), 1 m (N, SE, SW, and W), 5 m (NE and E), and 20 m (NW) from the pollinator. Approximately 56% of cumulative gene flow occurred to the N, W, and NW of the pollinator. Prevailing wind direction was associated with elevated gene flow rates to the N, W, and NW of the experiment for only 2 of 7 d (29%) of pollination (Table 1 and 3).
Long-Distance Intra- and Interspecific Gene Flow
Long-distance intraspecific pollen-mediated gene flow was not detected beyond 300 m of the pollinator blocks (Table 4). In 2000, a trace level of long-distance intraspecific gene flow was confirmed in one of 34 samples of CDC Teal. That is, a trace gene flow rate of 0.005% was detected in CDC Teal sampled at 300 m to the NW of the pollinator. The detection of this intraspecific seed was observed within the 400- by 400-m experimental field of CDC Teal and not within an adjacent field. This gene flow occurrence was associated with prevailing winds and the elevated gene flow rates to the NW of the pollinator (Tables 1 and 2). In 2001, long-distance intraspecific gene flow was not confirmed in any of the 34 samples of CDC Teal screened. Long-distance interspecific gene flow was not confirmed in any of the four AC Navigator samples screened in 2000 or any of the six samples screened in 2001 (Table 4).
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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Pollen dispersal during flowering varies with environmental factors such as prevailing winds, wind speed, temperature, humidity, and precipitation (de Vries, 1971, 1972, 1974). Gene flow was generally dependent on wind direction during the flowering period of the crop (Tables 2 and 3). In 2000, elevated gene flow rates were generally observed to the N, W, and NW of the pollinator. Wind direction and gene flow rates showed a weaker association in 2001 compared with 2000, indicating that gene flow rates should not be based on experiments oriented in only one direction from the pollinator. Pollination periods in 2001 were generally hotter, less humid, and drier relative to pollination periods in 2000, indicating that the elevated gene flow rates observed in 2000 were probably promoted by cooler, more humid, and wetter 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, the weather conditions in 2000 fall 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 be the researcher. Consequently, the weather conditions in some years will promote relatively high gene flow rates and conditions in other years will depress gene flow rates.
Intra- and interspecific pollen-mediated gene flow rates remained below 0.5% and declined rapidly with distance from the pollinator. Confirmed intraspecific pollen mediated gene flow rates declined from 0.44 to 0.01% over a distance of 100 m from the pollinator in 2000 (Table 2). In 2001, confirmed gene flow rates declined from 0.24 to 0.01% over a distance of 80 m from the pollinator. In both years, intraspecific gene flow declined to trace levels (0.01%) by 60 m from the pollinator. Confirmed interspecific pollen mediated gene flow rates declined from 0.19% to 0.05% in 2000 and from 0.11 to 0.01% in 2001 over a distance of 20 m from the pollinator (Table 3). Interspecific gene flow declined to trace levels of 0.05% in 2000 and 0.01% in 2001 by 20 m from the pollinator. In Canada, a maximum impurity tolerance of 0.01% (one off-type per 10 000 plants x 100) in Foundation or Registered seed and 0.05% in Certified seed has been deemed acceptable in wheat production (Anonymous, 1994). Minimum isolation requirements, from another genotype of the same wheat species, of 3 m for the production of Foundation, Registered, or Certified seed are currently recommended. Recently, we recommended increasing the isolation distance from 3 m to at least 30 m to mitigate out-crossed derived off-types in the subsequent generation of pedigreed seed (Hucl and Matus-Cádiz, 2001). Our current results, based on a 50- by 50-m pollen source, support our earlier findings, based on a small 5- by 5-m pollen source (Hucl and Matus-Cádiz, 2001), that an isolation distance of at least 30 m may be required to limit off-type impurities to 0.01%.
One case of long-distance intraspecific pollen-mediated gene flow was confirmed at a rate of 0.005% at a distance of 300 m to the NW of the 2000 pollinator block (Table 4). This gene flow event probably resulted from wind-mediated pollen movement promoted by strong and prevalent winds. The possibility of insect-mediated pollen movement cannot be discarded, but the amount of insect pollination, if any, in wheat is currently not known. The detection of trace gene flow rates at distances of 300 m from the pollinator field suggest that attaining 100% genetic purity in pedigreed seed lots is unattainable without geographic separation. Another implication of these findings is that geographic separation of wheat with various novel traits or genetic backgrounds, including transgenic and nontransgenic material, may be required for maintaining purity within breeding programs. Genetic purity of breeder, select, foundation, registered, and certified seed is of great importance since the degree of impurity within these seed lots, used to produce pedigreed seed or establish a commercial crop, can become a source from which crop cultivars can become contaminated with novel traits.
The genetic stability, fitness, and fertility of hybrids have been listed as important factors affecting gene flow in plants (Barton and Dracup, 2000). Intraspecific pollen mediated gene flow was confirmed over longer distances from the pollinator compared with interspecific gene flow (Tables 2, 3, and 4). The shrunken endosperm of the interspecific hybrids, unlike the fully developed endosperm of intraspecific hybrids, probably reduces the possibility of gene introgression in subsequent generations because of reduced seedling establishment. Little research is available regarding the stability, fitness, and fertility of intra- and interspecific wheat hybrids. Our study will be useful in assessing the risk posed by transgenic wheat cultivars. When considering the risk posed by the introduction of a new wheat cultivar, the distance and frequency to which pollen mediated gene flow occurs is considered, as is any potential ecological consequence of gene transfer.
Pollen dispersal during flowering varies with pollinator field size (de Vries, 1974). Hybrid wheat production studies have studied optimum sizes of pollinator blocks for cross-pollination. These studies generally use small pollinator plots and, thus, have limited application in estimating the amount of gene flow taking place between commercial-scale fields. Caution must be applied to extrapolating the gene flow rates reported in the present study to larger-scale field studies using commercial-scale pollinator sources because our study may underestimate the level of gene flow rate that may be occurring between commercial-scale fields. In addition, trace gene flow rates of 0.005% may occur at distances exceeding 300 m from commercial-scale pollinator sources. Further research is needed to determine the level of gene flow occurring between neighboring commercial-scale wheat fields.
In summary, our results suggest that (i) gene flow will be a minor contributor to product admixture and (ii) 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, probably ranging from 1 to 5%, will have to be established on the basis of the impurities arising from various transgene contributors such as breeder and certified seed purity, gene flow from neighboring fields, crop volunteers, occurrence of gene introgression from related or weedy interspecific hybrids, mechanical admixture, and grain handling.
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ACKNOWLEDGMENTS |
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Received for publication March 14, 2003.
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REFERENCES |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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R. G. Lawrie, M. A. Matus-Cadiz, and P. Hucl Estimating Out-Crossing Rates in Spring Wheat Cultivars Using the Contact Method Crop Sci., January 24, 2006; 46(1): 247 - 249. [Abstract] [Full Text] [PDF]
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D. I. Gustafson, M. J. Horak, C. B. Rempel, S. G. Metz, D. R. Gigax, and P. Hucl An Empirical Model for Pollen-Mediated Gene Flow in Wheat Crop Sci., May 27, 2005; 45(4): 1286 - 1294. [Abstract] [Full Text] [PDF]
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