risk assessment and gene flow
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Risk assessment and Gene flow. Mike Wilkinson United Kingdom. Contents. Current state of GM technology Principles of risk assessment Defining hazards Measuring exposure Conclusions. Current state of GM technology. Most crops can be genetically modified - PowerPoint PPT PresentationTRANSCRIPT
Risk assessment and Gene flow
Mike Wilkinson
United Kingdom
Contents
1. Current state of GM technology2. Principles of risk assessment3. Defining hazards4. Measuring exposure5. Conclusions
Current state of GM technology
• Most crops can be genetically modified
• Four crops dominate (Maize, Cotton, Soya, oilseed rape)
• Herbicide tolerance and insect resistance account for nearly all GM cultivars
• Four countries dominate GM production (USA, China, Argentina, Canada)
Future
• More GM crops will be commercialised
• More countries will approve commercial release
• The trend is towards transgene stacking
• The next wave of GM crops will target stress tolerance (drought and salt)
Implications for risk assessment
As the number of crop-cultivar-gene-gene mixture-location-construct mixes grows,
So does the need for generic information to assess risks
•RISK
•HAZARD
•EXPOSURE
Principles of Risk Assessment
Hazard, exposure & risk
• Hazard– Adverse effect
• Exposure– Frequency/intensity of contact with agent
causing adverse effect
• Risk– Magnitude and likelihood of an adverse effect
A ‘bad thing’
Is it likely the ‘bad thing’ will happen?
a judgement based on ‘how bad’ and ‘how likely’
Risk = f (Hazard, Exposure)
Ecological hazards
That a transgene from a GM crop will move into a wild relative
AND
lead to some form of unwanted ecological change
First generation hybrid
Seed
Seed
cross-fertilise
cross-fertilise
Second generation hybrid
Crop
Wild species
Invasion of new habitats
Replacement of existing genotypes
Changed abundance/mix of herbivores
Out-compete other species in same habitat
Change pollinator mix in community
Changed abundance/mix of predators
Categories of ecological hazard
• Relating to the recipient (Direct hazards)– Increased population size within habitat
– Invasion of new habitat
– Replacement of native genotypes
• Relating to other organisms (Indirect hazards)– Decline in sympatric plant species
– Changed pollinator abundance
– Changed herbivore abundance
– Changed predator/parasitoid abundance
Note:
The ‘end point’ species may not be the wild recipient
Having defined the hazard
What is the exposure?
Exposure pathway concept
In chemical toxicology, exposure is to a toxic chemical is simple and direct
The ecological hazards represent endpoints in a pathway or matrix of linked events
GM crop
F1 hybrid in region
Transgene stabilises by introgression
Transgene spreads to most populations
Enhanced resistance to herbivore depresses herbivore numbers
Depressed herbivore numbers cause extinction of specialist parasitoid of the herbivore
Wilkinson et al 2003. Trends in Plant Science
GM crop
F1 hybrid in region
Transgene stabilises by introgression
Transgene spreads to most populations
GM crop
F1 hybrid in region
Transgene stabilises by introgression
Transgene spreads to most populations
Generic exposure elements (applies to most transgenes)Specific exposure elements (applies to 1-few genes)
Hazard
The risk assessment process
1. Specify and rank the hazards2. Quantify generic aspects of exposure3. Evaluate aspects of exposure specific to
one transgene or GM cultivar4. Assess the risks
Stage 1: Specify and rank hazards
1. Identify cross-compatible recipients
2. Rank crudely according to likelihood (sympatry, ease of hybridization)
3. Specify hazards relating to recipient (direct hazards)
4. Specify hazards relating to sympatric organisms (indirect hazards)
1. Which species are interfertile with wheat?
The tribe Triticeae contains around 330 species in 18 genera and shows ‘an exceptional capacity for intergeneric hybridisation’ Clayton and Renvoize (1986)
Breeders define ‘Gene Pool’ groupings of species related to wheat based on taxonomy and ease of crossing
Gene pool 1: Wild species within Triticum
‘Crossing is easy and hybrids fertile’
Triticum monococcum ssp. aegilopoides
Triticum urartu
Triticum turgidum ssp. dicoccoides
Triticum timopheevii ssp. armeniacum
……..Not found in the USA
Gene Pool 2: Closely related Genera
‘All species that will cross with a crop, although with more difficulty than GP1. Hybrids tend to be sterile’
Comprises of species from the following genera:
Aegilops (22 species)
Amblyopyrum (1 species)
Agropyrum (4 species)
Dasypyrum (2 species)
……Several species introduced into USA
Gene Pool 3: ‘Wide hybrids’
‘Gene transfer is not possible without radical techniques’
Includes species from:
Agropyrum
Elymus (eg couch grass)
Hordeum
…….Natural hybrids not recorded
Evidence of natural hybrids in Gene Pool 2 ?
At least 12 species
Which of these occur in USA?
Aegilops triuncialis
Aegilops geniculata
Aegilops tauschii
Aegilops neglecta
Aegilops cylindrica
Wheat production in the USA: Harvested area per county for 2000
Aegilops triuncialis
Aegilops geniculata
Aegilops tauschii
Aegilops neglecta
(positions approximate)
Species interfertile with wheat found in USA
Ffffff
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Distribution of Aegilops cylindrica in USA: 1993 USDA survey
Other records (positions approximate)
Present
Moderate to dense infestations
Hazards for Aegilops cylindricawhen exposed to GMHT wheat
1. The transgene (herbicide tolerance) will cause A. cylindrica to become a more aggressive weed (Direct Hazard)
2. Changed herbicide use on A. cylindrica will cause decline in a named plant or animal species (Indirect Hazards)
Prioritising hazards
For any crop-wild relative combination, there are many possible and fewer plausible hazards
But comprehensive risk assessment is both expensive and slow
So, we need to prioritise hazards
Possible options for ranking hazards
1. Scarcity of the ‘end point’ species2. Cultural importance (Bald Eagle in the
USA)3. Ecological importance of ‘end point’
species4. Agronomic or medicinal value of ‘end
point’ species5. Cuddliness of ‘end point’ species
Stage 2: quantify exposure
1. Generic exposure elements
2. Specific exposure elements
Quantify generic elements of exposure
1. Quantify hybridization
2. Quantify introgression
3. Quantify gene spread
Quantifying hybrid formation
Important to define1. Context of contact (weed or adjacent wild
population)2. Distribution of crop and recipient species3. Relationship between hybrid frequency
and separation, donor/recipient population size
4. Crop rotation patterns
Cultivated and wild Helianthus annuus, Nebraska USA, in sympatry
1. Context of contact
2. Crop and recipient distribution
??
Sunflower production in the USA: Harvested area per county for 2000
wild Helianthus annuus
Wild Helianthus annus in the USA (data incomplete) – USDA NRCS 2001
This scale is too crude
So use
• Direct surveys
• Literature
• Remote sensing
• Herbarium specimens
3. Importance of separation and population size on hybrid frequency
• Direct measures of gene flow – Seed collections – Hybrid plant screens
• Modelled gene flow– Pollen dispersal models– Seed dispersal models
Direct measures
• Sampling strategy– Representative sample of the field/population
• Need for markers to handle large numbers– Transgene
– Physiological/ phenotypic screen
– Flow cytometry
• Confirmation of hybrid status– Molecular analysis (microsatellites/ locus-specifc PCR)
Modelling gene flow
• Need pollen dispersal curves
• Population sizes
• Life history details
4. Crop rotation patterns
• Farmer’s records
• Remote sensing
Crop rotation in the target area/nation
2. Quantify Introgression
Locus transmission rates in the field will be influenced by drag imposed by crop genes
• Gametic disequilibrium
• Linkage disequilibrium
Possibly countered by fitness advantage of transgene
Measuring introgression
• Map historic introgression of mapped neutral markers to establish position effects
• Note introgression profiles of mapped markers in glasshouse conditions to study effects of chromosome pairing
3. Transgene spread
• Detailed distribution of recipient
• Proportion of populations exposed to gene flow
• Gene exchange rates between populations
• Demography of recipient
Combine to hybridization introgression and gene spread
data to quantify generic elements of exposure
Stage 3: Quantifying specific exposure elements
• Tiered approach– First tier (worst case scenario conditions)– Second tier (more realistic laboratory
experiments)– Third tier (field/ microcosm experiments)
Losey et al. 1999, Nature
Tier 1
Larvae exposed to GM Bt pollen on milkweed leaves died
Tier 2
Sears et al 2001 PNAS 98 (21): 11937-11942
• Expression levels in most GM maize is low in pollen
• Laboratory experiments exposing larvae to pollen concentrations found in the field showed no acute effects on the larvae
Tier 3
Stanley-Horn etal (2001) PNAS 98: 11931-11936
• Monarch larvae exposed to 3 different Bt pollen sources in field conditions
• Effects varied with event and position
• Negligible effects of Bt11 and Mon810 pollen on larvae survivorship feeding 14 to 22 days on milkweeds in fields
Conclusions
• Quantitative risk assessment require 2 processes – Stage 1: Specify and rank hazards
– Stage 2: Quantify exposure
• Exposure comprises a matrix of linked events• Early elements of exposure relate to hybrid
formation and spread and are largely generic• Late elements are transgene-specific and require a
tiered approach to evaluate
Thanks to
Joel Allainguillaume, Luisa Elliott,
David Mason, Rob Treu, Chris Smith