Gene flow between genetically modified crops and wild relatives

One of the most controversial aspects of farming in recent years has been the actual or proposed introduction of genetically modified (GM) crops. Genetically modified crops have been altered using molecular biology techniques that allow scientists to isolate genes from one species and then incorporate these into the genome of a second species. Plants are genetically modified for a number of reasons, such as a desire to confer resistance to insect pests or herbicides such as glyphosate, to increase yield, or to make plants more resistant to harsh environmental conditions such as frost. One reason why genetically modified crops have been the subject of intensive debate is concern over the possibility that gene flow may lead to hybridization and gene introgression between domestic GM crops and

Table 8.3 Some examples of hybridization between crops and their wild relatives that have been detected using molecular markers. With the exception of sorghum, transgenic varieties of all of these crops have already been developed, although most are not commercially available. Adapted from Stewart, Halfhill and Warwick (2003) and references therein

Crop-weed

Table 8.3 Some examples of hybridization between crops and their wild relatives that have been detected using molecular markers. With the exception of sorghum, transgenic varieties of all of these crops have already been developed, although most are not commercially available. Adapted from Stewart, Halfhill and Warwick (2003) and references therein

Crop-weed

Crop

Main species

Main wild relatives

hybridization risk

Barley

Hordeum vulgare

H. spontaneum

Low

Sunflower

Helianthus annuus

H. annuus, H. petiolaris

Moderate

Canola

Brassica napus

B. rapa

Moderate

Wheat

Triticum aestivum

Aegilops cylindrica

Moderate

Sugar beet

Beta vulgaris vulgaris

B. v. maritime

Moderate

Alfalfa

Medicago sativa sativa

M. s. sativa, M. s. falcate

Moderate

Sorghum

S. bicolor

S. halepense, S. almum,

High

S. propinquum

their wild relatives. Some people believe that hybridization between GM crops and weeds could lead to aggressive 'superweeds' with an elevated fitness that would allow them to outcompete other wild plant species and so alter the make-up of communities and ecosystems.

Twelve of the thirteen most important crops worldwide often grow near wild plants to which they are closely related and therefore have the potential to hybridize with. By assessing levels of gene flow and hybridization, biologists have been able to identify which GM crops they believe are most likely to hybridize with wild relatives, and this has led to a classification of very low, low, moderate and high-risk crops (Stewart et al., 2003). This approach may be particularly important for identifying high-risk crops for which experimental field trials could be inappropriate. Some examples of crop-weed hybrids that have been identified by molecular markers such as microsatellites, AFLPs, and RAPD are shown in Table 8.3.

Gene flow and hybridization have been particularly well studied between cultivated beets (Beta vulgaris ssp. vulgaris) and their wild relative the sea beet (B. v. maritima). In this case there may be a high potential for gene flow from crops to wild populations because sugar beets are outcrossing, wind pollinated, genetically compatible with sea beets, and are often grown in proximate locations. An early study that used allozyme and RFLP data to quantify patterns of gene flow among wild beet populations found that the genetic structure of wild beets was patchy and showed a pattern that could be explained by founder effects within a metapopulation structure. Under this scenario, long-distance seed dispersal may be an important mechanism of gene flow between beet populations (Raybould et al., 1996). A slightly later study that investigated the possibility of gene flow between wild and cultivated beet populations found that alleles typical of the cultivated beets occurred at high frequencies in adjacent wild beet populations but were rare in geographically distant wild beets. This finding suggested that gene flow between cultivated and wild beets is relatively high when they are separated by short distances (Bartsch et al., 1999).

The finding of crop--weed hybridization across small geographical distances did not preclude the possibility that rare long-distance dispersal via seeds -- in a manner compatible with the metapopulation structure that was found in wild beets -- was leading to occasional hybridization between cultivated and wild plants over relatively broad spatial scales. The possibility of seed-mediated gene flow from domestic to wild populations was investigated in a later study that used a chloroplast marker plus nuclear microsatellites to quantify gene flow between cultivated beets and a neighbouring coastal wild population in France (Arnoud et al., 2003). Because chloroplasts are inherited maternally in beets, this combination of nuclear and chloroplast data allowed the researchers to compare pollenmediated gene flow with seed-mediated gene flow, and they concluded that the latter was the most important mechanism for the movement of genes from beet crops into wild populations.

This was an important finding because studies on gene flow between crops and wild populations have typically concentrated on pollen-mediated gene flow. The occurrence of seed-mediated gene flow means that the potential effects of long-lived seed banks on the dispersal of transgenic organisms need to be incorporated into future studies, as does the possibility of long-distance transportation of seeds by humans and other animals (Arnaud et al., 2003). Although estimates of gene flow and the likelihood of hybridization events cannot give us precise answers about the spread of transgenic crops, they do provide us with some valuable baseline data that can be added to more detailed models that may help us to predict the probability that GM crops will escape into the wild.

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