生命科學(xué)概論論文

1、An introduction to life scienceCourse assignmentsBt and MonarchDespite widespread adoption of genetically modified crops in many countries, heated controversies about their advantages and disadvantages continue. Especially for developing countries, there are concerns that genetically modified crops

2、fail to benefit smallholder farmers and contribute to social and economic hard-ship. Many economic studies contradict this view, but most of them look at short-term impacts only, so that uncertainty about longer-term effects prevails.So, Bt or not Bt, that is a question.Plant resistance to insect pe

3、sts has evolved naturally over many millions of years and involves (i) both constitutive and inducible phytochemical and morphological mechanisms in plants, (ii)counteradaptations to plant defenses by the herbivores,and(iii)biotic interactions of the multitrophic level communities of insect pathogen

4、s, parasites, and predators (14). The outcomes of such complex biotic interactions are sometimes determined by local mosaics of abiotic environmental conditions or regional climate changes that directly influence the component participants and their behavioral, physiological, and genetic adaptations

5、 (57). The intentional selection and breeding of insect andyor pathogen-resistant plant genotypes such as corn (Zea mays) has resulted in slow, but steady, progress against stalk-boring larvae such as the European corn borer and other such species, and host plant resistance in general has significan

6、tly reduced the need for broad-spectrum insecticides across agro-ecosystems and forests (8, 9).It has recently been possible to genetically engineer plants, such as Z. mays, to avoid problems with insecticidal spray drift and more efficiently express toxins in particular plant tissues against lepido

7、pterous pests. Such efficient, effective, and relatively safe tools for pest management have eluded classical plant breeders and integrated pest management practitioners for many decades (10). However, even though safe for most other taxonomic groups of insects other than Lepidoptera, the nontarget

8、impacts of Bt require careful and thorough evaluation combined with extensive publicizing. If not done, the real risks, which may be minimal, can be significantly amplified in the public minds by perceived risks and distrust of biotech corporations and the regulatory federal agencies and similar goa

9、l-affiliated agricultural research institutions and universities (11, 12). Additional risks for crop pest management such as the rapid development of insect resistance to the Bt insecticides also need serious attention and consideration in long-term agroecosystem design as well (13, 14)Recent public

10、 concerns over the trans-genic (genetically modified) plants and nontarget impacts such as those from Bt-toxin expressing corn pollen on the monarch butterfly populations have escalated, despite good pest management intentions and good science(1523).A single laboratory study on monarch butterflies h

11、as prompted widespread concern that corn pollen, engineered to express Bacillus thuringiensis (Bt) endotoxin, might travel beyond corn fields and cause mortality in nontarget lepidopterans. Among the lepidopterans at high potential risk from this technology is the black swallowtail butterfly, Papili

12、o polyxenes, whose host plants in the midwestern U.S. are located principally in narrow strips between roads and crop fields. A field study was performed to assess whether mortality of early instar black swallowtails was associated either with proximity to a field of Bt corn or by levels of Bt polle

13、n deposition on host plants. Potted host plants were infested with first instar black swallowtails and placed at intervals from the edge of a field of Bt corn (Pioneer 34R07 containing Monsanto event 810) at the beginning of anthesis. They confirmed by ELISA that pollen from these plants contained C

14、ry1Ab endotoxin (2.125 6 0.289 ngyg). Although many of the larvae died during the 7 days that the experiments were run, there was no relationship between mortality and proximity to the field or pollen deposition on host plants.Moreover, pollen from these same plants failed to cause mortality in the

15、laboratory at the highest pollen dose tested (10,000 grainsycm 2 ), a level that far exceeded the highest pollen density observed in the field (200 grainsycm 2 ). they conclude that Bt pollen of the variety tested is unlikely to affect wild populations of black swallowtails. Thus, their results sugg

16、est that at least some potential nontarget effects of the use of transgenic plants may be manageable.Laboratory tests were conducted by Richard L to establish the relative toxicity of Bacillus thuringiensis (Bt) toxins and pollen from Bt corn to monarch larvae. Toxins tested included Cry1Ab, Cry1Ac,

17、 Cry9C, and Cry1F. Three methods were used: (i) purified toxins incorporated into artificial diet, (ii) pollen collected from Bt corn hybrids applied directly to milkweed leaf discs, and (iii) Bt pollen contaminated with corn tassel material applied directly to milkweed leaf discs. Bioassays of puri

18、fied Bt toxins indicate that Cry9C and Cry1F proteins are relatively non- toxic to monarch first instars, whereas first instars are sensitive to Cry1Ab and Cry1Ac proteins. Older instars were 12 to 23 times less susceptible to Cry1Ab toxin compared with first instars. Pollen bio-assays suggest that

19、pollen contaminants, an artifact of pollen pro-cessing, can dramatically influence larval survival and weight gains and produce spurious results. The only transgenic corn pollen that consistently affected monarch larvae was from Cry1Ab event 176 hybrids,currently<2%cornplantedandforwhichre-regist

20、rationhas not been applied. Results from the other types of Bt corn suggest that pollen from the Cry1Ab (events Bt11 and Mon810) and Cry1F, and experimental Cry9C hybrids, will have no acute effects on monarch butterfly larvae in field settings.To assess the likelihood that monarch larvae will be ex

21、posed to Bacillus thuringiensis (Bt) pollen, Karen S. et al studied milkweed and monarch densities in habitats which comprise much of the land available to breeding monarchs, e.g., cornfields, cornfield edges, other agricultural fields, and nonagricultural areas, in four regions of the monarch breed

22、ing range. They found that monarchs use milkweed in cornfields throughout their breeding season, and that per plant densities are as high or higher in agricultural habitats as in nonagricultural habitats. As a result of the prevalence of agricultural land, most of the monarchs produced in the upper

23、Midwest are likely to originate in cornfields or other agricultural habitats. There was a greater temporal overlap between susceptible monarchs and corn anthesis in the northern than the southern part of the summer breeding range, because of earlier pollen shed in the south. The importance of agricu

24、ltural habitats to monarch production suggests that, regardless of the impact of genetically modified crops, agricultural practices such as weed control and foliar insecticide use could have large impacts on monarch populations.A collaborative research effort by scientists in several states and in C

25、anada has produced information to develop a formal risk assessment of the impact of Bt corn on monarch butterfly (Danaus plexippus) populations. Information was sought on the acute toxic effects of Bt corn pollen and the degree to which monarch larvae would be exposed to toxic amounts of Bt pollen o

26、n its host plant, the common milkweed, Asclepias syriaca, found in and around cornfields. Expression of Cry proteins, the active toxicant found in Bt corn tissues, differed among hybrids, and especially so in the concentrations found in pollen of different events. In most commercial hybrids, Bt expr

27、ession in pollen is low, and laboratory and field studies show no acute toxic effects at any pollen density that would be encountered in the field. Other factors mitigating expo-sure of larvae include the variable and limited overlap between pollen shed and larval activity periods, the fact that onl

28、y a portion of the monarch population utilizes milkweed stands in and near cornfields,andthecurrentadoptionrateof Bt corn at 19% of North American corn-growing areas. Their 2-year study suggests that the impact of Bt corn pollen from current commercial hybrids on monarch butterfly populations is neg

29、ligibleConceptual Model. Risk assessment requires knowledge of four essential components: (i) hazard identification, (ii) nature of dose responsetoatoxin,(iii)probability of exposure to an effective dose, and (iv) characterization of risk (24). Components of a risk assessment approach as applied to

30、the case of Bt corn and monarch butterfly are depicted in Fig. 1. Bt proteins expressed in corn plant tissues can bring about specific reactions in the gut of lepidopteran larvae(25) ,including nontarget larvae that consume Bt corn pollen.The magnitude of the reaction will depend on the type of prot

31、ein produced by various transgenic events o-f hybrid Bt corn,the amount of protein expressed in pollen grains from different events, the amount of pollen consumed by larvae of different developmentalstages, and the susceptibility of larvae to the Bt protein. That a hazard may exist was suggested by

32、Losey etal.(26).Characterization of toxic effects is necessary to establish the first component of risk.Laboratory and field assays of lethaland sublethal toxicity resulting from exposure to doses of Bt pollen are required to establish toxicity thresholds for comparison against the dose encountered

33、within the environment. These toxicity thresholds will vary based on expression levels for individual Bt corn events in conjunction with environmental factors determining ecological exposure.Only three papers concerning the impact of Bt corn pollen on nontarget Lepidoptera have been published (26,33

34、,34) before , and they are limited in their application to risk assessment (32). For example, the dose of pollen was not specified in the exposure study by Losey et al. (26), and the study by Jesse and Obrycki (33) used pollen collection and handling techniques that probably resulted in contaminatio

35、n from corn anthers or tassel fragments, which contain significantly higher levels of Cry protein than the pollen (28). Finally, neither study addressed the spatial or temporal potential for exposure by monarch larvae to pollen in cornfields, thereby precluding a risk assessment.Mark et al develop a

36、 weight-of-evidence approach to the risk of exposure of monarch larvae to Bt corn pollen and the impact of such exposure on populations of the monarch butterfly in eastern North America by using recently published information based on collaborative research by scientists in the U.S. and Canada (2831

37、). They use an approach to risk assessment that has been performed for many nontarget species in relation to pesticides (3539), industrial by-product (40, 41), and other potential toxicants found in the environment (42). The approach to this process is consistent, well documented, and standardized (

38、http:yNCEAyecorsk.htm). It requires consideration of both the expression of toxicityand the likelihood of exposure to the toxicant as the basic components for a risk assessment procedure.At the same time,Genetically modified (gm) crops especially Bt cotton made great contribution positi

39、ve economic and social development in India.Impact on Cotton Yield. Results of panel fixed-effects specifications of a cotton yield function are shown in Table 2 (full mode results with all control variables are shown in Table S2). The positive and significant coefficient of Bt in column 1 indicates

40、 that Bt has a positive net impact on cotton yield per acre. Controlling for all other factors, Bt increases cotton yield by 126 kg per acre, which is equivalent to a 24% gain over mean yields on conventional cotton plots. The Bt dummy variable captures Bt adoption in any year, whereas the additiona

41、l Bt 20062008 dummy takes a value of one only when Bt was used in the 2006 or 2008 survey waves. In the first column, the Bt 20062008 coefficient is insignificant, indicating that the Bt yield effect was stable over time and did not increase or decrease in the later compared with the earlier period.

42、Table 1. Net impact of Bt on cotton yield and profit per acreTable 2. Net impact of Bt on household living standardImpact on Household Living Standard. During 20062008, Bt-adopting households increased their annual consumption expenditures by 15,841 Rs (321 US$) on average. Compared with nonadopters

43、, this finding implies a net increase of 18%, which underlines that Bt cotton has significantly raised living standards of smallholder farm households.Hence, Bt cotton contributes to positive economic and social development.Everything has two sides, as well as genetically modified (gm) crops, In my

44、opinion, We should treat gm scientifically. All in all,with the development of economy, the transgenic technology will bring benefits to the people further. Countries will take more perfect relevant law, to protect the interests of the people.Rerference:1.Rosenthal, G. A. & Berenbaum, M. R. (199

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