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Fitness of Hybrids between Two Types of Transgenic Rice and Six and Weed Rice Accessions
Crop Science
Abstract
Studies of hybrid fitness can help in evaluating the potential introgression of a transgene from transgenic rice (Oryza sativa L.) to weedy rice. The objective of this study was to assess the composite fitness of intra- (within the same subspecies either indica or japonica) and inter- (between individuals of different subspecies) hybrids between two transgenic glufosinate-resistant rice lines and six weedy rice accessions in the field. Compared with their weedy rice counterparts, intrahybrids may be similar or greater, and the interhybrids may be lower, similar, or greater in fitness. Using japonica transgenic rice Y0003 as male progenitor, composite fitness of intrahybrids increased over generations, while that of interhybrids kept variable over generations. Using indica transgenic rice 86B as paternal plants, the composite fitness of all intrahybrids decreased, but that of interhybrids increased over generation. Regardless of japonica or indica transgenic rice being paternal plant, pollen viability, and seed set were greater in intrahybrids but lower in interhybrids than their weedy rice counterparts. Moreover, inter-F2 and inter-F3 hybrids segregated for plant height, heading period, and seed set. Therefore, potential risk of gene flow from transgenic rice to weedy rice was dependent on the genotype of both weedy rice and transgenic rice. Gene flow from transgenic rice to weedy rice of the same or different subspecies should be prevented either through mitigation or stewardship owing to the similar or greater fitness of intraspecific hybrids compared with parental weedy rice and the similar or increased composite fitness of majority interhybrids over generations.
... The fitness of hybrids might vary widely, depending on the parental genotype, testing environment, and their interaction (Campbell and Waser 2001;Johnston et al. 2001;Johannessen et al. 2006;Whitney et al. 2006;Yang et al. 2011;Liu et al. 2016;Huang et al. 2019). Therefore, it is important to evaluate the fitness of hybrids of transgenic crops and wild relatives under different conditions. ...
... Separation of the means of each measured variable was performed following the method described by Liu et al. (2016) using SPSS II 25.0 Software (IBM). Without weed competition, the means of every measured variable of wild soybeans and F1 hybrids were calculated. ...
... The reproductive ability of hybrids is one of the most important traits for assessing fitness (Liu et al. 2016). In the present study, reproductive variables of 10 F1 hybrids, including pollen viability and filled seed number per plant, were significantly lower than those of their wild progenitors. ...
The releasing of transgenic soybeans ( Glycine max (L.) Merr.) into farming systems raises concerns that transgenes might escape from the soybeans via pollen into their endemic wild relatives, the wild soybean ( Glycine soja Sieb. et Zucc.). The fitness of F1 hybrids obtained from 10 wild soybean populations collected from China and transgenic glyphosate-resistant soybean was measured without weed competition, as well as one JLBC-1 F1 hybrid under weed competition. All crossed seeds emerged at a lower rate from 13.33–63.33%. Compared with those of their wild progenitors, most F1 hybrids were shorter, smaller, and with decreased aboveground dry biomass, pod number, and 100-seed weight. All F1 hybrids had lower pollen viability and filled seeds per plant. Finally, the composite fitness of nine F1 hybrids was significantly lower. One exceptional F1 hybrid was IMBT F1, in which the composite fitness was 1.28, which was similar to that of its wild progenitor due to the similarities in pod number, increased aboveground dry biomass, and 100-seed weight. Under weed competition, plant height, aboveground dry biomass, pod number per plant, filled seed number per plant, and 100-seed weight of JLBC-1 F1 were lower than those of the wild progenitor JLBC-1. JLBC-1 F1 hybrids produced 60 filled seeds per plant. Therefore, F1 hybrids could emerge and produce offspring. Thus, effective measures should be taken to prevent gene flow from transgenic soybean to wild soybean to avoid the production F1 hybrids when releasing transgenic soybean in fields in the future.
... F 1 , F 2 and F 3 hybrids between weedy rice and transgenic glufosinate-resistant rice had a lower seed set (P<0.05) or non-significant differences in seed set (P>0.05) as well as non-significant differences in composite agronomic performance (P>0.05) compared to their weedy rice parents Liu et al. 2016). Moreover, the fitness of intrasubspecies (within the same subspecies, either indica or japonica) hybrids presented higher values (P<0.05) or non-significant differences (P>0.05), and that of intersubspecies hybrids (between individuals of different subspecies) presented lower values (P<0.05), ...
... nonsignificant differences (P>0.05), or higher values (P<0.05) than their weedy rice counterparts, respectively (Liu et al. 2016). Insect-resistant Bt-transgenic plants generally have competitive advantages under selective pressures of target insects, but they also show fitness costs as a result of the expression of Bt transgenes in the absence of insects (Liu et al. 2015;Liang et al. 2018). ...
... Regarding to hybrids F 1 + (intra-subspecies) heterosis, we analyzed performance of the three hybrids F 1 + and weedy rice under non-insect monoculture planting, it was found that the three F 1 + hybrids displayed heterosis on 1 000-grain weight, F 1 YY+ on panicle number per plant, filled grain number per panicle and yield per plant, and F 1 TZ+ on plant height, filled grain number per panicle and yield per plant. Similarly, Liu et al. (2016) found that intrasubspecies (within the same subspecies, either indica or japonica) hybrids presented heterosis in plant height, filled grains per panicle and yield per plant. ...
Compared to single-trait transgenic crops, stacked transgenic plants may be more prone to become weedy, and transgene flow from stacked transgenic plants to weedy relatives may pose a potential environmental risk because these hybrids could be more advantageous under specific environmental conditions. Evaluation of the potential environmental risk caused by stacked transgenes is essential for assessing the environmental consequences caused by crop-weed transgene flow. The agronomic performance of fitness-related traits was assessed in F1+ (transgene positive) hybrids (using the transgenic line T1c-19 as the paternal parent) in monoculture and mixed planting under presence or absence glufosinate pressure in the presence or absence of natural insect pressure and then compared with the performance of F1– (transgene negative) hybrids (using the non-transgenic line Minghui 63 (MH63) as the paternal parent) and their weedy rice counterparts. The results demonstrated that compared with the F1– hybrids and weedy rice counterparts, the F1+ hybrid presented higher performance (P<0.05) or non-significant changes (P>0.05) under natural insect pressure, respectively, lower performance (P<0.05) or non-significant changes (P>0.05) in the absence of insect pressure in monoculture planting, respectively. And compared to weedy rice counterparts, the F1+ hybrid presented higher performance (P<0.05) or non-significant changes (P>0.05) in the presence or absence of insect pressure in mixed planting, respectively. The F1+ hybrids presented non-significant changes (P>0.05) under the presence or absence glufosinate pressure under insect or non-insect pressure in monoculture planting. The all F1+ hybrids and two of three F1– hybrids had significantly lower (P<0.05) seed shattering than the weedy rice counterparts. The potential risk of gene flow from T1c-19 to weedy rice should be prevented due to the greater fitness advantage of F1 hybrids in the majority of cases.
... Since the introduction of Clearfield rice in 2002 (Croughan 2003), there are several research on its agronomic performance in comparison with conventional cultivars. It has been reported that Clearfield rice grain yields were lower than conventional cultivars (Sha et al. 2007;Shivrain et al. 2009;Deliberto and Salassi 2010;Sudianto et al. 2013;Shengnan et al. 2016). Despite this, Clearfield cultivars showed greater head rice yield than their regular counterparts. ...
Herbicides are used in modern agriculture to control weeds that pose threat to agricultural crops by competing with them for light and nutrients. Continuous use of the same type of herbicide results in the evolution of weeds that are resistant to herbicides. Indiscriminate use of herbicides can also cause environmental pollution. Development of transgenic rice that is resistant to more than one herbicide is a solution to these problems. The use of herbicide-resistant variety of rice also reduces the cost for weed control. Genetic engineering involves the manipulation of an organism’s genetic material using various molecular biological techniques. Source of transgenes for herbicide resistance includes bacteria and plants. Development of transgenic rice that is herbicide-resistant will result in the development of wild rice. Genetically engineered rice that is herbicide-resistant has changed the usage pattern of herbicide worldwide. Examples for herbicide-resistant rice varieties include Clearfield, Roundup Ready, and LibertyLink. Clearfield is imidazolinone-resistant, Roundup Ready glyphosate-resistant, and LibertyLink glufosinate-resistant. Despite having so many benefits, genetically engineered herbicide-resistant rice varieties have some drawbacks. One such drawback is that genetically improved crops are also responsible for the problems associated with herbicide-resistant weeds. Another issue is related to the acceptance of genetically modified herbicide-resistant rice variety by the public. It is also not that easy to find domestic and international markets with herbicide-resistant rice varieties. Hence, proper production and marketing strategies should be adopted along with the development of herbicide-resistant rice varieties that are developed using genetic engineering.
... In 2009, the Ministry of Agriculture of the People's Republic of China approved two transgenic rice strains, namely Huahui-1 (cry1Ab/ cry1Ac) and Shanyou-63 (cry1Ab/cry1Ac), as restorer lines for three-line hybrid rice. In addition, other transgenic rice varieties have reached the safety declaration and certification stage, which include T1c-19(Bar/cry1c), T2A-1, and II-You 86 B (Sun et al. 2015, Huang et al. 2016, Liu et al. 2016, which are also restorer lines for three-line hybrid rice that were obtained via receptor transformation. Therefore, once China approves the release of transgenic rice, it is expected to be used for the restorer lines in many cultivars of three-line hybrid rice combinations. ...
Weedy rice (Oryza sativa f. spontanea) is a conspecific weedy relative of cultivated Asian rice (O. sativa L.) that occurs in paddy fields worldwide. The mechanism underlying the persistence of weedy rice in rice fields remains unclear. We tested for evidence that southern Chinese weedy rice has originated through pollen flow from weedy rice that has resulted in hybridization with three‐line hybrid rice cultivars, which have been extensively cultivated in southern China for the last 40 yr and now account for over half the total rice cultivation in China. To test the hypothesis, we screened for the presence of the maternally inherited WA352 gene in weedy rice populations in southern China. WA352 is a mitochondrial gene that controls wild abortive cytoplasmic male sterility (CMS‐WA) and is present in three‐line hybrid rice cultivars. Screening of 48 weedy rice populations (939 accessions) indicates that 36 of 48 populations (75%) and 325 of 939 accessions (34.6%) harbor the WA352 gene. The rate is significantly higher than the 1% outcrossing rate between weedy rice and rice cultivars. Further analysis indicates significance between the frequency of the WA352 gene in weedy rice populations and depth of plowing. Our results suggest that outcrossing with rice cultivars is one of the important mechanisms underlying the persistence of weedy rice in southern China and that deep plowing may be conducive to promoting the persistence of weedy rice derived from hybridization (three‐line hybrid rice × weedy rice).
The introduction of herbicide-tolerant (HT) transgenic soybeans (Glycine max (L.) Merr.) into farming systems raises great concern that transgenes may flow to endemic wild soybeans (Glycine soja Sieb. et Zucc.) via pollen, which may increase the ecological risks by increasing the fitness of hybrids under certain conditions and threaten the genetic diversity of wild soybean populations. In order to demonstrate the potential risk of gene flow from the HT soybean to the wild soybean, the fitness of F2 and F3 hybrids obtained from two wild soybean populations (HLJHRB-1, JSCZ) collected from China and the HT soybean was measured under farmland and wasteland soil conditions, as well as with or without weed competition. Compared with their wild progenitors, the F2 and F3 hybrids of HLJHRB-1 displayed a higher emergence rate, higher aboveground dry biomass, more pods and filled-seed plants, as well as better composite fitness under four planting conditions. The F2 and F3 hybrids of JSCZ also displayed a higher emergence rate, higher aboveground dry biomass, more pods, and more filled seeds per plant under mixed planting, whereas these characteristics were lower under pure planting conditions in wasteland and farmland soil. Therefore, the composite fitness of JSCZ hybrids was higher or lower depending on the planting conditions. Furthermore, the soil microbial communities of the F3 of HLJHRB-1, JSCZ, and the wild soybean were investigated with 16S rDNA sequencing, which showed that low alpha diversity of rhizobacteria was relative to high fitness, and Rhizobium played an important role in promoting F3 plant growth.
Hybridization of transgenic crops with their weedy relatives may spread transgenes into weedy populations and alter the ecological fitness of such hybrids. Our study objective was to quantify the rate of gene flow from herbicide-resistant transgenic rice to weedy rice and compare the performance of hybrids between them over several generations. The transgenic plants used in these experiments are resistant to protoporphyrinogen IX oxidase (PPO)-inhibiting herbicides such as oxyfluorfen, oxadiazon, and butafenacil. The rate of gene flow from those plants to five weedy accessions ranged from 0.025% to 0.139%. The F1 hybrids that resulted from gene flow from transgenic to weedy rice and subsequent F2 progeny were shorter than the weedy parents but produced significantly more and heavier grains. The F3 progeny resulting from gene flow were resistant to oxyfluorfen, which indicated that the gene associated with PPO-inhibiting herbicide resistance is dominantly inherited. The GR50 values (dose required for 50% shoot fresh weight reduction) for F3 progeny were > 69-fold greater than for the weedy rice. These results suggest that transgenes that could escape from PPO-inhibiting herbicide-resistant rice and be transferred to weedy rice might persist and disperse into weedy populations over several generations due to herbicide resistance and improved reproductive traits in the hybrids.
Natural hybrid zones between related species illustrate processes that contribute to genetic differentiation and species formation. A common viewpoint is that hybrids are essentially unfit, but they exist in a stable tension zone where selection against them is balanced by gene flow between the parent species. An alternative idea is that selection depends on the environment, for example, by favoring opposite traits in the two parental habitats or favoring hybrids within a bounded region. To determine whether selection of hybrids is environment dependent, we crossed plants of naturally hybridizing Ipomopsis aggregata and I. tenuituba in the Colorado Rocky Mountains and reciprocally planted the seed offspring into a suite of natural environments across the hybrid zone. All types of crosses produced similar numbers and weights of seeds. However, survival of the offspring after 5 years differed markedly among cross types. On average, the F1 hybrids had survival and growth rates as high as the average for their parents. But hybrid survival depended strongly on the direction of a cross, that is, on which species served as the maternal parent. This fitness difference between reciprocal hybrids appeared only in the parental environments, suggesting cytonuclear gene interactions that are environment specific. These results indicate that complex genotype-by-environment interactions can contribute to the evolutionary outcome of hybridization. Corresponding Editor: S. Tonsor
Red rice is an important weed in paddy rice fields. The use of ammonium glufosinate - resistant genetically modified (GM) rice cultivars is a viable alternative for controlling red rice in cultivated rice crops. The objective of this study was to evaluate the behavior of F2 hybrid population derivates of crossing between ammonium glufosinate - resistant GM rice and red rice. The F2 hybrid population derivates of crossing between GM rice and red rice are viable but do not present any apparent competitive advantage over red rice. The morphological characteristics of the F2 hybrid populations, such as tillering capacity, number of leaves produced and average plant height were partly reduced and partly unaffected by the introgression of the bar gene. The period between rice planting and 50% flowering increased in the red rice x GM rice F2 hybrids, compared to the parental red rice and GM rice. Average seed shattering rate in the four hybrid families was lower than in the red rice ecotypes, with such aspect also being unfavorable to the persistence of red rice in the environment. Seed dormancy of the hybrids was similar to that of the red rice ecotypes. However, management practices should be adopted to avoid crossing between transgenic rice and red rice.
Weed species are known to evolve rapidly with their associated crops. A better understanding of the mechanisms and rates of weed evolution could aid in limiting or at least anticipating this process. Spontaneous hybridization between crops and related weed species can transfer crop genes coding for fitness-enhancing traits to wild populations, but little is known about how easily this takes place in various weed-crop complexes. We studied interspecific hybrids between wild and cultivated radishes (Raphanus raphanistrum × R. sativus), which often co-occur and share pollinators: To determine whether the F1 generation represents a strong barrier to subsequent introgression, we compared the fitness of wild plants and wild-crop hybrids. Two experiments were carried out in Michigan, USA, one with potted plants and the other involving four artificially established populations. In the artificial populations, we used white flower color, a dominant, crop-specific allele, to document the persistence of crop genes over time. Wild plants had yellow flowers, which is a recessiye trait. F1 hybrids had lower fitness than wild plants due to lower pollen fertility, fewer seeds per plant, and delayed flowering. Despite these disadvantages, hybrids contributed substantially to each population's gene pool. After 3 yr, frequencies of white-flowered plants in the artificial populations ranged from 8% to 22%, demonstrating that crop genes persisted. Other studies of flower color variation in wild populations of R. raphanistrum provide circumstantial evidence for frequent crop-to-wild gene flow. We predict that, if cultivated radish is engineered to possess transgenes coding for traits such as resistance to insect herbivores, disease, herbicides, or environmental stress, these fitness-related crop genes will easily spread to R. raphanistrum.
Within the next few years, many types of transgenic rice (Oryza sativa) will be ready for commercialization, including varieties with higher yields, greater tolerance of biotic and abiotic stresses, resistance to herbicides, improved nutritional quality, and novel pharmaceutical proteins. Although rice is primarily self-pollinating, its transgenes are expected to disperse to nearby weedy and wild relatives through pollen-mediated gene flow. Sexually compatible Oryza species often co-occur with the crop, especially in tropical countries, but little is known about how quickly fitness-enhancing transgenes will accumulate in these populations and whether this process will have any unwanted environmental consequences. For example, weedy rice could become much more difficult to manage if it acquires herbicide resistance, produces more seeds, or occurs in a wider range of habitats because of the spread of certain transgenes. Rice-growing countries urgently need publicly available ecological assessments of the risks and benefits of transgenic rice before new varieties are released.
Weedy rice has been becoming a notorious weed in the paddy field of China in recent decades due to its increasing damage to
rice yield and rice quality. In this study, a microsatellite technique with 21 pairs of SSR markers was utilized to estimate
the genetic structure of two biotypes of weedy rice with Japonica and Indica rice characteristics, collected from Liaoning
and Guangdong provinces, respectively. The genetic diversity of the weedy rice in the two provinces was relatively low (Liaoning
h = 0.086; Guangdong h = 0.160), and distinctly large genetic differences existed between these two provinces (Gcs = 0.623). The genetic diversity was found primarily within populations, and genetic differentiation was relatively low within
the same province. Both cluster analysis (UPGMA) and principle component analysis (PCA) showed that weedy rice had a closer
relationship with the cultivated rice collected from the sample field than with other cultivated rice and common wild rice
varieties in China. Thus, the results of this study on samples from the Liaoning and Guangdong provinces in China support
the de-domestication hypothesis that weedy rice most probably originated from local cultivated rice.
Background:
Competition from weedy rice can cause serious yield losses to cultivated rice. However, key traits that facilitate competitiveness are still not well understood. To explore the mechanisms behind the strong growth and competitive ability, replacement series experiments were established with six genotypes of weedy rice from different regions and one cultivated rice cultivar.
Results:
(1) Weedy rice from southern China had the greatest impact on growth and yield of cultivated rice throughout the entire growing season. Weedy rice from the northeast was very competitive during the early vegetative stage while the competitive effects of eastern weedy rice were more detrimental at later crop-growth stages. (2) As the proportion of weedy rice increased, plant height, tillers, above-ground biomass, and yield of cultivated rice significantly declined; the crop always being at disadvantage regardless of proportion. (3) Weedy biotypes with greater diversity as estimated by their Shannon indexes were more detrimental to the growth and yield of cultivated rice.
Conclusion:
Geographic origin (latitude) of weedy rice biotype, its mixture proportion under competition with the crop and its genetic diversity are determinant factors of the outcome of competition and the associated decline in the rice crop yield. © 2013 Society of Chemical Industry.
The commercialization of imazethapyr-resistant (Clearfield™, CL) rice in the southern United States has raised serious concerns about gene flow to red rice, producing imazethapyr-resistant red rice populations. Our objectives were to determine the impact of planting date, CL cultivars, and red rice biotypes on outcrossing rate; and to investigate the relative contribution of flowering time of CL rice and red rice biotypes, together with air temperature and relative humidity (RH), on outcrossing rate. Field experiments were conducted at Stuttgart, Rohwer, and Kibler, AR, from 2005 to 2007, at three or four planting times from mid-April to late May. ‘CL161’ (inbred cultivar) and ‘CLXL8’ (hybrid) rice were planted in nine-row plots, with red rice planted in the middle row. Twelve red rice biotypes were used. The flowering of red rice and CL rice, air temperature, and RH were recorded. Red rice seeds were collected at maturity. To estimate outcrossing rate, resistance to imazethapyr was evaluated in subsequent years and confirmed using rice microsatellite markers. CLXL8 rice flowered 2 to 4 d earlier than CL161 rice, and flowering was completed within 1 wk in all plantings. The flowering duration of most red rice biotypes ranged from 4 to 17 d. Flowering synchrony of red rice biotypes and CL rice ranged from 0 to 100% at different plantings. In general, CLXL8 had greater flowering overlap and higher outcrossing rate with red rice than did CL161 rice. The outcrossing rate of red rice biotypes ranged from 0 to 0.21% and 0 to 1.26% with CL161 and CLXL8 rice, respectively. The outcrossing rate differed within each planting date (P < 0.05). Outcrossing was generally lower in mid-May and late May than in mid-April and late April planting times. Flowering synchrony and outcrossing rate were not correlated (r2 < 0.01). Outcrossing with CL161 was primarily influenced by red rice biotype. A minimum air temperature of > 24 C in the evening also favors outcrossing with CL161. With CLXL8 rice, outcrossing was most affected by RH. When RH was < 54%, outcrossing was less (0.12%) than when RH was ≥ 54% (0.38%). With CLXL8 rice, a minimum RH of ≥ 54%, from mid-morning to noon, increased outcrossing with red rice. To fully understand the interaction effects of these factors on outcrossing with red rice, controlled experiments are needed. Nomenclature: Imazethapyr; Red rice, Oryza sativa L.; Rice, Oryza sativa L. ORYSA.
Certified Crop Advisors of Arkansas and members of the Arkansas Crop Consultants Association were surveyed in fall 2006 through direct mail to assess the current situation of the red rice problem and early impact of imidazolinone-resistant (IMR) rice technology on red rice infestation. The information generated represented 40% (226,800 ha) of rice production areas in Arkansas. Barnyardgrass and red rice were the most problematic weeds, with 62% of fields infested with red rice. The estimated economic loss due to red rice averaged $274/ha. Red rice infestation was prevented mostly by crop rotation (96%) and use of certified seed (86%). Of the red rice–infested fields, 38% had light infestation and 26% had severe red rice problems before adopting IMR rice. Thirty-seven percent of infested fields had been planted with IMR rice once and 43% at least twice. Approximately 85% of the consultants reported > 90% red rice control when using IMR rice. The majority (92%) of IMR rice growers rotate to other crops, mostly soybean. Unsuitable field condition was the main reason for growing only rice. After 3 seasons, the consultants perceived that red rice infestation level declined by 77% on average. The herbicide-resistance gene had escaped to red rice in some fields, and 90% of growers are exerting effort to mitigate outcrossing. Nomenclature: Barnyardgrass, Echinochloa crus-galli (L.) Beauv. ECHCG, red rice, Oryza sativa L. ORYSA, rice, Oryza sativa L, soybean, Glycine max (L.) Merr
Bushkiller was evaluated under inter- and intraspecific competition. In experiment 1, bushkiller, trumpetcreeper, and wild grape were greenhouse-grown alone and in two or three species mixtures in pots. Of the three species, bushkiller grew the tallest and had the greatest final biomass when grown alone. When all three species were grown together, bushkiller grew over twice the height of trumpetcreeper, over three times the height of wild grape, and over four times the biomass of either competing species. Plots of height over time showed that competition did not affect bushkiller or wild grape growth rate, but trumpetcreeper growth was reduced when grown with bushkiller. In experiment 2, bushkiller was grown in cultures of one, two, and three plants per pot to determine intraspecific competition effects on growth. Final height of bushkiller was not affected by intraspecific competition; however, bushkiller biomass decreased with increasing competition. Nomenclature: Bushkiller, Cayratia japonica (Thunb.) Gagnep.
Transgene integration into plant genomes is a complex process accompanied by molecular rearrangements. Classic methods that are normally used to study transgenic population genetics are generally inadequate for assessing such integration. Two major characteristics of transgenic populations are that a transgenic genome may harbor many copies of the transgene and that molecular rearrangements can create an unstable transgenic locus. In this work, we examined the segregation of T1, T2 and T3 transgenic tobacco progenies. Since transfer DNA (T-DNA) contains the NptII selectable marker gene that confers resistance to kanamycin, we used this characteristic in developing a method to estimate the number of functional inserts integrated into the genome. This approach was based on calculation of the theoretical segregation ratios in successive generations. Mendelian ratios of 3:1, 15:1 and 63:1 were confirmed for five transformation events whereas six transformation events yielded non-segregating progenies, a finding that raised questions about causal factors. A second approach based on a maximum likelihood method was performed to estimate recombination frequencies between linked inserts. Recombination estimates varied among transformation events and over generations. Some transgenic loci were unstable and evolved continuously to segregate independently in the T3 generation. Recombination and amplification of the transgene and filler DNA yielded additional transformed genotypes.
Conquering Rice Sterility
The hybrid sterility occurring among rice species has long been a puzzle and hampers progress in breeding crops with improved performance and yield characteristics. Yang et al. (p. 1336 ) have identified three linked genes encoding a killer, a partner, and a protector protein. The killer and partner work together to kill female gametes not carrying the functional protector, resulting in preferential transmission of gametes carrying the functional protector, which also causes segregation distortion in the progeny. This explanation for how reproductive isolation is maintained among species of rice, and perhaps other organisms, also offers approaches for boosting yields by intersubspecific heterosis.
Gene flow from transgenic crops allows novel traits to spread to sexually compatible weeds. Traits such as resistance to insects may enhance the fitness of weeds, but few studies have tested for these effects under natural field conditions. We created F2 and F3 crop–weed hybrid lineages of genetically engineered rice (Oryza sativa) using lines with two transgene constructs, cowpea trypsin inhibitor (CpTI) and a Bt transgene linked to CpTI (Bt/CpTI). Experiments conducted in Fuzhou, China, demonstrated that CpTI alone did not significantly affect fecundity, although it reduced herbivory. In contrast, under certain conditions, Bt/CpTI conferred up to 79% less insect damage and 47% greater fecundity relative to nontransgenic controls, and a 44% increase in fecundity relative to the weedy parent. A small fitness cost was detected in F3 progeny with Bt/CpTI when grown under low insect pressure and direct competition with transgene-negative controls. We conclude that Bt/CpTI transgenes may introgress into co-occurring weedy rice populations and contribute to greater seed production when target insects are abundant. However, the net fitness benefits that are associated with Bt/CpTI could be ephemeral if insect pressure is lacking, for example, because of widespread planting of Bt cultivars that suppress target insect populations.
The partial sterility of hybrids between the indica and japonica rice subspecies of Asian cultivated rice is a serious constraint
for utilizing inter-subspecific heterosis in hybrid rice breeding. In this study, we have investigated the relationship between
molecular-marker polymorphism and indica-japonica hybrid fertility using a diallel set involving 20 rice accessions including
9 indica and 11 japonica varieties. Spikelet fertility of the resulting 190 F1s and their parents was examined in a replicated field trial. Intra-subspecific hybrids showed much higher spikelet fertility
than inter-subspecific hybrids except in crosses involving wide-compatibility varieties. The parents were surveyed for DNA
polymorphism using 96 RFLP and ten SSR markers, which revealed extensive genetic differentiation between indica and japonica
varieties. A large number of markers detected highly significant effects on hybrid fertility. The chromosomal locations for
many of the positive markers coincided well with previously identified loci for hybrid sterility. The correlation between
hybrid fertility and parental distance was low in both intra- and inter-subspecific crosses. The results suggest that the
genetic basis of indica-japonica hybrid sterility is complex. It is the qualitative, rather than the quantitative, difference
between the parents that determines the fertility of hybrids.
Hybridization is hypothesized to promote invasiveness, but empirical tests comparing the performance of hybrid taxa versus parental taxa in novel regions are lacking. We experimentally compared colonization ability of populations of wild radish (Raphanus raphanistrum) with populations of advanced-generation hybrids between wild radish and cultivated radish (Raphanus sativus) in a southeast Texas pasture, well beyond the known invasive range of hybrid radish. We also manipulated the strength of interspecific competition to better generalize across variable environments. In both competitive environments, hybrid populations produced at least three times more seeds than did wild radish populations, a distinction that was driven by greater hybrid seedling emergence, earlier hybrid emergence, and more hybrid seedlings surviving to flower, rather than by greater individual fecundity. Flowering duration in hybrids was less negatively affected by competition than it was in wild radish, while early emergence was associated with subsequent high seed output in both biotypes. Our data show that hybridization can enhance colonization success in a novel region and, by comparison with previous studies, that the life-history traits enhancing hybrid success can differ across regions, even for lineages originating from the same hybridization event. These results imply a much larger arena for hybrid success than previously appreciated.
Liaoning and Jiangsu are main regions seriously infested by weedy rice (Oryza sativa f. spontanea) in China. Weedy rice samples collected from 29 sites in 14 cities of the two provinces were cultivated in Nanjing, and 23 traits during vegetative and reproductive growth stage were observed and measured in order to reveal the diversity and identification traits of weedy rice. The results showed that there were differences of traits in terms of plant height after one month, tiller number after one month or two months during vegetative stage, and in terms of pericarp color, grain length/width ratio, 100-seed weight, shattering habit, culm rigidity, flag leaf width, flag leaf length, yellow maturity, half-yellow maturity–half-heading, full yellow maturity during reproductive stage between weedy rice and cultivated rice in the two provinces. The scatter plot of the first and second principlal components (their cumulative contribution accounting for 43.24%) also showed that there were differences between Liaoning and Jiangsu weedy rice, as well as between weedy and cultivated rice of the two provinces. Cluster analysis based on Euclidean distance showed that weedy rice populations could be divided into two types: Japonica-like and Indica-like weedy rice. Liaoning weedy rice populations fall into first category. Jiangsu weedy rice populations fall into both categories. Weedy rice of the two provinces can be further divided into 6 classes: (I) the Japonica-like type with strong shattering, light grain weight, short stem and prematurity; (II) the Japonica-like type with no awn, many panicle, late maturity, narrow flag leave and strong dormancy; (III) the Japonica-like type with long awn, weak tillering, few pancle and weak shattering; (IV) the Indica-like type with hard culm, wide flag leave, late maturity and strong dormancy; (V) the Indica-like type with red pericarp, long grain, soft culm, and heavy grain weight; (VI) the Indica-like type with no awn, tall stem, long flag leave and long panicle. The study on the morphological index and classification of weedy rice in the two provinces may provide theoretical basis for prevention and control of weedy rice.
The compatibility and outcrossing rates between transgenic rice and weedy rice biotypes have been studied in some previous cases. However, few studies have addressed the reasons for these differences. The present study compared the compatibility and outcrossing rates between transgenic rice and selected weedy rice biotypes using manual and natural crossing experiments to elucidate the key innate factors causing the different outcrossing rates.
Hybrid seed sets from manual crossing between transgenic rice and weedy rice varied from 31.8 to 82.7%, which correlated directly with genetic compatibility. Moreover, the significant differences in the quantity of germinated donor pollens and pollen tubes entering the weedy rice ovule directly contributed to the different seed sets. The natural outcrossing rates varied from 0 to 6.66‰. The duration of flowering overlap was the key factor influencing natural outcrossing. Plant and panicle height also affected outcrossing success.
From this study, it is concluded that the likelihood of gene flow between transgenic rice and weedy rice biotypes is primarily determined by floral synchronisation and secondarily influenced by genetic compatibility and some morphological characteristics.
The rapid development of transgenic biotechnology has greatly promoted the breeding of genetically engineered (GE) rice in China, and many GE rice lines are in the pipeline for commercialization. To understand field performances of GE rice, key agronomic traits of two insect-resistant Bt rice lines that have been granted biosafety certificates for commercial production in China were evaluated together with their nontransgenic counterparts under environmental conditions with significant differences in insect pressure. Results from the experiments showed enhanced field performances of the Bt GE rice lines compared with the non-GE counterparts for yield-related traits such as number of panicles and filled seeds per plant, under environmental conditions with no insecticide application. No detectable underlying cost of the Bt transgene was observed in the two insect-resistant GE rice lines, particularly in the GE hybrid rice line. Results further indicated significantly greater yield performances of the two insect-resistant GE rice lines under environmental conditions with non-target insect control compared with no insect control. It is concluded from this study that insect-resistant Bt GE rice, particularly the hybrid line, has great potential to maintain its high yield when ambient insect pressure is high. In addition, proper application of insecticides to control non-target insects will guarantee optimal performance of insect-resistant Bt GE rice.
We strive to predict consequences of genetically modified plants (GMPs) being cultivated openly in the environment, as human and animal health, biodiversity, agricultural practise and farmers' economy could be affected. Therefore, it is unfortunate that the risk assessment of GMPs is burdened by uncertainty. One of the reasons for the uncertainty is that the GMPs are interacting with the ecosystems at the release site thereby creating variability. This variability, e.g. in gene flow, makes consequence analysis difficult. The review illustrates the great uncertainty of results from gene-flow analysis.
Many independent experiments were performed on the individual processes in gene flow. The results comprise information both from laboratory, growth chambers and field trials, and they were generated using molecular or phenotypic markers and analysis of fitness parameters. Monitoring of the extent of spontaneous introgression in natural populations was also performed. Modelling was used as an additional tool to identify key parameters in gene flow.
The GM plant may affect the environment directly or indirectly by dispersal of the transgene. Magnitude of the transgene dispersal will depend on the GM crop, the agricultural practise and the environment of the release site. From case-to-case these three factors provide a variability that is reflected in widely different likelihoods of transgene dispersal and fitness of introgressed plants. In the present review, this is illustrated through a bunch of examples mostly from our own research on oilseed rape, Brassica napus. In the Brassica cases, the variability affected all five main steps in the process of gene dispersal. The modelling performed suggests that in Brassica, differences in fitness among plant genome classes could be a dominant factor in the establishment and survival of introgressed populations.
Up to now, experimental analyses have mainly focused on studying the many individual processes of gene flow. This can be criticised, as these experiments are normally carried out in widely different environments and with different genotypes, and thus providing bits and pieces difficult to assemble. Only few gene-flow studies have been performed in natural populations and over several plant generations, though this could give a more coherent and holistic view.
The variability inherent in the processes of gene flow in Brassica is apparent and remedies are wished for. One possibility is to expose the study species to additional experiments and monitoring, but this is costly and will likely not cover all possible scenarios. Another remedy is modelling gene flow. Modelling is a valuable tool in identifying key factors in the gene-flow process for which more knowledge is needed, and identifying parameters and processes which are relatively insensitive to change and therefore require less attention in future collections of data. But the interdependence between models and experimental data is extensive, as models depend on experimental data for their development or testing.
More and more transgenic varieties are being grown worldwide harbouring genes that might potentially affect the environment (e.g. drought tolerance, salt tolerance, disease tolerance, pharmaceutical genes). This calls for a thorough risk assessment. However, in Brassica, the limited and uncertain knowledge on gene flow is an obstacle to this. Modelling of gene flow should be optimised, and modelling outputs verified in targeted field studies and at the landscape level. Last but not least, it is important to remember that transgene flow in itself is not necessarily a thread, but it is the consequences of gene flow that may jeopardise the ecosystems and the agricultural production. This emphasises the importance of consequence analysis of genetically modified plants.
dSupply and demand for resources change dynamically throughout the growing season. If we are to understand the differences in competitiveness among crop cultivars, we need to study the dynamics of species interactions, not just their outcomes. We examined the temporal patterns in competition between wheat (Triticum aestivum L.) and oat (Avena sativa L.) in a series of field experiments. The combined effects of shifts in height growth and phenological development were described for monocultures and mixtures by varying the sowing dates of the species. Similar observations were made for oat competing with wheat lines differing in flowering time and for wheat competing with a range of Avena taxa. In contrast to earlier work, no reversals of competitive hierarchy were seen during the season. The species that achieved the greater biomass early on remained the better competitor throughout growth. The hypothesis that a delay in emergence will move the timing of such reversals was thus not supported. There were few differences in patterns of the dynamics of competition among Avena taxa and no observable differences in competitiveness among wheat lines differing by up to 6 d in flowering date. Overall, the results support the idea that reversals in competitive hierarchy during the season are caused by relative patterns in plant height growth of competitors and may occur only in systems in which competition for light is dominant. Otherwise, an early competitive hierarchy will be maintained throughout the growing season.
Wild oat reduced light penetration and growth of dwarf hard red spring wheat in field experiments performed under nonlimiting nitrogen and moisture conditions. Wild oat grew taller than wheat and had a greater portion of its canopy above 60 cm at maturity. Light penetration in a mixed canopy was similar to that in a monoculture wheat canopy when wild oat was clipped to the height of the wheat. A mathematical model was developed which accurately predicted the reduction in the growth rate of wheat from wild oat interference. The model also predicted that interference from wild oat was due to reduced leaf area of wheat at early growth stages and low wild oat densities, and reduced light penetration to wheat leaves at later growth stages and higher densities of wild oat.
Previous studies in the United Kingdom (UK) and Australia have led to the hypotheses that reversals in competitive hierarchy between wheat (Triticum aestivum L.) and oat (Avena spp.) could occur only in situations in which (i) there is a reversal in the relative heights of the competitors during the year and (ii) there is strong competition for light. Reversals in competitive hierarchy should not, therefore, occur where the wheat is either taller or shorter than oat throughout growth. We grew near-isogenic lines of wheat for height in monoculture and in replacement mixtures with oat (A. sativa L. and A. strigosa Schreb.) in 2 yr. In the drier year and with a late-maturing oat cultivar, height of wheat had no influence over its competitive superiority, which was maintained throughout the season. In a somewhat wetter year and with an earlier-maturing oat cultivar, the shortest wheat lines were less competitive than corresponding taller near-isolines. Results were consistent for near-isogenic lines in three contrasting genetic backgrounds. Partial reversals in competitive hierarchy were seen in a late sowing, but these did not correspond with patterns in the relative height growth of the species.
Reproductive barriers between distant groups of rice are anticipated to be overcome through a systematic use of widely compatible varieties in breeding programs. For this purpose a total of 74 varieties were screened for compatiblity types in terms of F1 fertility by crossing them to one each of indica and japonica testers. The compatibility of a variety to a tester was rated by pollen and spikelet fertility of the F1 hybrid. The pollen fertility of more than 90 percent and the spikelet fertility of 75-80 percent were rated normal. The compatibility tests showed that the pollen fertility is independent of the spikelet fertility. Therefore, each variety was inspected for four fertility scores, namely, the pollen and the spikelet fertility in each cross to an indica and a japonica tester. Out of 24 Indonesian upland varieties, 15 showed normal pollen fertility with both testers, and normal spikelet fertility with a japonica tester, but gave remarkable spikelet sterility with an indica tester. Six showed semi-sterility in all the four scores. Only one variety, Padi Bujang Pendek revealed normal fertility in all the four scores. The remaing two seemed to belong to an exceptional type. A total of 27 Aus varieties showed many types of compatibility including six varieties of indica type and five of japonica. Five varieties revealed sterility in all the four scores. Only two, Aus 373 and Dular seemed to be widely compatibe. The remaining nine were not classified into any difinite categories. In the test of 15 varieties which are identified tolerant of salinity, drought or peat soil, ten of strongly photoperiod-sensitive varieties were classified into typical indica rice. Two photoperiod-nonsensitiwe varieties were clearly identified to be japonica. In additional tests of some varieties which are notable from previous works, Calotoc, CPSLO 17 and Ketan Nangka were confirmed to be widely compatible. The screening results indicate that the Aus group of rice is a complex of various compatibility types, while a majority of Indonesian upland rices seemed to be of a type which is closer to japonicas rather than iledicas. Only a few varieties were identified as the wide-compatibility type, contrary to the expectation based on earlier works. It was discussed that the upland cultivation has permitted the Aus group to preserve the diverse compatibility types, while photoperiod-sensitive lowland varieties are predominantly of iledica type.
Weedy rice has been becoming a notorious weed in the paddy field of China in recent decades due to its increasing damage to rice yield and rice quality. In this study, a microsatellite technique with 21 pairs of SSR markers was utilized to estimate the genetic structure of two biotypes of weedy rice with Japonica and Indica rice characteristics, collected from Liaoning and Guangdong provinces, respectively. The genetic diversity of the weedy rice in the two provinces was relatively low (Liaoning h = 0.086; Guangdong h = 0.160), and distinctly large genetic differences existed between these two provinces (Gcs = 0.623). The genetic diversity was found primarily within populations, and genetic differentiation was relatively low within the same province. Both cluster analysis (UPGMA) and principle component analysis (PCA) showed that weedy rice had a closer relationship with the cultivated rice collected from the sample field than with other cultivated rice and common wild rice varieties in China. Thus, the results of this study on samples from the Liaoning and Guangdong provinces in China support the de-domestication hypothesis that weedy rice most probably originated from local cultivated rice.
Genetic diversity and differentiation in indica and japonica groups of the cultivated rice (Oryza sativa L.) were studied by assaying DNA restriction fragment length polymorphisms of 12 indica and 14 japonica rice lines digested with three restriction endonucleases. A total of 49 probes were selected to represent the entire RFLP map at intervals of 20-30 cM. It was shown that 95 of the 145 possible probe/enzyme combinations, involving 43 probes and all three enzymes, detected restriction fragment length variation, and the degree of polymorphism varied greatly from one probe/enzyme combination to another. These results demonstrate that indica rice is genetically more diverse than japonica type. Significant differentiation between the two rice groups was detected by 33 probes representing 11 of the 12 rice chromosomes. It was deduced that the processes leading to differentiation involved a combination of molecular events that include base substitutions and insertion/deletions.
Understanding the potential for the evolution of herbicide tolerance in weeds following gene flow from transgenic crops is important for their management. The inter-specific hybrids formed between four Brassica juncea populations with different glyphosate-susceptibility and transgenic oilseed rape was studied, emphasizing on the possible population difference. Compared with both parents, the fitness-related components and photosynthetic capacity of hybrids decreased dramatically, but significant differences were observed among different hybrids. All F1 hybrids exhibited highly enhanced but similar herbicide tolerance level regardless of the wild B. juncea parent, indicating that genetically engineered herbicide tolerance may over-dominate phenotype ones by wild-crop hybridization. The analysis of parental loci transmission revealed a higher transfer ratio of male-specific loci detected in F1 hybrids, suggesting that oilseed rape genetic markers can be transferred at relatively high frequencies to the next generation. Therefore, higher transfer ratio of oilseed rape-specific loci, coupled with variation of populations in fitness-related parameters in F1 hybrids, could complicate environmental risk assessment of transgenic oilseed rape, especially in current agroecosystems with increasing application of glyphosate.
Supply and demand for resources change dynamically throughout the growing season. If we are to understand the differences in competitiveness among crop cultivars, we need to study the dynamics of species interactions, not just their outcomes. We examined the temporal patterns in competition between wheat (Triticum aestivum L.) and oat (Avena sativa L.) in a series of field experiments. The combined effects of shifts in height growth and phenological development were described for monocultures and mixtures by varying the sowing dates of the species. Similar observations were made for oat competing with wheat lines differing in flowering time and for wheat competing with a range of Avena taxa. In contrast to earlier work, no reversals of competitive hierarchy were seen during the season. The species that achieved the greater biomass early on remained the better competitor throughout growth. The hypothesis that a delay in emergence will move the timing of such reversals was thus not supported. There were few differences in patterns of the dynamics of competition among Avena taxa and no observable differences in competitiveness among wheat lines differing by up to 6 d in flowering date. Overall, the results support the idea that reversals in competitive hierarchy during the season are caused by relative patterns in plant height growth of competitors and may occur only in systems in which competition for light is dominant. Otherwise, an early competitive hierarchy will be maintained throughout the growing season.
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In this study, we simulate a transgenic rice crop highly infested with red rice to examine transgene transfer from a transgenic line (A2504) resistant to glufosinate ammonium to cohabitant red rice. The red rice was sown along with the transgenic line at the highest density found in naturally infested crops in the region. Agricultural practices similar to those used to control red rice infestation in northern Italy rice fields were used to reproduce the local rice production system. During the first 2 years, the field was treated with herbicide at the appropriate time; in the first year the dosage of herbicide was three times the recommended amount. In this first year, detectable red rice plants that escaped herbicide treatment were manually removed. Nevertheless, two herbicide-resistant hybrid plants (named 101 and 104) were identified in the experimental field during the second year of cultivation. Phenotypic and molecular characterisation suggests the hybrid nature of these two plants, deriving from crossing events involving A2504, respectively, with red rice (plant 101) and the buffer cultivar Gladio (plant 104). The progeny of two subsequent generations of the two plants were examined and the presence of the transgene detected, indicating stable transfer of the transgene across generations. In conclusion, despite control methods, red rice progeny tolerant to the herbicide can be expected following use of transgenic rice and, consequently, difficulties in controlling this weed with chemicals will emerge in a relatively short time.
As a weed of rice paddy fields, weedy rice has spread worldwide. In northern China, the expansion of weedy rice has been rapid over the past two decades. Its evolutionary history and adaptive mechanisms are poorly understood.
Evolutionary relationships between northern weedy rice and rice cultivars were analyzed using presumed neutral markers sampled across the rice genome. Genes involved in rice domestication were evaluated for their potential roles in weedy rice adaptation. Seed longevity, a critical trait of weedy rice, was examined in an F 2 population derived from a cross between weedy rice and a rice cultivar to evaluate weedy rice adaptation and the potential effect of candidate genes.
Weedy rice in northern China was not derived directly from closely related wild Oryza species or from the introgression of indica subspecies. Introgression with local cultivars, coupled with selection that maintained weedy identity, shaped the evolution of weedy rice in northern China.
Weedy rice is a unique system with which to investigate how weedy plants adapt to an agricultural environment. Our finding that extensive introgression from local cultivars, combined with the continuing ability to maintain weedy genes, is characteristic of weedy rice in northern China provides a clue for the field control of weedy rice.
Imidazolinone-herbicide-resistant Clearfield™ (CL) rice allows the selective chemical control of red rice (Oryza sativa L.), a major weed problem in rice-growing areas of the southern US, using herbicides that otherwise are toxic to the crop. However, selection imposed by the herbicide of resistant individuals resulting from gene flow, and of naturally tolerant individuals, is a major concern. Experiments were conducted at the Rice Research and Extension Center, Stuttgart, AR, US in 2002–2004 to evaluate the effects of cultivar, distance from pollen source, and planting date on natural outcrossing between CL rice and Stuttgart strawhull red rice. The experiments were planted in a split-plot design with planting date (April and May) as main plot and cultivar (CL121 and CL161) as subplots. An encircle-population combination technique was used to determine distance from the rice pollen source at which hybrids were detected. Resistance of the F1 plants was determined by repeated spraying of imazethapyr, and DNA analysis using SSR primer RM 180 to confirm hybrids. All resistant red rice plants were confirmed hybrids. Outcrossing was higher with CL161 (0.008%) than with CL121 (0.003%) regardless of planting date. Averaged over cultivars, outcrossing rate between April (0.004%) and May (0.006%) plantings did not differ. The majority of hybrids were located within 1 m from CL rice. However, a few hybrids were found up to 6 m from the CL rice, which was the limit of detection in these experiments. All F1 hybrids were taller and had longer flag leaves than their parents. F1 hybrids had rough texture and pale-colored leaves similar to the red rice parent. Gene flow between CL rice and red rice is a concern, and even at the low outcrossing rates in these experiments, approximately 170 resistant plants/ha resulted.
Abstract Natural hybrid zones between related species illustrate processes that contribute to genetic differentiation and species formation. A common viewpoint is that hybrids are essentially unfit, but they exist in a stable tension zone where selection against them is balanced by gene flow between the parent species. An alternative idea is that selection depends on the environment, for example, by favoring opposite traits in the two parental habitats or favoring hybrids within a bounded region. To determine whether selection of hybrids is environment dependent, we crossed plants of naturally hybridizing Ipomopsis aggregata and I. tenuituba in the Colorado Rocky Mountains and reciprocally planted the seed offspring into a suite of natural environments across the hybrid zone. All types of crosses produced similar numbers and weights of seeds. However, survival of the offspring after 5 years differed markedly among cross types. On average, the F1 hybrids had survival and growth rates as high as the average for their parents. But hybrid survival depended strongly on the direction of a cross, that is, on which species served as the maternal parent. This fitness difference between reciprocal hybrids appeared only in the parental environments, suggesting cytonuclear gene interactions that are environment specific. These results indicate that complex genotype-by-environment interactions can contribute to the evolutionary outcome of hybridization.
There is much interest in the inter-varietal fertility of Asian cultivated rice. In this study, we analyzed the fertility of hybrids in a diallel set of 210 crosses involving 21 parents representing a broad range of the cultivated rice germplasm including landraces, primitive cultivars, modern elite cultivars and parents of hybrid rice. The materials were also carefully selected with respect to indica-japonica classification including typical indica/japonica, more or less intermediate and wide compatibility varieties. The level of hybrid fertility varied widely among the crosses from almost completely sterile to fully fertile. In general, hybrid fertility of intra-subspecific crosses, i.e., indica by indica (II) and japonica by japonica (JJ), is much higher than inter-subspecific crosses (IJ or JI). The fertility varied widely in hybrids involving wide compatibility varieties and also in inter-subspecific crosses not involving wide compatibility varieties. An analysis of variance showed that both the main effects of indica and japonica parents and the interaction between the parents are highly significant in determining hybrid fertility. We speculate that, in addition to wide compatibility, the overall genetic difference between the indica and japonica parents, that may involve a series of minor mutations, play an important role in determining the hybrid fertility. There are also genes of sizable effect that influence fertility in hybrids resulting from specific combinations of the parents.
The objective of this study was to assess the frequency of pollen-mediated gene flow from a transgenic rice line, harbouring the gusA and the bar genes encoding respectively, -glucuronidase and phosphinothricin acetyl transferase as markers, to the red rice weed and conventional rice in the Spanish japonica cultivar Senia. A circular field trial design was set up to investigate the influence of the wind on the frequency of pollination of red rice and conventional rice recipient plants with the transgenic pollen. Frequencies of gene flow based on detection of herbicide resistant, GUS positive seedlings among seed progenies of recipient plants averaged over all wind directions were 0.036 0.006% and 0.086 0.007 for red rice and conventional rice, respectively. However, for both red rice and conventional rice, a clear asymmetric distribution was observed with pollination frequency favoured in plants placed under the local prevailing winds. Southern analyses confirmed the hemizygous status and the origin of the transgenes in progenies of surviving, GUS positive plants. Gene flow detected in conventional rice planted at 1, 2, 5 and 10 m distance revealed a clear decrease with increasing distance which was less dramatic under the prevailing wind direction. Consequences of these findings for containment of gene flow from transgenic rice crops to the red rice weed are discussed. The precise determination of the local wind conditions at flowering time and pollination day time appear to be of primary importance for setting up suitable isolation distances.
Clearfield rice technology is a popular method for controlling noxious red rice weeds (Oryza sativa L.) in commercial rice fields in the southern U.S. Previous research has detected red rice-Clearfield variety F1 hybrids at low rates in Louisiana and Arkansas. The first research objective was to determine genetic control of imazethapyr
resistance in F1 hybrids and F2 populations derived from natural and controlled hybridizations of red rice and Clearfield rice. The second objective was
to characterize and compare agronomic performance of the hybrids and their progeny with Clearfield varieties. Genetic analysis
showed that imazethapyr resistance was dominant in all tested F1 hybrids with single and two-gene inheritance observed across composite F2 populations. F1 hybrids exhibited high levels of variation for plant height, heading date, and seven reproductive traits. Heterosis was observed
in the hybrids versus Clearfield varieties for plant height, heading date, seed-bearing tillers, panicle length, and spikelets/panicle.
While seed production of the F1 hybrids was generally inferior to that of the commercial varieties, one red rice/CL121 hybrid produced greater seeds/panicle
than the CL121 commercial parent. Extensive variation was observed for all measured traits in the F2 populations derived from either natural or controlled crosses. Results from this study indicate that red rice/Clearfield F1 hybrids normally exhibit low reproductive seed capacity, but a small proportion of the subsequent F2 progeny can produce high fecundity levels. Effective stewardship practices are therefore warranted to reduce the occurrence
of such hybrids and their offspring to ensure continued success of the Clearfield technology.
In this paper, we described the breeding of transgenic rice restorer line for multiple resistance against bacterial blight,
striped stem borer (SSB) and herbicide by conventional crossing of two transgenic parental lines transformed independently
with different genes. Two stable transgenic rice lines used as donor parents were developed, one was Zhongguo91 which contained
cry1Ab gene (for insect resistance) and bar gene (for tolerance of herbicide), and the other was Yujing6 which contained Xa21 gene (resistance to bacterial blight). The elite restorer line Hui773 was used as recipient and crossed with the two stable
transgenic rice lines. Then five successive backcrosses were made using Hui773 as recurrent parent. Two rice elite restorers,
T773-1 expressing cry1Ab and bar genes and T773-2 expressing Xa21 gene, were obtained, which were confirmed by PCR analysis and testing selectable marker genes in the hybrid progenies. The
cross was made between T773-1 and T773-2 to select stable restorer line carrying Xa21, cry1Ab and bar genes. Finally, we obtained transgenic restorer line T773 with good agronomic traits and obvious multiple resistance to Xanthomonas oryzae pv. oryzae, striped stem borer (Chilo suppressalis) and herbicide. The hybrid F1 generation produced from the cross between transgenic restorer line T773 and a corresponding male sterile line Zaohua2A maintained
obvious resistance to rice bacterial blight, rice leaffolder and striped stem borer, and showed significant heterosis. Our
results indicate that it is feasible to develop transgenic hybrid rice cultivar through breeding transgenic restorer lines.
Hybridization between Clearfield rice and weedy red rice would have a direct impact on management and long-term strategies of imazethapyr technology for rice weed control. The objective of this research was to determine rates and agronomic consequences for outcrossing between Clearfield rice and red rice. Red rice populations showed extensive variation for plant height, panicle length, tillers/plant, seeds/plant, seed set and grain weight. Outcrossing was detected from all Clearfield rice cultivars (‘CL121’, ‘CL141’, ‘CL161’, and ‘CLXL8’) to red rice and was confirmed by phenotypic and DNA marker analyses. An overall outcrossing frequency of 0.17% was observed in 2002 red rice samples with a range from 0% to 0.46%. Tolerance of 2002 red rice samples to imazethapyr corresponded to levels of acetohydroxyacid synthase (AHAS) activity. A majority (94%) of the progeny from the 2002 samples segregated 3 resistant:1 susceptible for tolerance to imazethapyr, indicating that a single dominant gene from Clearfield rice was associated with tolerance in the hybrid material. The remaining samples did not segregate for tolerance, suggesting that spontaneous mutations for tolerance were present in this material before or after crossing with Clearfield rice. A four-fold increase in outcrossing frequency of 0.68% was observed in 2003 red rice samples with the highest outcrossing frequency for a single location at 3.2%. Results from this study indicate that outcrossing between Clearfield and red rice will occur rapidly at rates that warrant early-season field scouting and a crop rotation scheme to prolong usefulness of the Clearfield technology.
A critical step in the development of a robust Agrobacterium tumefaciens-mediated transformation system for a recalcitrant species, like barley, is the establishment of optimal conditions for efficient T-DNA delivery into target tissue from which plants can be regenerated. We used Agrobacterium strain LBA4404 harbouring either binary vector pUGAB7, which contains a T-DNA incorporating the bar gene and uidA gene, or pYF133, which contains hpt gene and gfp gene. These were used to investigate and optimize major T-DNA delivery and tissue culture variables in the immature embryos of barley (cv. Golden Promise). A number of factors produced significant difference in T-DNA delivery. These included pre-culture of immature embryos, co-cultivation, presence of acetosyringone and sonication and vacuum filtration assisted inoculation of 1-day precultured immature embryos. Plant regeneration was achieved via somatic embryogenesis. Regeneration media showed significant difference in their capacity to support regeneration of transgenic plants. Optimizing factors for T-DNA delivery resulted in bialaphos and hygromycin resistant barley transgenic plants with transformation efficiencies ranging from 2.6 to 5.6% and 3.3 to 6.7%, respectively. Molecular analysis of transgenic plants demonstrated that transgenes were integrated into the barley genome and subsequently transmitted into progeny at Mendelian ratios. The developed protocol will facilitate the insertion of desirable traits into barley via Agrobacterium-mediated transformation.
Over 30 root and shoot growth traits of nine diverse rice cultivars (cvs) grown under competitive conditions were analyzed to exhaustively quantify cv differences and identify which traits most affected rice growth in competition. The cvs included a traditional Oryza glaberrima, CG14, and selections from three isozyme groups of O. sativa: aus, indica, and tropical japonica. Single rice plants were grown in pots with and without nitrogen fertilizer, surrounded by six plants of either barnyardgrass (Echinochloa crus-galli), in trials 1 and 2, or themselves, in trial 3. Growth traits were measured weekly or in harvests at 21 and 48 days after seeding (DAS). Significant differences amongst cvs were found for nearly every response variable, and the most productive cvs lacked uniform growth traits. Total dry mass (DM) was primarily a function of leaf area (LA, m−2 per plant), but height growth rates (HGRs, m per day) at 7 and 21 DAS and tillering ability were also consistently important. Non-destructively measured traits, such as tillering, and HGRs at 7 and 21 DAS, accounted for at least 75% of the variation in DM across all trials. Overall, rice cv growth in competition depended on multiple traits, and in particular was not simply a function of specific leaf area (SLA, m2 g−1 leaf). Early vigor was highly repeatable in both competition and monoculture trials, indicating that it may be used to discriminate between more and less competitive cvs even in monoculture. Therefore, screening lines in standard nurseries for competitiveness against weeds seems possible using non-destructive scoring of plant cover, very early heights (i.e., vigor), tillering ability, and early HGRs.
The genetic and agronomic consequences of transferring glufosinate (Liberty™) herbicide resistance from transgenic rice (Oryza sativa L.) lines to the noxious weed red rice (Oryza sativa L.) were evaluated under field conditions. Replicated field trials in Louisiana (LA) and Arkansas (AR) were conducted in 1997 to evaluate ten vegetative and reproductive traits of eight F2 populations produced from controlled crosses of two transgenic, glufosinate-resistant rice lines and four red rice biotypes. Plant vigor and plant density at both locations were similar among populations derived from either transgenic or non-transgenic parents. Significant differences in plant height and maturity were observed among LA populations produced from transgenic lines when compared to corresponding populations developed from non-transgenic material. However, values for these traits were not greater than those detected in the red rice biotypes. Seed dormancy and seed production were not significantly different at either location among transgenic and non-transgenic populations. Dominant Mendelian segregation of glufosinate resistance was detected in 40% of the populations evaluated. Results of this study indicated that those populations segregating for glufosinate resistance responded in a location-specific manner with respect to life history and fecundity traits.
The model DSRICE1 was developed for analyzing integrated weed management strategies for direct-seeded rice. We have shown that DSRICE1 predicts monoculture rice growth well and accounts for water-depth effects on growth. Here, the model is used to simulate competition for light between rice and two weeds, Echinochloa oryzoides (early watergrass) and Ammannia spp. (redstem). Except for minor differences in phenology, weed growth was simulated as described for rice. Direct competition for light depended on the species' vertical distributions of leaf and stem areas (live and dead) and their extinction coefficients. Water also attenuates light, so species' early height growth rates were important because they determined when plants emerged into full light. Structural sensitivity analyses of rice in competition with the two weeds revealed that water-depth effects and leaf area distributions strongly affected competition, and shading by dead leaf and stem dry mass reduced total production. Validation was based on independent data sets for redstem and watergrass competition using several statistical tests and indices. For rice–redstem competition, DSRICE1 simulated rice growth well because redstem competitive effects were small, but predictions of redstem growth were good only when observed heights were matched in simulations. Redstem competitiveness depended on height growth rate, perhaps due to its small seed size. For rice–watergrass competition, the growth of both species was predicted well, except that watergrass growth in plots with early-season drainage was underpredicted. Watergrass parameters were similar to those for rice except for faster height growth and higher photosynthesis rates. In a model application, simulations in which rice seeding was delayed for a time after flooding led to greater yield losses from redstem than from watergrass because delays reduced the advantage of rice over redstem. The usefulness of DSRICE1 for drained fields will be improved by better simulation of plant growth responses to drainage, but rice competition with redstem and watergrass in continuously-flooded fields was simulated well.
Two representative weedy rice lines, three typical japonica varieties and three typical indica varieties were used for 6 pairs of reciprocal crosses. The morphological traits of twelve F(1) hybrid lines, their parents and four elite cultivars were investigated for heterosis over mid-parent (HM), over parent (HP) and competitive heterosis (CH) analysis. Traits detected in weedy rice lines seemed larger than those in cultivars and excellent heterosis was produced in weedy rice crossing with japonica rice. Although weedy rice kept closer relationships with japonica rice compared to indica rice. But the heterosis of reciprocal crosses between weedy rice and japonica was closed to those of crosses between indica rice and japonica rice. In six of one hundred and eighteen weedy rice lines, the fertility restore gene for BT type cytoplasmic male sterility (BT-CMS) were detected. Weedy rice was very valuable germplasm resources with the abundant polymorphism. Meanwhile, the disadvantage, lodging, shattering and incompact plant type, should be modified by hybridization, backcross and multiple cross with japonica rice. Although it is difficult to use weedy rice directly, weedy rice may be available to breed both male sterile line and restorer line through improvement, developing japonica hybrid rice.
Studies of hybrid fitness, of which agronomic performance may be an indicator, can help in evaluating the potential for introgression of a transgene from a transgenic crop to wild relatives. The objective of this study was to assess the agronomic performance of reciprocal hybrids between two transgenic glufosinate-resistant rice lines, Y0003 and 99-t, and two weedy rice accessions, WR1 and WR2, in the greenhouse.
F1 hybrids displayed heterosis in height, flag leaf area and number of spikelets per panicle. The agronomic performance of F1 between WR1 and Y0003 was not affected by crossing direction. The tiller and panicle numbers of F1 individuals were higher than their F2 counterparts. However, these traits did not change significantly from the F2 to the F3 generation or in hybrids with weedy rice as maternal or paternal plants. For all hybrids, the in vitro germination rates of fresh pollen were similar and significantly lower than those of their parents, seed sets were similar to or of lower value than those of weedy rice parents and seed shattering characteristics were partially suppressed, but the survival of hybrids over winter in the field was similar to that of weedy rice parents. All F1, F2 and F3 hybrids had similar composite agronomic performance to weedy rice parents.
There was no significant decrease in the composite agronomic performance of any of the hybrids compared with weedy rice. This implies that gene flow from transgenic cultivated rice to weedy rice could occur under natural conditions.
Gene flow from genetically modified (GM) crops to non-GM cultivars or weedy relatives may lead to the development of more aggressive weeds. We quantified the amount of gene flow from herbicide-tolerant GM rice (Protox GM, derived from the cultivar Dongjin) to three cultivars (Dongjin, Aranghyangchal and Hwaseong) and a weedy rice line. Gene flow frequency generally decreased with increasing distance from the pollen donor. At the shortest distance (0.5 m), we observed a maximum frequency (0.039%) of gene flow. We found that the cultivar Dongjin received the greatest amount of gene flow, with the second being weedy rice. Heterosis of F2 inbred progeny was also examined between Protox GM and weedy rice. We compared growth and reproduction between F2 progeny (homozygous or hemizygous for the Protox gene) and parental rice lines (GM and weedy rice). Here, transgene-homozygous F2 progeny was significantly taller and produced more seeds than the transgene-hemizygous F2 progeny and parental lines. Although the gene flow frequency was generally low, our results suggest that F2 progeny between GM and weedy relatives may exhibit heterosis.
The main environmental concerns about genetically modified (GM) crops are the potential weediness or invasiveness in the crop itself or in its wild or weedy relatives as a result of transgene movement. Here we briefly review evidence for pollen- and seed-mediated gene flow from GM crops to non-GM or other GM crops and to wild relatives. The report focuses on the effect of abiotic and biotic stress-tolerance traits on plant fitness and their potential to increase weedy or invasive tendencies. An evaluation of weediness and invasive traits that contribute to the success of agricultural weeds and invasive plants was of limited value in predicting the effect of biotic and abiotic stress-tolerance GM traits, suggesting context-specific evaluation rather than generalizations. Fitness data on herbicide, insect, and disease resistance, as well as cold-, drought-, and salinity-tolerance traits, are reviewed. We describe useful ecological models predicting the effects of gene flow and altered fitness in GM crops and wild/weedy relatives, as well as suitable mitigation measures. A better understanding of factors controlling population size, dynamics, and range limits in weedy volunteer GM crop and related host or target weed populations is necessary before the effect of biotic and abiotic stress-tolerance GM traits can be fully assessed.
Transgenic herbicide-resistant rice is needed to control weeds that have evolved herbicide resistance, as well as for the weedy (feral, red) rice problem, which has been exacerbated by shifting to direct seeding throughout the world-firstly in Europe and the Americas, and now in Asia, as well as in parts of Africa. Transplanting had been the major method of weedy rice control. Experience with imidazolinone-resistant rice shows that gene flow to weedy rice is rapid, negating the utility of the technology. Transgenic technologies are available that can contain herbicide resistance within the crop (cleistogamy, male sterility, targeting to chloroplast genome, etc.), but such technologies are leaky. Mitigation technologies tandemly couple (genetically link) the gene of choice (herbicide resistance) with mitigation genes that are neutral or good for the crop, but render hybrids with weedy rice and their offspring unfit to compete. Mitigation genes confer traits such as non-shattering, dwarfism, no secondary dormancy and herbicide sensitivity. It is proposed to use glyphosate and glufosinate resistances separately as genes of choice, and glufosinate, glyphosate and bentazone susceptibilities as mitigating genes, with a six-season rotation where each stage kills transgenic crop volunteers and transgenic crop x weed hybrids from the previous season.
Herbicide-resistant rice cultivars allow selective weed control. A glufosinate indica rice has been developed locally. However, there is concern about weedy rice becoming herbicide resistant through gene flow. Therefore, assessment of gene flow from indica rice cultivars to weedy rice is crucial in Tropical America. A field trial mimicking crop-weed growing patterns was established to assess the rate of hybridization between a Costa Rican glufosinate-resistant rice line (PPT-R) and 58 weedy rice accessions belonging to six weedy rice morphotypes. The effects of overlapping anthesis, morphotype, weedy accession/PPT-R percentage, and the particular weedy accession on hybridization rates were evaluated. Weedy rice accessions with short overlapping anthesis (4-9 days) had lower average hybridization rates (0.1%) than long anthesis overlapping (10-14 days) accessions (0.3%). Hybridization also varied according to weedy rice morphotype and accession. Sativa-like morphotypes (WM-020, WM-120) hybridized more readily than intermediate (WM-023, WM-073, WM-121) and rufipogon-like (WM-329) morphotypes. No hybrids were identified in 11 of the 58 accessions analyzed, 21 accessions had hybridization rates from 0.01% to 0.09%, 21 had rates from 0.1% to 0.9%, and 5 had frequencies from 1% to 2.3%. Another field trial was established to compare the weedy rice-PPT-R F(1) hybrids with their parental lines under noncompetitive conditions. F(1) hybrids had a greater phenotypic variation. They had positive heterosis for vegetative trait and reproductive potential (number of spikelets and panicle length) traits, but negative heterosis for seed set. This study demonstrated the complexity of factors affecting hybridization rates in Tropical America and suggested that the phenotype of F(1) hybrids facilitate their identification in the rice fields.