Detecting gene flow from GM crops to wild relatives

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Detecting gene flow from GM crops to wild
relatives

• Mike Wilkinson

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Interspecific hybrids between crops and wild
relatives, worldwide
Ellstrand et al. 1999
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In the United Kingdom

• Of 30 crops reviewed
• 7 have no cross-compatible wild relatives
• 11with the potential for hybrid formation
• 12 with a history of hybridization
• From Raybould and Gray, 1993, amended by
  Wilkinson 2002

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In the United Kingdom

• First wave of GM crops
• Maize No relatives
• Oilseed rape Many relatives
• Sugar beet 1 close relative
• Potato 3 relatives, none compatible

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The process of transgene recruitment and spread

• Initial Hybridization
• Introgression
• Gene flow between populations
• Changed fitness leading to change in population
  size, density or distribution
• Effects on other organisms
• Bilateral interactions(e.g. bitrophic)
• Trilateral interactions (Tritrophic)
• Changed community

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Initial hybridization
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Why quantify hybridization?
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1. Hybrid frequency affects the likelihood of all
subsequent consequences
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Example Impact
GM crop
A specialist parasitoid
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GM crop
F1 hybrid in region
Insect resistant transgene stabilises by
introgression
Transgene spreads to most populations
Enhanced resistance to herbivore depresses
herbivore numbers
Depressed herbivore numbers depress specialist
parasitoid abundance
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2. Hybrid abundance dictate the value of measures
to repress/prevent hybrid formation

• Isolation distance
• Male sterility
• Integration site
• Chloroplast transformation
• Inducible promotors/ terminator technology
• Transgene excision

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Estimates of hybrid abundance and other exposure
terms have most value for regulation when applied
at national scale
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How do you estimate F1 hybrid formation between
crop and wild relatives across the UK?

• Identify recipients
• Quantify local gene flow rates
• Estimate long-range gene flow
• Combine 1-3 to estimate frequency and location of
  hybrids

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Possible primary recipientsScheffler Dale
(1994) Transgenic Res. 3, 263-278

• Brassica rapa
• B. oleracea
• B. carinata
• B. juncea
• B. nigra
• B. adpressa

• Raphanus raphanistrum
• Diplotaxis erucoides
• D. muralis

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Brassica rapa

• Casuals of disturbed land
• A weed of B. napus
• Stable wild populations of river banks

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Weedy B. rapa in oilseed rape
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Hybrid seed formation

• 9-93 of seeds from B. rapa are hybrids
• Average 60
• Surely lots of hybrids will be everywhere
• Jorgensen and Andersen (1994)

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No, because

• B. rapa is an infrequent weed
• Crop rotation and weed control

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Weedy B.rapa
In Yorkshire and the Humber region 37.4 of OSR
fields contain weedy rapa
In all other regions 0.54 of OSR fields contain
weedy rapa
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Hybrid survival

• Hybrid seeds show lt10 dormancy
• Weedy B. rapa shows 60-90 dormancy
• So in next year, 90 of hybrids germinate but
  only 10-40 of B. rapa
• WEED CONTROL IS EFECTIVE IN CEREALS
• Linder (1998) Ecological Applications 8 (4)
  1180-1195

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Wild B. rapa

• Local hybrids
• Long-range hybrids

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Screen seed offspring for hybrids

• 15341 seeds sown from two populations
• 8647 seeds germinated
• 46 hybrids morphology, flow cytometry,
  chromosome counts, ISSR (0.5)
• Hambledon (5m separation- 0.4)
• Culham (1m separation- 1.5)
• Wilkinson et al (2000)

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How often do populations coincide?
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(Wilkinson et al., 2000)
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Sympatry Likelihood Map
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Total hybrid number per annum

• Weed and wild hybrid numbers are now being
  calculated with error estimates for the entire UK

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Conclusions

• The number and distribution of hybrids determine
  the scale, speed and possibility of any
  subsequent ecological change
• Hybrid numbers in a country determines the
  feasibility of corrective measures
• Error estimates will be fairly large initially
  but can be improved
• Hybrid frequency estimates should be followed by
  measures of other parts of the pathway to change

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Thoughts

• Quantification of each stage in the pathway to
  change is possible but requires consideration of
  location, context and biology of both crop and
  recipient
• Quantification requires effort to integrate data
  from different disciplines
• Information generated is generic for the crop in
  the country concerned