GENE FLOW FROM CULTIVATED RICE: ECOLOGICAL CONSEQUENCES
Undoubtedly, biotechnology and GM crops will provide new opportunities for global food security and development in life sciences. However, the uses of GM crops have also aroused tremendous concerns about their biosafety world wide. The potential ecological risks associated with transgene escape through gene flow are the foremost among these concerns4.
When alien transgenes escape to and express in weedy or wild relatives of GM rice, transgenes may persist and disseminate within the weedy or wild populations through sexual reproduction and/or vegetative propagation. Transgenes that are responsible for resistance to biotic and abiotic stresses (such as disease and insect resistance, drought and salt tolerance, and herbicide resistance) can significantly enhance the ecological fitness of weedy and wild populations. The escape of these transgenes may cause ecological problems, for instance, by producing aggressive weeds, the spread of which might result in unpredictable consequences to local ecosystems. On the other hand, when transgenes escape to and persist in wild rice populations, the rapid dissemination of transgenic hybrid individuals (or progeny) might change the original wild rice populations. In some cases, the aggressive spreading of hybrid swarms with better ecological fitness could even lead to the extinction of endangered wild species populations locally5. Therefore, knowledge of the likelihood of gene flow from rice to its weedy and wild relatives will help to predict the magnitude of the potential ecological consequences caused by transgene escape. This knowledge will also facilitate the effective management and safe use of the transgenic crops.
Cultivated rice and its weedy and wild relatives
As close relatives of cultivated rice, some wild rice species such as O. rufipogon, O. nivara, O. longistaminata, and O. glumaepatula are commonly found or coexist in rice farming systems of many Asian, African, and American countries. The weedy rice (also referred to as red rice, Oryza spontanea) is frequently observed in rice fields as an accompanying weed, particularly in the rice fields with direct-seeding cultivation practices. These AA-genome weedy and wild relatives are highly compatible sexually with cultivated rice. Their interspecific F1 hybrids could form complete chromosome pairing in meiosis and have relatively high pollen and seed fertility to produce viable offspring. Thus, studying gene flow from rice to its weedy and wild relatives becomes an important component for the potential ecological risk assessment of GM rice, because gene flow is the primary step from which potential ecological consequences of transgene escape may follow.
Gene flow from cultivated rice to its weedy and
Gene flow from transgenic rice to weedy rice was measured using a transgenic rice variety (Nam29/TR18, as a pollen donor) with herbicide resistance (bar) and 13 accessions of weedy rice collected from Asia and America. The experimental plot was designed as complete random blocks where Nam29/TR18 was planted and mixed with one of the 13 weedy rice accessions in each block, respectively (Fig. 1). Each block consisted of eight weedy rice plants. For identification of hybrids between Nam29/TR18 and weedy rice, seedlings generated from different weedy rice plants were sprayed with herbicide Basta at the 34-leaf stage. The surviving seedlings with resistance to herbicide Basta were considered hybrids and were subject to PCR detection of the herbicide resistance bar gene to confirm their hybridity. Gene flow frequencies were estimated by calculating the number of hybrids against the total number of seedlings germinated. The average frequencies of weedy rice seedlings with herbicide resistance were very low and varied among different blocks, but with no significant differences among the replications. The experimental results indicated that the detectable rate of herbicide resistance gene flow from the transgenic rice to weedy rice plants varied between 0.011~0.046%.
Gene flow from cultivated rice to perennial common wild rice was measured using the Minghui-63 rice variety, and wild rice O. rufipogon (as pollen recipient) was planted in different models to allow outcrossing to occur naturally. Co-dominant simple sequence repeats (SSRs) were used as molecular markers for accurate identification of hybrids between cultivated rice and O. rufipogon. The selected SSR primer pair amplified polymorphic alleles from the two species, which were easily distinguishable with electrophoresis in agarose gels. O. rufipogon presented a consistent fast-migrating allele (F) and Minghui-63 a slow-migrating allele (S) in the gels. The hybrids between the two species displayed stable heterozygous (FS) alleles. Leaf samples of germinated seeds from O. rufipogon populations were collected from individual seedlings for SSR examination. Gene flow frequencies were estimated by calculating the number of seedlings with the FS heterozygote SSR pattern against the total number of seedlings examined. As a result, the frequencies of detected interspecific hybrids varied from 1.21~2.94% in different planting models. Gene flow frequency from cultivated rice to O. rufipogon was therefore expectedly high, up to ca. 3%, although humidity and wind strength and direction significantly affected the rate of gene flow.
Potential consequences of transgene escape from GM rice to its weedy and wild relatives
The detected gene flow frequencies from GM rice line Nam29/TR18 to various weedy rice accessions were very low, ranging from 0.011~0.046% in one generation, when a weedy rice strain occurred simultaneously in a rice field. However, the gene flow frequency from cultivated to weedy rice in large populations might be more significant than the data observed in this experiment. Actually, rice cultivars cross easily with their related weedy forms (red rice) found in direct-seeded paddy fields and produce viable and fertile hybrids with a reasonable rate. In addition, when weedy rice consistently occurs simultaneously with a cultivated rice variety in the same field, the number of hybrids resulting from gene flow could accumulate and increase through generations. If GM rice varieties are released to environments where weedy rice occurs abundantly, the transferred alien genes could spread and accumulate in weedy populations. This may pose a severe problem for weedy rice control and management in rice production. Therefore, release of transgenic rice with genes that can significantly increase weediness and can resist weed control measures is not recommended in regions where weedy rice is already a serious weed problem.References
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