How does GMO plant pollination affect biodiversity?

Expert response from Peter J. Davies

Professor of Plant Physiology and International Professor of Plant Biology, Cornell University, Ithaca New York, USA

Friday, 31/07/2015 16:05

To address this multifaceted question we need to start with a consideration of types of pollination:

 

•          Open pollination is when a flower is pollinated by any other flower of the same species.  If there are separate male and female flowers all pollination is, by definition, open.  At the extreme, plants may be self-incompatible, such that if pollen is from another genetically-identical plant the pollen does not germinate, and only pollen of a different genetic constitution can fertilize the egg cell to produce the seed.
•          Closed or self-pollination is when the pollen comes from the same flower.  Sometimes this occurs even before the flower opens, a process termed cleistogamy.  Self-pollinated plants breed true (they retain their characteristics), and do not have the same genetic or generational variety as openly pollinated plants.  For example canola (rapeseed) is predominantly (approx. 70%) self-pollinated. Some soybean cultivars are entirely cleistogamous.  Pollen dispersal by insects or wind in these crops appears to be limited to very short distances, which is important not only for pollination but also with respect to the movement of genes between genetically modified (GM) and conventional crops.
•          Hybrid pollination is controlled pollination by plant breeders between two defined lines of plants in an effort to select certain characteristics and produce superior progeny.  The parental lines are sometimes inbred lines that then result in hybrid-vigor in the progeny, so that the progeny is bigger, stronger and more productive than either parent.  Many of the fruits and vegetables that appear in supermarkets are hybrid varieties; almost all corn (maize) produced in the USA and other developed countries is hybrid from two inbred parents, resulting in much higher yields than can be produced from non-hybrid varieties.

Heirloom plants may be open-pollinated or self-pollinated (but not all open-pollinated plants are heirlooms).  According to the Seed Savers Exchange the term “heirloom” signifies “varieties that have a history of being grown and shared within a family or community”.  An old variety, including long-standing heirloom varieties, are referred to as Heritage varieties.

 

An heirloom maize variety would be a population that is heterogeneous, as otherwise it would suffer from inbreeding depression, so a large number of individuals would need to be crossed at every generation.  It would also need to undergo selection at every generation to prevent genetic drift.  An heirloom pea variety would be a population that is either nearly clonal or a mix of clones. Because peas are cleistogamous, they self-pollinate to a great extent, no matter the diversity of their neighbors. The diversity would be dependent on the number of individuals.  Heirloom tomatoes are typically grown by saving the open pollinated seeds of one generation of tomatoes, to be planted in the next growing season.  As they rely on nature they evolve and change season after season, though usually retaining one or more characteristics for which they are selected.  Thus the population of plants is not genetically uniform, but is morphologically similar for the traits of importance to the grower that are perpetuated by random mating.

 

Preventing Cross-Pollination

 

For open-pollinated crops you always have to isolate one cultivar from another when saving seed otherwise you'll just get mixes.  Preventing cross-pollination between these varieties can be difficult.  Corn, being wind-pollinated, is notably promiscuous. There are a few ways to prevent cross-pollination among corn varieties, the most basic of which is to grow only one variety each year. If growing more than one variety one can choose varieties that vary dramatically in time-to-maturity, though this does not account for what a neighbor is growing.  Most control over seed purity is hand-pollination.  In the case of corn this involves putting bags over the ears before the silks emerge. Pollen is then collected from bagged tassels and used to hand-pollinate the silks on each ear.  The other method for preventing cross-pollination is to utilize distance to maintain purity by growing each variety.  Corn pollen is quite heavy and most pollination is within a short distance of the tassels.  A distance of two hundred yards is standard for preventing cross pollination of corn, but at closer distances relative genetic isolation can be achieved by eliminating border rows during seed collection of the variety to be saved, as the central field will have been flooded with the desired varietal pollen which will dilute any adventitious pollen.  Either method will ensure not more than 0.5% cross contamination, and probably far less with greater distances or border rows combined.

 

On papaya

 

As the questioner cited papaya we should take a look at papaya.  Papaya plants may be male, female or hermaphrodite.  Male plants do not produce fruit and are eliminated by the grower.  Female plants are also frequently eliminated as being less productive because they have to receive pollen from another plant.  Growers therefore prefer hermaphroditic plants in which self-pollination most frequently occurs, so giving a higher yield, and enabling the plants to breed true.

 

GMO papaya was developed to be resistant to papaya ringspot virus, against which there is no other defense, either genetic or chemical.  The GMO papaya has within it the gene for the coat-protein of the virus.  This forms an immune-type defense mechanism (technically known as RNAi): it never produces the virus coat-protein it just prevents the virus from doing so, and in this way the papaya plant becomes virus resistant.  This alarms some people, but if you have ever eaten a non-GMO papaya you have probably consumed more ringspot virus protein and DNA than ever would be the case in GMO papaya, which is virus free!

 

Will GMO plants take over?

 

Just being genetically engineered does not make a plant any more likely to become an invasive or a persistent weed.  A huge decade-long study by a team of British scientists found that genetically modified potatoes, beets, corn and oilseed rape (canola) planted in natural habitats were as feeble at spreading and persisting in the wild as their traditional counterparts.  One might expect that insect resistant plants would have greater fitness and dominate whenever cross-pollination occurred.  However this has not proved to be the case when Bt canola was in proximity with wild Brassica rapa with which the cultivated crop can easily hybridize.

 

On Organic Rules

 

It is interesting to note that the above information on preventing cross pollination comes from a web site entitled “preventing GMO contamination” and yet the methods described apply to general techniques of maintaining heirloom varietal purity.  So why the emphasis on GMO?  Indeed although GMOs are excluded from the definition of organic produce this was not always the case as the initial organic rules did not require such an exclusion.  However in 2000 the U.S. Secretary of Agriculture announced that due to the large number of submitted comments, mainly as form letters, the proposed rules would specifically exclude genetically engineered seeds.  The Secretary did so despite his finding that “there is no current scientific evidence that [genetic engineering] presents unacceptable risks to human health or the environment.”  Thus the exclusion of genetically engineered crops from organic agriculture by the USDA rests not on any scientific grounds, but solely on opinions and politics.  I consider this regrettable as the aims of both organic growers and plant genetic engineers is to produce healthier food through the decreased use of pesticides, achieved in the case of genetic engineering by the incorporation of insect resistance (e.g., in corn) or disease resistance, as in the case of papaya.

 

Biodiversity

 

It is quite natural to think of GMO crops as being one monolithic entity for each crop.  However even the big seed companies sell many varieties adapted to local conditions.  Pioneer Hybrid Seed Company claims that the diversity they offer exceeds the diversity available prior to the adoption of hybrid corn in the 1930’s.  But an even better example is in India where farmers have bred local varieties for local conditions and preferences.  According to a recent analysis, GMO cotton varietal diversity in India, where over 90 percent is transgenic, is now at the same level than it was before the adoption of transgenic insect resistance.  In addition there is not just one gene that gives, for example, insect resistance; there are many different forms of just Bt and plant genetic engineers and breeders are continually producing and releasing improved varieties with genetically-engineered characteristics.

 

Conclusion

 

So will all crops be GMO in the future?  No more than any crop now possesses only any one single characteristic.  Genetic diversity will be maintained, and the more GMO crops are adopted the greater the genetic diversity of these crops.  However would you not prefer to have wholesome food crops without the need to spray insecticides, or in some cases, the fruit itself as there is often no other defense against the disease?

I thank Professor Michael Mazourek for explaining some of the intricacies of pollination of heirloom varieties, and Professor Dennis Gonsalves for information about papaya.

Expert response from Peter J. Davies

Professor of Plant Physiology and International Professor of Plant Biology, Cornell University, Ithaca New York, USA

Friday, 31/07/2015 16:05

To address this multifaceted question we need to start with a consideration of types of pollination:

 

•          Open pollination is when a flower is pollinated by any other flower of the same species.  If there are separate male and female flowers all pollination is, by definition, open.  At the extreme, plants may be self-incompatible, such that if pollen is from another genetically-identical plant the pollen does not germinate, and only pollen of a different genetic constitution can fertilize the egg cell to produce the seed.
•          Closed or self-pollination is when the pollen comes from the same flower.  Sometimes this occurs even before the flower opens, a process termed cleistogamy.  Self-pollinated plants breed true (they retain their characteristics), and do not have the same genetic or generational variety as openly pollinated plants.  For example canola (rapeseed) is predominantly (approx. 70%) self-pollinated. Some soybean cultivars are entirely cleistogamous.  Pollen dispersal by insects or wind in these crops appears to be limited to very short distances, which is important not only for pollination but also with respect to the movement of genes between genetically modified (GM) and conventional crops.
•          Hybrid pollination is controlled pollination by plant breeders between two defined lines of plants in an effort to select certain characteristics and produce superior progeny.  The parental lines are sometimes inbred lines that then result in hybrid-vigor in the progeny, so that the progeny is bigger, stronger and more productive than either parent.  Many of the fruits and vegetables that appear in supermarkets are hybrid varieties; almost all corn (maize) produced in the USA and other developed countries is hybrid from two inbred parents, resulting in much higher yields than can be produced from non-hybrid varieties.

Heirloom plants may be open-pollinated or self-pollinated (but not all open-pollinated plants are heirlooms).  According to the Seed Savers Exchange the term “heirloom” signifies “varieties that have a history of being grown and shared within a family or community”.  An old variety, including long-standing heirloom varieties, are referred to as Heritage varieties.

 

An heirloom maize variety would be a population that is heterogeneous, as otherwise it would suffer from inbreeding depression, so a large number of individuals would need to be crossed at every generation.  It would also need to undergo selection at every generation to prevent genetic drift.  An heirloom pea variety would be a population that is either nearly clonal or a mix of clones. Because peas are cleistogamous, they self-pollinate to a great extent, no matter the diversity of their neighbors. The diversity would be dependent on the number of individuals.  Heirloom tomatoes are typically grown by saving the open pollinated seeds of one generation of tomatoes, to be planted in the next growing season.  As they rely on nature they evolve and change season after season, though usually retaining one or more characteristics for which they are selected.  Thus the population of plants is not genetically uniform, but is morphologically similar for the traits of importance to the grower that are perpetuated by random mating.

 

Preventing Cross-Pollination

 

For open-pollinated crops you always have to isolate one cultivar from another when saving seed otherwise you'll just get mixes.  Preventing cross-pollination between these varieties can be difficult.  Corn, being wind-pollinated, is notably promiscuous. There are a few ways to prevent cross-pollination among corn varieties, the most basic of which is to grow only one variety each year. If growing more than one variety one can choose varieties that vary dramatically in time-to-maturity, though this does not account for what a neighbor is growing.  Most control over seed purity is hand-pollination.  In the case of corn this involves putting bags over the ears before the silks emerge. Pollen is then collected from bagged tassels and used to hand-pollinate the silks on each ear.  The other method for preventing cross-pollination is to utilize distance to maintain purity by growing each variety.  Corn pollen is quite heavy and most pollination is within a short distance of the tassels.  A distance of two hundred yards is standard for preventing cross pollination of corn, but at closer distances relative genetic isolation can be achieved by eliminating border rows during seed collection of the variety to be saved, as the central field will have been flooded with the desired varietal pollen which will dilute any adventitious pollen.  Either method will ensure not more than 0.5% cross contamination, and probably far less with greater distances or border rows combined.

 

On papaya

 

As the questioner cited papaya we should take a look at papaya.  Papaya plants may be male, female or hermaphrodite.  Male plants do not produce fruit and are eliminated by the grower.  Female plants are also frequently eliminated as being less productive because they have to receive pollen from another plant.  Growers therefore prefer hermaphroditic plants in which self-pollination most frequently occurs, so giving a higher yield, and enabling the plants to breed true.

 

GMO papaya was developed to be resistant to papaya ringspot virus, against which there is no other defense, either genetic or chemical.  The GMO papaya has within it the gene for the coat-protein of the virus.  This forms an immune-type defense mechanism (technically known as RNAi): it never produces the virus coat-protein it just prevents the virus from doing so, and in this way the papaya plant becomes virus resistant.  This alarms some people, but if you have ever eaten a non-GMO papaya you have probably consumed more ringspot virus protein and DNA than ever would be the case in GMO papaya, which is virus free!

 

Will GMO plants take over?

 

Just being genetically engineered does not make a plant any more likely to become an invasive or a persistent weed.  A huge decade-long study by a team of British scientists found that genetically modified potatoes, beets, corn and oilseed rape (canola) planted in natural habitats were as feeble at spreading and persisting in the wild as their traditional counterparts.  One might expect that insect resistant plants would have greater fitness and dominate whenever cross-pollination occurred.  However this has not proved to be the case when Bt canola was in proximity with wild Brassica rapa with which the cultivated crop can easily hybridize.

 

On Organic Rules

 

It is interesting to note that the above information on preventing cross pollination comes from a web site entitled “preventing GMO contamination” and yet the methods described apply to general techniques of maintaining heirloom varietal purity.  So why the emphasis on GMO?  Indeed although GMOs are excluded from the definition of organic produce this was not always the case as the initial organic rules did not require such an exclusion.  However in 2000 the U.S. Secretary of Agriculture announced that due to the large number of submitted comments, mainly as form letters, the proposed rules would specifically exclude genetically engineered seeds.  The Secretary did so despite his finding that “there is no current scientific evidence that [genetic engineering] presents unacceptable risks to human health or the environment.”  Thus the exclusion of genetically engineered crops from organic agriculture by the USDA rests not on any scientific grounds, but solely on opinions and politics.  I consider this regrettable as the aims of both organic growers and plant genetic engineers is to produce healthier food through the decreased use of pesticides, achieved in the case of genetic engineering by the incorporation of insect resistance (e.g., in corn) or disease resistance, as in the case of papaya.

 

Biodiversity

 

It is quite natural to think of GMO crops as being one monolithic entity for each crop.  However even the big seed companies sell many varieties adapted to local conditions.  Pioneer Hybrid Seed Company claims that the diversity they offer exceeds the diversity available prior to the adoption of hybrid corn in the 1930’s.  But an even better example is in India where farmers have bred local varieties for local conditions and preferences.  According to a recent analysis, GMO cotton varietal diversity in India, where over 90 percent is transgenic, is now at the same level than it was before the adoption of transgenic insect resistance.  In addition there is not just one gene that gives, for example, insect resistance; there are many different forms of just Bt and plant genetic engineers and breeders are continually producing and releasing improved varieties with genetically-engineered characteristics.

 

Conclusion

 

So will all crops be GMO in the future?  No more than any crop now possesses only any one single characteristic.  Genetic diversity will be maintained, and the more GMO crops are adopted the greater the genetic diversity of these crops.  However would you not prefer to have wholesome food crops without the need to spray insecticides, or in some cases, the fruit itself as there is often no other defense against the disease?

I thank Professor Michael Mazourek for explaining some of the intricacies of pollination of heirloom varieties, and Professor Dennis Gonsalves for information about papaya.