detecting gene flow from gm crops to wild relatives mike wilkinson
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Detecting gene flow from GM crops to wild relatives
Mike Wilkinson
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Crop Species Interspecific hybrids in field
Wheat Triticum aestivum,T.turgidum
+
Rice Oryza sativa,O. glaberrima
+
Maize Zea mays +
Soybean Glycine max +
Barley Hordeum vulgare + (m)
Cotton Gossypium hirsutumG. barbadense
+
Sorgham Sorgham bicolour +
Millet Eleusine coracana,Pennisetum glaucum
+ (m)
Beans Phaseolus vulgaris +
Oilseed rape/canola Brassica napus,B. rapa
+
Ground nut Arachis hypogaea -
Sunflower Helianthus annuus +
Sugar cane Saccharum officinarum +
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
1. Initial Hybridization2. Introgression3. Gene flow between populations4. Changed fitness leading to change in population
size, density or distribution5. Effects on other organisms
1. Bilateral interactions(e.g. bitrophic)2. Trilateral interactions (Tritrophic)
6. 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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GM crop
A specialist parasitoid
Example Impact
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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 formation1. Isolation distance2. Male sterility3. Integration site4. Chloroplast transformation5. Inducible promotors/ ‘terminator technology’6. 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?
1. Identify recipients
2. Quantify ‘local’ gene flow rates
3. Estimate long-range gene flow
4. 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 hybridsAverage 60%
Surely lots of hybrids will be everywhere
Jorgensen and Andersen (1994)
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No, because
1. B. rapa is an infrequent weed
2. Crop rotation and weed control
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In Yorkshire and the Humber region37.4% of OSR fields contain weedy rapa
In all other regions0.54% of OSR fields contain weedy rapa
Weedy B.rapa
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Hybrid survival
Hybrid seeds show <10% 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