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CROP BREEDING, GENETICS & CYTOLOGY-NOTE

Estimating Out-Crossing Rates in Spring Wheat Cultivars Using the Contact Method

Dep. of Plant Sciences and Crop Development Centre, Univ. of Saskatchewan, 51 Campus Dr., Saskatoon, SK, Canada S7N 5A8

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

	ABSTRACT

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Wheat (Triticum aestivum L.) is a self-pollinated species with out-crossing (OC) rates assumed to be less than 1%. The objective of this study was to evaluate the OC rates for 35 Canadian spring wheat cultivars grown under greenhouse conditions. Recipient cultivars were crossed with Purendo-38, a blue aleurone wheat, using direct spike contact inside glassine bags. Four seeding dates were used in each of two greenhouse experiments (2001 and 2002). Out-crossing rates ranged from 0 to 2.8% (2001) and 0 to 3.5% (2002), with the exception of ‘Glenlea’ and ‘Wildcat’. Only Glenlea (10.6% [2001]; 8.6% [2002]) and Wildcat (6.3% [2001]; 4.2% [2002]) displayed consistently elevated levels of OC.

Abbreviations: CPS, Canada Prairie Spring • CWAD, Canada Western Amber Durum • CWES, Canada Western Extra Strong • CWRS, Canada Western Red Spring • OC, out-crossing

	INTRODUCTION

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WHEAT IS A SELF-POLLINATED species with OC rates that are assumed to be <1%, but OC rates > 1% have been reported for wheat plants grown in close proximity. The literature on intraspecific OC in wheat has recently been reviewed in detail by Waines and Hegde (2003). Research on the tendency of western Canadian spring wheat cultivars toward OC is based on a limited number of cultivars (Hucl, 1996; Briggs et al., 1999; Hucl and Matus-Cádiz, 2001). The present research was conducted to assess the effectiveness of estimating OC rates in a larger number of wheat cultivars using greenhouses. There do not appear to be any reports on this approach in the published literature. If OC rates of wheat plants grown in close proximity can be estimated using greenhouse experiments, then estimating the OC rates of a larger cross-section of cultivars should be possible without relying on laborious field-based research. If the tendency of a large number of cultivars to OC can be assessed using only greenhouse experiments, then this information may be useful for developing guidelines for maintaining line purity in breeding programs and pedigreed seed production (Hucl and Matus-Cádiz, 2001). The objective was to estimate the OC rates of 35 Canadian wheat cultivars grown in direct spike contact under greenhouse conditions.

	MATERIALS AND METHODS

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Seed of Purendo-38, an awnless, spring type blue-aleuroned wheat was obtained from the Crop Development Center, Saskatoon, SK. The blue-grained trait is a dominant gene marker that has been applied within gene flow studies (Hucl and Matus-Cádiz, 2001; Matus-Cádiz et al., 2004). Certified seed was used for all 35 cultivars tested (Table 1). Of the cultivars tested, 34 were Canadian cultivars including 19 of the Canada Western Red Spring (CWRS) class, seven of the Canada Prairie Spring (CPS) wheat class, four of the Canada Western Extra Strong (CWES) class, and four of the Canada Western Amber Durum (CWAD; Triticum turgidum L.) class. Rongotea, a New Zealand cultivar (Griffin, 1987), was grouped with CPS cultivars for presentation purposes because Rongotea has similar agronomic and quality characteristics when compared to CPS cultivars.

When seedlings reached the two-leaf stage (ZGS 12), up to three seedlings were removed from each pot to allow three plants to establish per pot. Under these growing conditions, plants within pots produced an average of five to six tillers. All tillers were used for crossing. Each recipient cultivar was crossed with Purendo-38 by covering up to three non-emasculated spikes of a given cultivar and one Purendo-38 spike as pollen source with an individual glassine bag (5 by 19 cm). The number of recipient spikes in a bag was based on spike availability. Awns were clipped to 1 cm above their respective glume before crossing to allow glassine bags to slip easily over multiple spikes. The bags were removed 7 d after pollination to allow for normal grain development and ripening. Crossed spikes were harvested at ZGS 92 (seeding dates were maintained separate), air-dried at 24°C (40% relative humidity) for 7 d, and then individually hand threshed.

Cross-pollination events from Purendo-38 to recipient cultivars were identified by the expression of a light-blue pigment in the aleurone layer of putative F₁ seed. Seeds possessing a light-blue aleurone were visually identified using a fluorescent light box source and separated from the remaining seed lot. Putative light-blue F₁ seeds were sown in the greenhouse and resulting plants were selfed and grown to maturity to confirm the light-blue F₁ seed was the result of an out-crossing event. Before planting, the seeds were surface sterilized for 8 min using a 2.5% sodium hypochlorite and 0.1% (v/v) Tween 20 solution, rinsed for 5 min with water, followed by a rinse with 70% ethanol, and then air-dried at room temperature. Seeds were pregerminated in the dark at 15°C for 10 d in a petri dish (each containing a Whatman No. 1 filter paper) and subsequently transferred to soil. The pregerminated seeds were planted (2.5-cm depth) in 15-cm-diameter pots (three plants per pot) as described above and grown to maturity using the greenhouse conditions described for 2002. Spikes from individual plants were harvested and hand threshed separately. The F₁–derived F₂ seed was classified as segregating (3:1 blue/nonblue seed ratio) or nonsegregating (all nonblue seeds) for the blue-aleurone trait.

Out-crossing rates were calculated for each cultivar as follows: OC (%) = 100(total number of confirmed light-blue seeds observed/total number of seeds collected) where confirmed light-blue seeds included only light-blue F₁ seeds that segregated for the blue-aleurone trait in greenhouse grow outs. In 2001 and 2002, OC rates were calculated by pooling values across the four seeding dates. Means were tested for significance (P 0.05) using one-tailed t tests (Minitab Version 13; Minitab Inc., State College, PA).

	RESULTS AND DISCUSSION

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This 2-yr greenhouse-based study indicates that OC rates in spring wheat cultivars grown in direct spike contact under greenhouse conditions are generally at levels below 2.8% but can exceed 10% (Table 1). Generally, OC rates were less than 2.8% in 2001 and 2002, with the exception of CPS cv. Genesis (3.5% in 2002), CWES cv. Wildcat (6.3% in 2001; 4.2% in 2002), and CWES cv. Glenlea (10.6% in 2001; 8.6% in 2002). In 2001, OC rates ranged from 0 to 2.8% for CWRS cultivars (mean = 0.6), 0 to 1.5% for CPS cultivars (mean = 0.4), 0 to 10.6% for CWES cultivars (mean = 4.3), and 0 to 0.4% for CWAD cultivars (mean = 0.2). In 2002, OC rates ranged from 0 to 2.0% for CWRS cultivars (mean = 0.7), 0 to 3.5% for CPS cultivars (mean = 1.7), 1.5 to 8.6% for CWES cultivars (mean = 4.1), and 0.2 to 1.4% for CWAD cultivars (mean = 0.5).

Eighteen out of the 35 cultivars tested were prone to OC levels of 1% in at least 1 yr of the study while Rongotea, Glenlea, and Wildcat were prone to OC, with rates of 1% in both years. Out-crossing levels in the CPS class were significantly lower in 2001 (mean = 0.4) compared with 2002 (mean = 1.7) while OC levels in the three other wheat classes were similar in both years. These results suggest that OC rates in wheat are cultivar and greenhouse environment dependent. Twenty of the cultivars tested possessed awns (Table 1). Research in hybrid wheat production has shown that the free-falling movement of pollen is impeded by the awns of male-sterile plants (de Vries, 1971), suggesting that their removal before crossing in our study may have promoted higher OC rates than would have been expected had the awns been retained. In contrast, interspecific barriers to hybridization were likely of greater importance in impeding OC rates in the CWAD class.

In wheat, successful OC depends on the receptivity of the stigma, the viability of the pollen, and availability of pollen during the receptive period (Johnson and Schmidt, 1968). These factors vary with genotype and environment (de Vries, 1971, 1972, 1974). Pollen dispersal during flowering varies with environmental factors including prevailing winds, wind speed, temperature, humidity, and precipitation (de Vries, 1971, 1972, 1974). Higher OC rates have been associated with higher wind speeds and prevailing wind directions during the flowering period of a wheat crop (Hucl and Matus-Cádiz, 2001). De Vries (1972) reported that the highest concentration of pollen dispersal occurred at temperatures between 16 and 20°C and 70 to 75% relative humidity. Daily humidity and temperature, during anthesis were not recorded in the present study and may have been useful in explaining, in part, the higher OC rates in the CPS class in 2002 relative to 2001 (Table 1). Seed production in 2002 was fourfold higher than in 2001. The higher grain production is evidence of more vigorous plant growth, likely a result of the 67% higher photosynthetically active radiation levels and cooler temperatures experienced with the planting in the fall of 2002.

Direct spike contact combined with the four seeding dates used in our current study, unlike the one (Hucl and Matus-Cádiz, 2001) and three (Hucl, 1996) seeding dates in our previous field studies, maximized the level of flowering synchrony between the donor blue-aleuroned pollen source and recipient cultivars. Hucl (1996) reported maximum OC rates for 11 wheat cultivars ranging from 5.2% (Oslo) to 0.2% (Columbus and CDC Makwa) in a field study. In the present greenhouse study, Glenlea (10.6% in 2001; 8.6% in 2002), Wildcat (6.3% in 2001; 4.2% in 2002), and Genesis (0.2% in 2001; 3.5% in 2002) tended to be prone to OC while Oslo tended toward low levels of OC (0.7% in 2001; 1.2% in 2002). In contrast, Hucl (1996) reported that under field conditions Glenlea (0.6%), Wildcat (1.1%), and Genesis (0.3%) tended to be less prone to OC relative to Oslo (5.2%). Similarly, Hucl and Matus-Cádiz (2001) also reported higher OC rates for Oslo under field conditions (3.2%) relative to the rate reported in our current study. Other research has reported that grain yield in Oslo responds differently to moisture stress under controlled environment (Baker, 1996) versus field conditions (Hucl and Graf, 1990), with Oslo showing changes in genotype rank from one environment to another only under field conditions.

Based on this initial research, some cultivars appear to be consistently low out-crossers regardless of environment while others appear to be prone to OC depending on the environment. In both years, the CWES class had the highest mean OC rates of all classes studied (4.3% in 2001; 4.1% in 2002), in large part due to the cultivars Glenlea and Wildcat. Spike laxness appears to result in a greater degree of floret opening in wheat (de Vries, 1971). The CWES cultivars Glenlea and Wildcat have laxer, fusiform spikes and lower pollen stainability relative to other spring wheat cultivars (Hucl, 1996). Hucl (1996) suggested that these morphological traits may, at least in part, explain the proneness of the two cultivars toward OC under field conditions. Out-crossing rates estimated under greenhouse conditions appear to be poor predictors of OC rates under field conditions. Further research is needed to quantify genetic variability for OC and assess the nature of the G x E interaction in determining OC in wheat.

	ACKNOWLEDGMENTS

 
Appreciation is expressed to L. Ehman, M. Grieman, W. Schatz, K. Jackle, A. Overlid, and S. Campbell for their technical assistance.

Received for publication April 20, 2005.

	REFERENCES

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION REFERENCES

 

• Baker, R.J. 1996. Oslo and Biggar spring wheats respond differently to controlled temperature and moisture stress. Can. J. Plant Sci. 76:413–416.
• Briggs, K.G., O.K. Kiplagat, and A.M. Johnson-Flanagan. 1999. Floret sterility and outcrossing in two spring wheat cultivars. Can. J. Plant Sci. 79:321–328.
• de Vries, A. p. 1971. Flowering biology of wheat particularly in view of hybrid seed production: A review. Euphytica 20:152–170.[CrossRef][ISI]
• de Vries, A.P. 1972. Some aspects of cross pollination in wheat (Triticum aestivum L). 1. Pollen concentration in the field as influenced by variety, diurnal pattern, weather conditions, and level as compared to the height of the pollen donor. Euphytica 21:185–203.[CrossRef]
• de Vries, A.P. 1974. Some aspects of cross pollination in wheat (Triticum aestivum L). 4. Seed set on male sterile plants as influenced by distance from pollen source, pollinator: Male sterile ratio, and width of the male sterile strip. Euphytica 23:601–622.[CrossRef]
• Griffin, W.B. 1987. Out-crossing in New Zealand wheats measured by occurrence of purple grain. N. Z. J. Agric. Res. 30:287–290.
• Hucl, P. 1996. Out-crossing rates for 10 Canadian spring wheat cultivars. Can. J. Plant Sci. 76:423–427.
• Hucl, P., and R.J. Graf. 1990. Evidence of cross-over interaction involving four spring wheat cultivars. p. 135–142. In New Frontiers in Prairie Agriculture. Proc. Soils and Crops Workshop. 22–23 Feb. 1990. Univ. of Saskatchewan Ext. Press, Saskatoon, SK.
• Hucl, P., and M.A. Matus-Cádiz. 2001. Isolation distances for minimizing out-crossing in spring wheat. Crop Sci. 41:1348–1351.[Abstract/Free Full Text]
• Johnson, V.A., and J.W. Schmidt. 1968. Hybrid wheat. Adv. Agron. 20:199–232.
• Matus-Cádiz, M.A, P. Hucl, M.J. Horak, and K. Blomquist. 2004. Gene flow in wheat at the field scale Crop Sci. 44:718–727.[Abstract/Free Full Text]
• Waines, J.G., and S.G. Hegde. 2003. Intra-specific gene flow in bread wheat as affected by reproductive biology and pollination ecology of wheat flowers. Crop Sci. 43:451–463.[Abstract/Free Full Text]
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