Impacts Of Introgressed Insect-and Herbicide-Resistance Transgenes On Fitness And Potential Evolution Of Rice Wild Relatives

The environmental release of genetically modified organisms and its potential biosafety risks have caused widespread concerns and controversies in the grobal, especially for the transgene escape through gene flow into the wild relatives of cultivated crops and its potential ecological risks. Rice is an important food crop in the world which has launched a large number of pilot studies on transgenic biotechnology. The government of China has approved biosafety certificates for two insect-resistant Bt rice lines in2009, and locally bred rice varieties with Bt transgenes is likely to be widely grown within the next few years. In this context, scientific evaluation of the rice transgenes escape into its wild relatives and its related Environmental Biosafety issues is very urgent.Scientific evaluation of the ecological risks brought by transgene escape on wild relatives of crops requires assessment on three aspects step by step:(1) the frequency of transgene escape,(2) the expression of transgene in wild relatives, and (3) the fitness impact brought by transgene on wild relatives. Given the studies on the first two aspects were already sufficient, the fitness impact brought by transgene on wild relatives and its long-term ecological consequences were lacking and required serious assessment. Noticeably, the fitness effect of transgene is the key to the biosafety risk by transgene escape. The wild relatives of cultivated rice, including weedy rice and common wild rice, were located separately in the cultivation environment with strong artificial selection and wild environment with intense natural selection. Their differences in biological characters and habitat selection may implicate distinct biosafety risks after transgene escape, which could also be used as model system for further biosafety study. In this thesis, we established multiple-generation hybridization system between the transgenic rice (insect-resistant and herbicide-resistant) and its wild relatives including weedy rice and common wild rice, to systematically study the long-term fitness effects of different transgenes on weedy rice and common wild rice populations. Based on above results, we could speculate the potential evolutionary impact of transgene escape on wild relatives as well as its long-term ecological risks, and provide useful references on biosafety assessment for the future commercialization and cultivation of genetically modified rice. The main findings of this thesis were listed as below:(1) Results from the F2and F3crop-weed hybrid progeny demonstrated that the Bt/CpTI transgene can result in lower insect damage and greater seed production when insect pressure was relatively high. The CpTI transgene was also associated with reduced insect damage, but to a much smaller extent than Bt/CpTI, and it was not associated with increased fecundity or survival. Therefore, the benefits of CpTI were minimum compared with those of the Bt transgene. A fitness cost was detected in F3progeny containing Bt/CpTI under conditions where reinforced competition between transgene-positive and transgene-negative was introduced under low insect pressure.(2) Results from the F4-F7crop-weed hybrid progeny of rice demonstrated that the Bt/CpTI transgene was still effective in later-generations while resulted in lower insect damage and increased survival and fecundity under high insect pressure. Furthermore, the increased density of transgenic plants in different cultivation plots could inhibit the insect occurrence and limit the fitness benefit brought by Bt/CpTI transgene.(3) Results from the F1-F7crop-weed hybrid progeny demonstrated the fitness effects of hybridization process combined with Bt/CpTI transgene on hybrid lineages. It revealed that the heterosis in vegetative traits and hybrid incompatibility in reproductive traits of early-generation hybrid progeny may mislead our judgement of the impact of Bt/CpTI transgene on weedy rice populations. With the disappearance of growth heterosis and recovery of fecundity in later-generation, the introgression of Bt/CpTI transgene may change the evolutionary potential of weedy rice populations. However, the ecological consequences are mainly relied on the cultivated conditions in which the weedy rice populations distribute and in which the corresponding insect occurrence is largely varied.(4) Results from the F1crop-wild hybrids demonstrated that the hybridization could produce F1hybrids with greater fitness in growth and fecundity than wild parent especially under competition condition, which may facilitate the introgression of Bt/CpTI transgene into common wild rice population. Meanwhile, the insect-resistant of Bt/CpTI transgene can result in lower insect damage and greater fitness performance in F1hybrids under high insect pressure, which may change the evolutionary potential of common wild rice population and lead to further ecological consequences.(5) Results from the F1and F2crop-wild hybrid progeny demonstrated that the EPSPS transgene can shorten the growing season and increase the vegetative growth ability, sexual reproduction ability and competition ability for hybrid progeny. Combined with the greater performance of early-generation hybrid progeny in most of the fitness-related traits than wild parents, the EPSPS transgene may eventrully introgress into the common wild populations and change their potential evolution, leading to unpredicted ecological consequences.The experimental findings obtained and analyzed above, can (1) improve the environmental biosafety evaluation system on the risks of transgene escape;(2) provide important basis for the study on long-term evolutionary and ecological impact of transgene into wild relatives of cultivated rice;(3) provide valuable references for the environmental biosafety management of transgenic rice after commercialization. For population genetics and evolutionary biology considerations, our findings can provide ideal model system for the study of hybridization-introgression process between crop and its wild relatives, as well as the potential evolutionary influences of the process. The role of transgene in hybridization-introgression could also serve as an ideal model system for the study of adaptive traits fixing and spreading in populations after the occurrence of gene flow.