Pollen Flow in Wheat Revisited

1
Pollen Flow in Wheat Revisited

• Joel Ransom
• Extension Agronomist Cereal Crops

2
Why renewed interest in pollen flow in wheat?

• Steady progress in the development of wheat with
  transgenic traits
• Certain markets have indicated that they require
  non-transgenic wheat
• Pollen drift is one of many factors to consider
  when maintaining segregation
• Information on out-crossing in wheat can help
  design effective identity preserved (IP) programs

3
How does gene flow via pollen drift occur?

• Some biology
• Pollen is produced in anthers
• Fertilization requires viable pollen to attach to
  a receptive stigma and the successful transfer to
  genetic material to the ovule.

4
Facts about wheat pollen

• Relatively heavy
• Viable for 2 to 20 minutes
• 2,000 to 4,000 pollen grains per flower

5
Factors affecting gene flow via pollen

• Distance between plants
• Temperature
• Humidity
• Wind
• Insects
• Variety
• Receptivity of the stigma
• Nick (synchrony of flowering)
• Pollen viability

6
Gene Flow via pollen in Wheat Current State of
Knowledge

• Review of pollen movement studies
• Review of information from out-crossing studies
• Isolation distances
• Varietal effects

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How far can wheat pollen move?
Adapted from Khan et al, 1973 (Kansas)
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Pollination of a male sterile
Adapted from Khan et al, 1973
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Summary on pollen movement

• Viable wheat pollen can move gt 150 ft
• Based on male sterile plants, cross pollination
  risk greatest in first 20 ft of isolation from
  source
• Fertilization success dependant on pollen
  concentration

10
Effect of variety and year on out-crossing in
adjacent plants in Kansas, HRWW
Adapted from Martin, 1990
11
Effect of variety and year on out-crossing
(92-93), HRSW, Canada
Adapted from Hucl, 1996
12
Effect of isolation distance on out-crossing of
four Canadian wheat cultivars, 1995
Adapted from Hucl Matus-Cadiz, 2001
13
Source Ostby et al., 2004
14
Factors conferring varietal differences in
cross-pollination propensity

• Glume opening
• Extrusion of anthers
• Duration of opening
• Open spikelets vs dense spikes

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What are the practical implications of these data?

• Environment and variety can influence level of OC
  
• In the two studies with spring wheat summarized a
  distance gt 33-59 ft sufficient gave zero
  outcrossing in HRSW
• Isolation distance gt90 high probability of zero
  or minimal out-crossing

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What are typical isolation distances in IP
systems in ND currently?

• Methodology
• Fields (within/between farms) sampled
• 8 Organic fields
• 8 certified/foundation seed production fields
• 3 IP fields
• Distance between closest wheat crop measured (all
  edges and corners)
• Distance of natural isolation distance measured

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Results

• Organic production fields (isolation required
  from non-organic fields - ? distance)
• Natural isolation
• Minimum distance 0 ft
• Maximum 250 ft
• Average 57 ft
• Median 45 ft
• Actual
• Minimum distance - 48
• Maximum 21,120
• Average - 2640
• Median - 2640

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Results

• Certified Seed Production (current regulations
  5 ft)
• Natural isolation
• Minimum distance 0 ft
• Maximum 165 ft
• Average 43 ft
• Median 42 ft
• Actual
• Minimum distance - 5
• Maximum 21,120
• Average 4,933
• Median 2,640

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Results

• Identity Preserved (isolation specified in
  contract)
• Natural isolation
• Minimum distance 0 ft
• Maximum 500 ft
• Average 97 ft
• Median 50 ft
• Actual
• Minimum distance 1 ft
• Maximum 15,840 ft
• Average 2,039 ft
• Median 152 ft

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Summary on isolation distances

• Natural boundaries typically 50 feet
• Fields are not always separated by natural
  boundaries
• If new standards of OC established for
  non-transgenic wheat requiring greater isolation
  (i.e. 60-90 ft)
• Most but not all IP fields currently close to
  these distances
• Seed production would be most impacted

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Conclusions

• With an isolation distance of 60 - 90 ft
  (conservative based on the most promiscuous
  cultivar) there is limited risk of gene flow via
  pollen between cultivars of HRSW
• Zero tolerance cannot be guaranteed with this
  distance, however, as pollen is capable of much
  farther movement
• Current IP systems frequently have isolation
  distances approaching 60 ft, but sometimes much
  less
• Natural boundaries alone for isolation is not
  workable due to layout of fields

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Conclusions

• Revised isolation distances in IP would likely
  not be too difficult to achieve
• Isolation distances in seed production would need
  to be revised to ensure increased purity
• Given limited out-crossing and current field
  layouts, gene flow from transgenic wheat to
  non-transgenic wheat will likely be minimal and
  manageable. Other factors in segregation process
  will present greater challenges?