AOBPreview originally published online on January 14, 2004
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Annals of Botany 93: 311-316, 2004
© 2004 Annals of Botany Company
Fitness Estimation through Performance Comparison of F1 Hybrids with their Parental Species Oryza rufipogon and O. sativa
Ministry of Education Key Laboratory for Biodiversity Science and Ecological Engineering, Institute of Biodiversity Science, Fudan University, Shanghai 200433, China
* For correspondence. Fax +86 021 65642468, e-mail jkch{at}fudan.edu.cn
Received: 22 August 2003;; Returned for revision: 3 October 2003; Accepted: 14 November 2003, Published electronically: 14 January 2004
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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• Background and Aims Introgression of crop genes into populations of wild relatives has important implications for germplasm conservation as well as for the persistence of novel transgenes in wild populations. Studies of hybrid fitness can be used to evaluate the potential for introgression to occur following episodes of interspecific hybridization.
• Methods This study estimated relative fitness of interspecific hybrids through performance comparison of F1 hybrids with their parental species, a cultivated rice (Oryza sativa) Minghui-63 and perennial common wild rice (O. rufipogon) under the cultivation conditions.
• Key Results Compared with their parents, the hybrids had the lowest values of seedling survival ability, pollen viability and seed production; intermediate values of seed germination, spikelet production and flag leaf areas; and the highest values of plant height, number of tillers and panicles. The hybrids performed poorly at the stage of sexual reproduction, although they had a slightly higher hybrid vigour at the vegetative growth stage and better tillering ability than their wild parent. There were no significant differences in composite fitness across the whole life-history between the hybrids and their wild parental species.
• Conclusions Rice genes, including transgenes, might persist in wild rice populations through vegetative and sexual reproduction. Further studies are needed to examine whether the extent of gene flow from rice is sufficiently significant to influence genetic diversity in wild populations of O. rufipogon, a species that has become endangered in some regions of south-east Asia.
Key words: Oryza rufipogon, O. sativa, rice, hybrid, fitness, introgression, ecological consequence.
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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Crop–wild/weed hybridization occurs frequently in a wide range of plant species and can be of great significance in evolution, because this process may result in crop–wild/weed gene flow that could alter the genetic make-up of both populations (Ellstrand et al., 1999). Recently, a great interest in crop–wild hybridization has been generated by the concerns of biosafety for genetically modified crops, simply because gene flow can be an avenue for transgene escape (Ellstrand and Holfman, 1990). As a direct method to assess the potential consequences of crop-to-wild/weed gene flow, estimation of fitness components of crop–wild hybrids and their parents, particularly the wild parents, is an important step (Snow et al., 1998). Among the large number of studies that have examined fitness-related traits of crop–wild/weed hybrids (e.g. Langevin et al., 1990; Klinger and Ellstrand, 1994; Linder and Schmitt, 1995; Jørgensen et al., 1996; Arriola and Ellstrand, 1997; Snow et al., 1998, 2001, 2003; Oard et al., 2000; Spencer and Snow, 2001; Burke and Rieseberg, 2003), most have found crop–wild/weed hybrids to be nearly as fit as their wild parents, and only one of the studies reported strong fitness decline among hybrid progeny (Jørgensen et al., 1996). These results generally suggest that crop genes might persist in wild populations. Thus the relative fitness, as estimated by the performance of hybrids in comparison with their parents, is essential for predicting the fate of hybrids and understanding the evolutionary significance of interspecific hybridization (Arnold and Hodges, 1995). It also serves as an effective approach for assessment of ecological consequences caused by transgene escape through gene flow (e.g. Burke and Rieseberg, 2003; Snow et al., 2003).
Data on the capability of crop–wild/weed hybrids for their long-term establishment and spread would be useful in predicting the potential fate of alien transgenes in wild populations. If it can de demonstrated that neutral or advantageous traits from traditionally improved crops can persist in wild or weedy populations, it is reasonable to infer that such traits which have been engineered may persist as well (Arriola and Ellstrand, 1997). It has been shown that even though they may demonstrate relatively low composite fitness across the whole life-history, hybrids usually do not perform with uniform inferiority at every life-history stage (Burke et al., 1998). Experiments designed to compare the overall hybrid plant vigour and reproductive capacity can reveal the evolutionary consequences of crop–wild/weed hybridization, although information specifically on germination and early establishment characteristics are essential as a baseline for comparison with parents (Arriola and Ellstrand, 1997). In addition, backcrosses of the hybrids with the crops and wild species are likely to happen, although some crop–wild species complexes exhibit strong barriers to gene flow and further introgression (e.g. Jiang et al., 2000). The fitness of hybrids will provide some indications of the likelihood and rate of spread of transgenes, because extreme hybrid sterility will make the establishment of crop genes unlikely (Linder and Schmitt 1995). It is therefore important to quantify fitness components of F1 crop–wild hybrids and to determine whether there is any barrier to subsequent introgression (Snow et al., 1998).
Common wild rice (Oryza rufipogon) is the putative ancestor of the Asian cultivated rice (O. sativa). It is, on the one hand, the most important genetic resource for rice improvement in terms of its accessibility for gene transfer through sexual means (Oka, 1988) and therefore needs urgent conservation due to its endangered status in many countries, such as China (Song et al., 2003b). However, on the other hand, it raises a serious biosafety concern for rice transgene escape and its potential consequences (Lu et al., 2003). Natural hybridization between O. rufipogon and O. sativa) has been frequently reported in many locations (Oka and Chang, 1961; Chu and Oka, 1970; Majumder et al., 1997; Song et al., 2002, 2003a; Lu et al., 2003), indicating that the alien transgenes could probably spread to this wild rice species from GM rice varieties through outcrossing. In spite of the high probability of transgene escape to this wild species (Lu et al., 2003; Chen et al., 2004), little is known about the ecological fitness of hybrids between O. sativa and O. rufipogon, even though this knowledge is important both for genetic conservation of the wild rice genetic resources and for risk assessment of rice transgene escape and its ecological consequences.
The present study was carried out to examine the performance of F1 hybrids produced between O. sativa and O. rufipogon under cultivation conditions in comparison with their parental species, in order to facilitate our understanding of potential consequences of transgene escape from GM rice to this wild relative under natural conditions.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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Plant material
Perennial wild rice, Oryza rufipogon Griff., was collected from the Chaling population (26°50'N, 113°40'E) in Hunan Province, China, and used as the maternal parent, following the main gene-flow pattern in nature. The rice variety Minghui-63 (a semi-dwarf cultivar with a growth period of 90–95 d) was provided by Prof. S. M. Mu from Hubei Academy of Agricultural Science (Wuhan City, China), and used as the pollen donor. Artificial hybridization was conducted following the method described by Song et al. (2002). Mature seeds from O. rufipogon, Minghui-63, and hand-pollinated panicles of O. rufipogon were collected separately in late November, air-dried naturally and stored at 4 °C for seed ripening until their use in late-March the following year.
Experimental layout and measurement of variables
Harvested seeds from the three studied types (i.e. O. rufipogon, Minghui-63 and hybrids) were germinated with six replicates of 200 seeds each. Seedlings of each type were transplanted separately into beds in a glasshouse after seed germination. The 50-d-old seedlings were then transplanted into experimental fields located at Wuhan University, China. Individuals of hybrids and O. rufipogon were mixed up and randomly planted in a 3 x 7 m plot, while individuals of Minghui-63 were grown as pure stands in a plot of the same size. All individuals were planted at 40 (hill) x 50 (row) cm intervals, thereby totalling 120 individuals per plot. Field management of the rice plants was conducted similarly to that for normal rice cultivation.
Measurements of variables were taken at three main life-history stages (Burke et al., 1998), during germination, growth and reproduction stages (Table 1). For the germination test, seeds were soaked in distilled water for approx. 30 h before being placed in Petri dishes at an alternating day/night temperature of 30/25 °C. The germination rate of seeds from different replicates was recorded every 7 d for a duration of 6 weeks. The seedling survival rates were recorded 30 d after transplanting to the glasshouse. The measured morphological characters included plant height, area of flag leaf, days to flowering, tiller production, panicle production, panicle length, number of spikelets and heading date. The details of measurements are listed in Table 1. Average values of different characters were determined from a total of 50 individuals from each type. In addition, self-pollination rates of the three types were examined by counting fertile spikelets against empty spikelets after bagging. Pollen viability and longevity of the three pollen grain types were measured following the method described by Song et al. (2001). Briefly, pollen was transferred to a gluten medium in Petri dishes at 1-min intervals after anther dehiscence (starting at 0 min) up to 5 min, and then at 2–5 min intervals up to 40 min post-dehiscence (up to 70 min for hybrids), with three replicates per treatment. After incubation at 34 °C for approx. 1 h, the number of germinated pollen grains was scored for ten random observations under a microscope per dish (Table 1).
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Data analysis
Because the data of measured variables showed a normal distribution, the means of each characteristic were separated using Duncan’s Multiple Range Test followed by a Bonferroni correction (
' = /k and = 0·05) (Rice, 1989), except for pollen longevity where regression analysis was conducted to highlight differences. For estimation of relative fitness, the three types were arranged in order from ‘low’ to ‘high’, and the relative fitness per characteristic of the plant at the top of the order was defined as 1·00. Each type was then assigned a performance value (0–1·00) based on its ratio to the highest (i.e. most fit) value. The fitness components were grouped according to characteristics associated with the three life-history stages, i.e. germination, growth and reproduction (Table 1), and the relative fitness related to each stage was calculated as the mean of the relative fitness of all characteristics within this stage. Composite fitness across the whole life-history was the mean of the fitness estimates of the three stages. Duncan’s Multiple Range Test and Bonferroni’s correction were also used to analyse the differences in relative fitness.
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RESULTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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Seed germination and seedling survival
Significant differences in seed germination were found among O. rufipogon, Minghui-63 and their hybrids (Table 2). The majority of seeds from Minghui-63 and hybrids germinated in the first week and their germination period lasted for 3 weeks. However, only 20 % of seeds from O. rufipogon germinated in the first week, and most of its seeds germinated in the second week (Fig. 1). The whole germination duration of O. rufipogon was more than 6 weeks. This indicated considerable differences in germination patterns among the three types of seeds and a certain degree of seed dormancy in O. rufipogon.
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Seedling survival was different between the three types. Nearly all seedlings of Minghui-63 survived after germination, and O. rufipogon seedlings had a rate of survival of over 90 %. Only approx. 70 % of seedlings of hybrids survived (Table 2).
Vegetative growth
The variables of plant height, flag leaf area and days to flowering were used to estimate the fitness associated with vegetative growth. Plant height varied from 182·5 to 206·2 cm for O. rufipogon, 60·1 to 74·9 cm for Minghui-63 and 186·9 to 199·8 cm for hybrids. The result showed that the plant height of hybrids was nearly the same as that of O. rufipogon, but was significantly greater than that of Minghui-63 (Table 2). Flag leaf area was greatest in Minghui-63, hybrids were intermediate and O. rufipogon had the smallest area. Days to flowering were approx. 92·5 for Minghui-63, and most individuals of this variety had synchronous flowering, more or less within a period of 5 d. Oryza rufipogon and the hybrids had a similar heading date, and both showed protracted flowering time with a 1-week delay compared with that of Minghui-63; however, the difference in time of anthesis between types was not statistically significant (Table 2).
Propagation and reproduction
It was shown that hybrids produced a significantly higher number of tillers than their parental species, and that cultivated Minghui-63 had the least number of tillers per individual (Table 2). This result demonstrates that hybrids have a slightly stronger capacity for clonal propagation than either parental species.
Hybrids had a significantly higher panicle production than their parents (Table 2). The wild parent O. rufipogon had a relatively higher number of panicles than Minghui-63. Minghui-63 had the highest number of spikelets of the three types, although no significant differences in spikelet production were revealed by the Duncan’s Multiple Range Test followed by Bonferroni’s correction.
The in vitro germination of fresh pollen of Minghui-63 was 85 % (pollen viability), and the time to reduction to 0 % germination was 30 min (pollen longevity) (Table 2). The in vitro germination of fresh pollen of O. rufipogon was 60 %, and the time to reduction to 0 % germination was 70 min. The in vitro germination of fresh pollen of the hybrids was 34 %, and the time to reduction to 0 % germination was approx. 40 min. The seed set of Minghui-63 was almost twice as high as that of O. rufipogon, and the seed set of O. rufipogon was two times higher than that of hybrids (Table 2).
Relative fitness
Among the three life-history stages, hybrids presented the highest values at growth and clonal reproduction stages. Minghui-63 had the highest value at the germination stage. Minghui-63 and O. rufipogon had the nearly the same fitness value at the sexual reproduction stage, which was significantly higher than that of hybrids. However, for the estimation of composite fitness across the whole life-history, there were no significant differences among the three types, although hybrids seemed to have a slightly higher value than their parents (Table 3).
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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Data on fitness of interspecific hybrids are extremely valuable for predicting the destiny of the hybrids in natural habitats, particularly in the context of environmental biosafety where alien transgenes might be included in the hybrids and cause safety concerns (Arriola and Ellstrand, 1996; Burke and Rieseberg, 2003; Snow et al., 2003). For estimating the fitness of such hybrids, it is very important to select the correct characteristics associated with vegetative growth and reproduction that can properly reflect their capacity for competition and reproductive potentials (Arriola and Ellstrand, 1997). The present study, involving the whole life-history, allows us to evaluate the possible consequences of hybridization and introgression between cultivated and wild rice species, which has implications for transgene escape from GM rice varieties.
Results from this study generally showed that hybrids generated between wild and cultivated rice had nearly the same fitness as their parental species across the whole life-history under the cultivation conditions imposed. The result accords with observations in other studies where hybrids do not always show consistent inferiority or superiority in comparison with their parental species (see Arnold and Hodges, 1995). It is notable that the hybrids between Minghuai-63 and O. rufipogon in this study were found to be slightly inferior at the stage of sexual reproduction, mainly due to lower seed set and pollen viability, but they showed slightly higher hybrid vigour during the growth stage and better clonal ability than their parents.
In terms of the hybrids’ ultimate prospects for survival, attention should be focused on the comparison between them and their wild parent (Snow et al., 1998). Compared with the wild parent O. rufipogon, the hybrids presented significantly lower seedling survival, pollen viability and seeding ability. The partial sterility in the F1 hybrids suggests the existence of barriers to the introgression of crop genes. If this is the case, hybrids between cultivated and wild rices would need to overcome these barriers to persist under natural conditions. The presence of such barriers can be used to interpret the observation that relatively few hybrid or introgressed populations of O. rufipogon become established under natural conditions (Oka, 1988), although hybridization between cultivated rice and O. rufipogon has frequently been reported. In addition, the differences in germination time between hybrids and their wild parents indicates a strong seed dormancy in O. rufipogon compared with the hybrids. This suggests that the hybrids might be inferior to their wild parent in terms of circumventing unfavourable conditions such as cold seasons, because dormancy is an important adaptive feature of wild rice species and apparently favours seeds that can live through the cold winter and can survive a relatively long period of remaining in water under natural conditions (Wu, 1978; Oard et al., 2000). For example, the temperature can reach a minimum of –4 °C in the severe winters in Chaling of Hunan Province, where O. rufipogon normally flowers in late autumn and produces mature seeds in early winter. In the absence of dormancy, the seeds might germinate soon after shedding from the plant and the seedlings would be winter-killed. Hence, the reduced degree of dormancy in hybrid seeds means that the winter might pose a significant selection pressure against them persisting under natural conditions. Seed dormancy of hybrids and their wild parents should be further studied under natural conditions for a more accurate evaluation of hybrid survival.
The absolute seed set of the hybrids reached a relatively high level of 20 % in this study, which demonstrates that the F1 barriers to the introgression of crop genes may be weak, even though they do exist (Chu and Oka, 1970), as in the wild–crop F1 hybrids of sunflower (Snow et al., 1998). This finding is also supported by studies of pollen competition and gene-flow between Minghui-63 and O. rufipogon, where the two species have been shown to possess considerable sexual compatibility (Song et al., 2002, 2003a). The hybrids showed a slightly better performance in growth and significantly higher tiller production than the wild parent, indicating hybrid vigour as mentioned by Oka and Chang (1961). The hybrids were morphologically similar to the wild parent and included its perennial character, although some intermediate characters were found in hybrids. The fact that hybrids produced more tillers than wild plants suggests that they may benefit from hybrid vigour and could be more competitive than their wild parents, with an enhanced probability for survival by vegetative reproduction. In addition, the average times to flowering for the hybrid and its wild parent were both approx. 100 d after germination; this synchronous flowering allowed hybrids to backcross with wild neighbours, despite having fewer seeds and lower pollen viability. To address the likelihood and rate of crop genes persisting and spreading in wild populations, more detailed examinations of fitness associated with subsequent generations of hybrids, including selfed- and backcrossed-progeny, should be conducted.
In conclusion, results from this study demonstrate that the F1 hybrids between cultivated rice and its close wild relative O. rufipogon could survive under natural conditions through sexual reproduction and vegetative propagation. However, the F1 hybrids did not show clear superiority compared with the wild parent under our experimental conditions. In other words, hybrids between cultivated rice and O. rufipogon may not necessarily have an advantage over the wild rice species under natural conditions. In the context of transgene escape and its ecological consequences, if the genetically modified genes are responsible for such traits as high protein content and special nutritional compounds that do not have natural selection advantages, the escape of such genes into wild rice would have minimum ecological consequences: thus, no special concerns need to be raised. However, the escape of genes that could confer advantages for natural selection and could significantly enhance the ecological fitness of wild rice species should be carefully assessed and monitored for their potential ecological consequences.
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ACKNOWLEDGEMENTS |
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We would like to thank Prof. Yinguo Zhu, Prof. Wei Li, and Mr Guihua Liu for their assistance in the field experiments, Prof. Allison A. Snow of Ohio State University for her valuable comments on the manuscript, and Carol Mallory-Smith and an anonymous referee for providing critical comments and suggestions on the manuscript. The National Nature Science Foundation of China (NSFC Grant Nos 30300019, 39893360 and 30125029), Shanghai Commission of Science and Technology (Grant Nos 02JC14022 and 03dz19309), and 211 Project (Biodiversity and Regional Ecosafety) supported this research.
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LITERATURE CITED |
|---|
|
TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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-
Arnold ML, Hodges SA. 1995. Are natural hybrids fit or unfit relative to their parents? Trends Ecology and Evolution 10: 67–71.
Arriola PE, Ellstrand NC. 1996. Crop-to-weed gene flow in the genus Sorghum (Poaceae): spontaneous interspecific hybridization between johnsongrass, Sorghum halapense, and crop sorghum, S. bicolor. American Journal of Botany 83:1153–1160.[CrossRef]
Arriola PE, Ellstrand NC. 1997. Fitness of interspecific hybrids in the genus Sorghum: persistence of crop genes in wild populations. Ecological Applications 7: 521–528.
Burke JM, Carney SE, Arnold ML. 1998. Hybrid fitness in the Louisiana irises: analysis of parental and F1 performance. Evolution 52: 37–43.
Burke JM, Rieseberg LH. 2003. Fitness effects of transgenic disease resistance in sunflowers. Science 300: 1250.
Chen LJ, Lee DS, Song, ZP, Suh, HS, Lu BR. 2004. Gene flow from cultivated rice (Oryza sativa) to its weedy and wild relatives. Annals of Botany 93: 67–73.
Chu Y, Oka HI. 1970. Introgression across isolating barriers in wild and cultivated Oryza species. Evolution 24: 344–355.[CrossRef]
Ellstrand NC, Holfman CA. 1990. Hybridization as an avenue of escape for engineered genes. BioScience 40: 438–442.[CrossRef]
Ellstrand NC, Prentice HC, Hancock JF. 1999. Gene flow and introgression from domesticated plants into their wild relatives. Annual Review of Ecology and Systematics 30: 539–563.[CrossRef][Web of Science]
Jiang C, Chee PW, Draye X, Morrell PL. 2000. Multilocus interactions restrict gene introgression in inter-specific populations of polyploid Gossypium (cotton). Evolution 54: 798–814.[CrossRef][Web of Science][Medline]
Jørgensen RB, Andersen B, Mikkelsen TR. 1996. Spontaneous hybridization between oilseed rape (Brassica napus) and weedy relatives. Acta Horticulturae 407: 193–200.
Klinger T, Ellstrand NC. 1994. Engineered genes in wild populations: fitness of weed–crop hybrids of Raphanus sativus. Ecological Applications 4: 117–120.
Langevin SA, Clay K, Grace JB. 1990. The incidence and effects of hybridization between cultivated rice and its related weed red rice (Oryza sativa). Evolution 44: 1000–1008.[CrossRef]
Linder CR, Schmitt J. 1995. Potential persistence of escaped transgenes: performance of transgenic, oil-modified Brassica seeds and seedlings. Ecological Applications 5: 1056–1068.[CrossRef]
Lu BR. 1999. Need to conserve wild rice species in Nepal. International Rice Research Notes 24: 41.
Lu BR, Song ZP, Chen JK. 2003. Can transgenic rice cause ecological risks through transgene escape? Progress in Natural Science 13: 17–24.[CrossRef]
Majumder ND, Ram T, Sharma AC. 1997. Cytological and morphological variation in hybrid swarms and introgressed population of interspecific hybrids (Oryza rufipogon Griff x O. sativa L.) and its impact on evolution of intermediate types. Euphytica 94: 295–302.[CrossRef]
Oard J, Cohn MA, Linscombe S, Gealy D, Gravois K. 2000. Field evaluation of seed production, shattering, and dormancy in hybrid populations of transgenic rice (Oryza sativa) and the weed, red rice (Oryza sativa). Plant Science 157: 13–22.
Oka HI. 1988. Origin of cultivated rice. Tokyo: Japan Scientific Societies Press.
Oka HT, Chang WT. 1961. Hybrid swarms between wild and cultivated rice species, O. perennis and O. sativa. Evolution 15: 418–430.[CrossRef]
Rice WR.1989. Analyzing tables of statistical tests. Evolution 43: 223–225.[CrossRef][Web of Science]
Snow AA, Moran-Palma P, Rieseberg LH, Wszelaki A, Seiler GJ. 1998. Fecundity, phenology, and seed dormancy of F1 wild-crop hybrids in sunflower (Helianthus annuus, Asteraceae). American Journal of Botany 85: 794–801.[Abstract]
Snow AA, Pilson D, Rieseberg LH, Alexander HM. 2003. A Bt transgene reduces herbivory and enhances fecundity in wild sunflowers. Ecological Application 13: 279–286.
Snow AA, Ythus Kl, Culley TM. 2001. Fitness of hybrids between weedy and cultivated radish: implications for weed evolution. Ecological Application 11: 934–943.[CrossRef]
Song ZP, Lu BR, Chen JK. 2001. A study of pollen viability and longevity in Oryza rufipogon, O. sativa, and their hybrid. International Rice Research Notes 26: 31–32.
Song ZP, Lu BR, Zhu YG, Chen JK. 2002. Pollen competition between cultivated and wild rice species (Oryza sativa and O. rufipogon). New Phytologist 153: 289–296.[CrossRef]
Song ZP, Lu BR, Zhu YG, Chen JK. 2003a. Gene flow from cultivated rice to the wild species Oryza rufipogon under experimental field conditions. New Phytologist 157: 657–665.[CrossRef]
Song ZP, Xu X, Wang B, Chen JK, Lu BR. 2003b. Genetic diversity in the northernmost Oryza rufipogon populations estimated by SSR markers. Theoretical and Applied Genetics 107: 1492–1499.[CrossRef][Web of Science][Medline]
Spencer LJ, Snow AA. 2001. Fecundity of transgenic wild-crop hybrids of Cucurbita pepo (Cucurbitaceae): implications for crop-to-wild gene flow. Heredity 86: 694–702.[CrossRef][Web of Science][Medline]
Wu L. 1978. The seed dormancy of a Taiwan wild rice population and its potential for rice breeding. Botany Bulletin Academia Sinica 19: 1–12.
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