Gene flow informs coexistence2. Enhanced understanding of gene flow informs practical strategies for coexistence in maize, oilseed rape and sugar beet.
Similar biological mechanisms govern the life histories of all crop plants. They produce structures that survive over time and disperse over space and by these means have the potential to transfer genes from one crop to another. Seed-borne genetic impurities can arise by several routes: from plants already present in the field as volunteers (weeds of the same species as the crop) and wild relatives; by seed brought to the field in the sown seed or on farm machinery; and by seed dispersed from feral plants or wild relatives growing around the the field. Volunteers or wild relatives growing in the same field can contribute their own seed to the harvest. Pollen-borne genetic impurities can arrive from another crop and from volunteers, ferals and wild relatives. The seed-borne genetic impurities can arise at any time of the year and from crops grown in the past, while pollen transmission occurs during the relatively short period that both receptor and donor plants are in flower.
Maize
Experiments relevant to coexistence of maize in Europe were almost exclusively on cross pollination between crops, since admixture through seeds and pollen from volunteers was thought to be low and relevant only in warmer regions. Maize has no wild relatives and few feral plants are found in Europe. However maize landraces are maintained in some regions, so special consideration was given to the cumulative outcrossing which may occur between modern varieties and landraces.
Cross pollination has been examined in great detail in several European countries, either using GM crops as a donor or using markers such as yellow and white grain colour. The database allows comparisons across scales, from small plots to full sized commercial fields, and in several contrasting climates in Germany, Spain, Switzerland and the UK. The studies are consistent and indicate a steep decline in cross pollination over three orders of magnitude (a 1000-fold) with distances to 100 m from the source of pollen, and an effect on percentage pollination of wind direction and related meterological factors. Cross pollination declined with distance in a similar manner in both experimental plots and full-sized fields. At 100 m from the donor, cross pollination was below 0.1% in most circumstances. Where donor and receptor fields were well dispersed in a landscape, and at a generally low overall density, the average cross pollination was typically 0.01% between 100 m and several kilometers. Where donor and receptor fields were grown close together in similar proportions (as in Spain, a region where commercial crops are grown without coexistence measures), cross pollination rates above 0.9% were sometimes found in situations where non-GM fields were completely surrounded by GM fields and both types flowered at the same time.
In summary, the potential for adventitious presence of GM material in non-GM maize production is:
- moderate for cross pollination between fields, and can be managed through separation, discards or buffers where crops are in close proximity;
- low through volunteers, and this is mainly in southern Europe;
- low for introgression to landraces from modern crop varieties;
- zero through wild relatives as none exist in Europe.
Over most of Europe, therefore, the biology, environment and agronomy of maize have been well characterised, so that coexistence (defined as complying with the official threshold) for hybrid varieties should be achievable through the use of high purity seed, the management of cross pollination by using varieties that flower at different times and/or spatially separating fields, or the installation of buffer zones or the practice of discarding where fields are in close proximity. However, a zero level of adventitious presence cannot be achieved or measured in practice. Volunteer maize still needs to be investigated thoroughly in climates where it occurs.
Oilseed rape
Genetic impurities in oilseed rape can arise from a wider range of sources than in maize. Pollen is dispersed by wind, hive bees, bumble bees and a variety of other insects. Transfer by seed following seed drop at harvest can be very high, as large seedbanks can form which survive for several years producing volunteers in subsequent crops. Also seed is transported on farm machinery, from which the small seeds are difficult to remove under normal agricultural conditions. Feral plants are widespread along waysides and margins, while wild relatives, notably Brassica rapa (the wild turnip), occur locally and cross pollination with crops, volunteers and ferals.
In total, results from over 50 field-experiments on oilseed rape from the Czech Republic, Denmark, France, Germany, Poland and the UK were re-examined in SIGMEA. Results on cross-pollination differed according to experimental designs, regions, cultivars and climates, but a dispersal function with a ‘fat’ tail (power-law) appears to be the most appropriate currently available to predict pollen movement at any scale. Over distances of tens of metres, cross pollination showed a similar decline to that in maize, and was typically less than 0.1% at 100 m from the edge of the donor; but crossing between commercial fields was sometimes as high as 0.1% even at distances between 100 m and 1000 m. The contribution of volunteers to admixture of harvested seed may range from <0.01% to more than 10% for the same crop variety in different management, soil and climatic conditions A range of agronomic practices can be deployed to limit transfer through seed banks such as delaying soil cultivation after harvest to allow germination and destruction of seedlings, increasing the interval between crops and stale-seedbed techniques. Comparison of feral oilseed rape in more than 20 growing seasons across 5 study areas enabled the definitive statement that, though widespread and sometimes persisting in the same place over several years, ferals are a negligible fraction of the total flowering oilseed rape in a region and contribute little to admixture in crops. The abundance of wild relatives differs between regions, and while their progeny may be fertile and as ecologically fit as the parents, they do not constitute a major route for transmission of traits between to crops.
In summary, the potential for adventitious presence of GM material in non-GM oilseed rape production is:
- moderate for cross pollination between fields, which can be managed through spatial separation and use of buffer or discard zones where crops are in close proximity;
- high through seedbanks resulting in volunteer populations that admix with and pollinate non-GM crops - volunteers are ubiquitous, mobile and commonly in high abundance and are of maximum importance to coexistence over time (when non-GM OSR is to be grown after a GM OSR in the same field);
- moderate through wild relatives in those localised areas of Europe where they occur in high abundance in the fields (e.g., B. rapa in Denmark)
- low through ferals (with some local exceptions) because of their low overall density compared to crops and volunteers in the landscape.
Problems of coexistence during the first few years of commercialisation can be reduced by management of cross pollination through separation and seed purity. However uncertainties remain over whether the cumulative movement and amplification of volunteers can be managed so as to achieve coexistence in of GM and non-GM oilseed rape in the longer term.
Beet
Crop varieties, in-field volunteers, ferals and wild types of beet are all sexually compatible variants of the species, Beta vulgaris, and together comprise the Beta complex. Crop beet plants are biennial, producing root bulk in the first season (after which they are usually harvested) and flowers in the second. By contrast most wild and weed beet forms are annual, producing flowers in the year they germinate. Flowers produce small wind-borne pollen that can disperse over large distances. The main source of genetic impurity in commercial crops arises from seed produced in localised areas of Italy and France in fields consisting of male fertile pollinators and male sterile seed mother plants. The male sterile mother plants can also receive pollen from volunteers, ferals and wild sea beet in the surrounding countryside and from other seed production fields in the area. The wild and weedy forms introduce annual genes into the seed crop, which give rise to annual plants that flower in the first year of the crop but produce little or no root and sugar yield. If allowed to set seed, these annual weedy beets give rise to seedbanks lasting many years, from which annual volunteers (bolters) will flower.
Annual traits, whether GM or otherwise, have the potential to spread in commercial production areas, but as indicated above, annual plants rarely give rise to tubers and so contribute little to adventitious presence in sugar beet. Their main importance is as weed. If herbicide tolerant (HT) beets are grown, HT weed beets will arise and pollinate non-GM weed beets and in this way introduce HT genes into non-GM fields. Since this does not translate in adventitious presence of GM in the final crop (roots), and therefore is not a coexistence issue sensu stricto, it could create weed management problems. For example, if the HT trait conferred tolerance to glyphosate, this same herbicide would become less effective for weed beet control in the non-GM beet crops.
SIGMEA drew together current and recent research on the Beta complex. Compared to maize and oilseed rape, there is little data on the form of the decline in cross pollination with distance, though in the studies examined pollen was found to move over at least several hundred metres. The work on beet in SIGMEA concentrated on weed and wild beet. Unlike in the other two species examined, the wild form, sea beet, is an important genetic resource within the Beta complex, and is used as a source of genetic traits by plant breeders. Genetic assessment of plants growing along both the Baltic and Adriatic coasts, confirmed populations remain highly diverse and distinct from crop varieties. Nevertheless, areas were identified where the crop, volunteer, feral and wild beets exist in proximity and exchange genetic material through movement of seed and pollen. It is considered essential to preserve the diversity of sea beet for any long term, plant breeding strategy, and for conservation and study in its own right.
In summary, the potential for adventitious presence of GM material in non-GM sugar beet production is:
- low through cross pollination between sugar beet crops since the harvest is vegetative,
- low through volunteer (weed beet) populations which arise from impurities in sown seed, since best management should minimise any harvest contamination with roots of these weed beets;
- low though cross pollination from feral plants and wild beet for the reasons given for volunteers.
The main source of adventitious presence is therefore through the seed sown to grow crops of sugar beet. Coexistence should still be achievable by best management of seed production crops, and by strategic siting and separation of seed production fields. Specifically, GM seed production crops need to be sufficiently separated from non-GM crops and from wild and weedy beet (which in time would contain GM individuals) both to keep the non-GM seed pure and to reduce the spread of transgenes into wild, weedy and feral populations. Separate areas or regions for GM and non-GM seed production may be required.
Wheat and rice
The knowledge-base for wheat and rice in Europe is much less than for the other crops, but tentative conclusions are that the potential for adventitious presence should be:
- low through cross pollination between crops,
- probably low in rice (to moderate in wheat) through volunteers, but their contribution needs to be clarified under European conditions;
- low in wheat through wild relatives, and low to moderate in rice through the red rice weed, in those areas where it occurs, provided agricultural practices to control this weed are applied.
Further research is needed on cross pollination and the life cycle of these species and their wild relatives in Europe.
General
In summary, the general conclusion drawn from gene flow studies of maize, oilseed rape and beet is that adventitious presence due to cross pollination alone can generally be managed through separation distance and related measures to comply with the official EU regulation. However it should be recognised that a zero level of adventitious presence cannot be achieved or measured in practice.
Oilseed rape (OSR) was identified by SIGMEA as having major problems in the management of coexistence. The problems arise principally because OSR seeds survive for several years in soil and give rise to volunteers that are competitive and difficult to eliminate,. Thus gene movement and persistence in seeds and volunteers is difficult to manage agronomically. Coexistence issues arising from maize volunteers are manageable using good agricultural practice. The problems associated with weed beet are mostly related to seed production which therefore needs careful management on a regional scale.
There remains uncertainty on the relevance to coexistence of transgenes that might confer differential fitness, for example by being associated with reduced pollen production or resistant to common herbicides. Further measurements at previous GM release sites are needed to assess the persistence and genetic structure of relevant populations (e.g. volunteers, wild relatives). State of the art modelling tools (individual based, spatially explicit, incorporating introgression of multiple events) have been developed to simulate the population dynamics around complex transgenic events, and could be adapted as aids to monitoring following commercialisation. 
Writing:
A. Messéan (INRA)
Creation date: 26 May 2009 Update: 28 May 2009 |