Wild Brassicas: a case study 

Mike Wilkinson

The risk assessment process

1. Specify and rank the hazards

2. Quantify generic aspects of exposure

3. Evaluate aspects of exposure specific to one transgene or GM cultivar

The risk assessment process

1. Specify and rank the hazards

2. Quantify generic aspects of exposure

3. Evaluate aspects of exposure specific to one transgene or GM cultivar

Hazards relating to Gene flow from oilseed rape in the United Kingdom

Rank 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

Rank 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

Oilseed rape Bargeman’s cabbage

Brassica napus Brassica rapa

GM crop

GM F1 hybrids in region

Transgene stabilises by introgression

Transgene spreads to most populations

Aim

To create spatially explicit models of hybridization, introgression and transgene spread

Parameterisation needs• F1 hybrid modelling

– Frequency of hybrids– Location of hybrids

• Introgression modelling– Fertility and fitness of hybrids– Past introgression rates for different parts of the

genome

• Transgene spread modelling– Population size and distribution– Demographic baseline data– Gene flow estimates between populations

F1 Hybrid abundance and distribution

1. Local hybrids

2. Long-scale hybrids

Local hybrids

1. Where do oilseed rape& B. rapa grow together?

2. How many hybrids form locally?

The importance of context

Brassica rapa

• Casuals of disturbed land

• A weed of B. napus

• Stable wild populations of river banks

Where is oilseed rape?

Where is oilseed rape?

Where is riverside B. rapa?

• >300km of 8 rivers (Thames, Nene, Avon, Derwent [Yorkshire], Ouse, Soar, The Great Ouse and Trent) and 4 canals surveyed by foot and boat.

• Survey of herbaria: 601 B. rapa, 138 identified on named river

• 82 local Floras• CEH country-wide survey

A. Rapeseed B. Waterside C B. rapa B. rapa within

30m of rapeseed

Measure local hybrid frequency

DNA fingerprintingMeasure DNA content

Determine hybrid frequency at sites of co-occurrence

Oilseed rape

B. rapa

Hybrid

Hybrid

Oilseed rape

B. rapa

Hybrid

On average 1.46% hybrids per site

Calculate long-range hybrid numbers

Calculate long range hybrids from..

• Separation distance between oilseed rape fields and B. rapa

• Pattern of decline in airborne pollen quantity with distance

• Relationship between amount of airborne pollen and hybrid numbers

Result

1. We calculate approximately 32,000 hybrids form annually between oilseed rape and riverside B. rapa in the UK

2. Most occur in eastern and central England

3. Riverside hybrids absent from Scotland and Northern Ireland

Wilkinson et al. (2003) Science 302, 457-459

Key assumption

• Insect pollination follows the same dispersal pattern as wind-mediated pollination or is unimportant

– Unsatisfactory and needs to be resolved

Introgression

• Fitness and fertility of hybrids

• Effect of integration site

Fitness and fertility of hybrids

• Male fitness – Viable pollen output– Total pollen output– Effectiveness as a male parent

• Female fitness– Seed return– Seed viability

Importance of integration site

Select populations where chloroplast capture has occurred (and so introgression has happened previously),

Compare introgression rates of selected C-genome and A-genome marker sets that are specific to B. napus

Spread between populations

• Distribution of populations

• Persistence of visible populations

• Self-incompatibility

• Observed gene flow between contemporary populations

• Seed versus pollen-mediated gene flow

Spread between B. rapa populations

• Population distribution patterns

Originally usedLiterature and surveys to identify rivers carrying

B. rapa

Survey data to establish patterns of distribution

Apply probabilistic approaches to predict B. rapa population distributions

But

• This assumes distributions on tributaries is similar to that on main rivers (where access is possible by footpaths)

• So– This assumption is being tested by remote

sensing using high-resolution aircraft-data

Real and False colour images

B. rapa mask

Population persistence

June 2001

March 2002

Jun-01

Mar-02

River

Fence

Disturb cover in variable populations

New B. rapa seedling germinate where plants are absent

New B. rapa populations germinate in extinct populations

So

• Presence or absence of plants is variable between years

• Populations more perhaps accurately defined by seedbank presence

Gene flow between generations

Time t+1

Seedbank

Germinated seeds

Seedbank

Outgoing seeds

Outgoing Pollen

Seedbank

Incoming Pollen

Germinated seeds

Germinated seeds

Incoming seeds

Incoming Pollen

Time t

Germinated seeds

Seedbank

Outgoing Pollen

Key: Sampling Direct transfer Loss from system Inter-population transfer

Conclusions• Trangene spread comprises three main stages that

need to be considered separately: F1 hybrid production; introgression and secondary spread to other populations

• Each element affecting gene spread needs to be carefully parameterised

• After initial parameterisation, there is often a need to collect additional data to correct for or test assumptions made in initial models

Thanks

• BBSRC

• NERC

• Perry Foundation

Joel Allainguillaume, Michael Shaw,

Jeremy Sweet, Luisa Elliott, Carol Norris,

Tom Harwood, David Mason,

Matt Alexander, Pippa Bell