Official 38 Passage 3


Transgenic Plants


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正确答案: A

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  • Genes from virtually any organism, from viruses to humans, can now be inserted into plants, creating what are known as transgenic plants. Now used in agriculture, there are approximately 109 million acres of transgenic crops grown worldwide, 68 percent of which are in the United States. The most common transgenic crops are soybeans, corn, cotton, and canola. Most often, these plants either contain a gene making them resistant to the herbicide glyphosate or they contain an insect-resistant gene that produces a protein called Bt toxin.

    On the positive side, proponents of transgenic crops argue that these crops are environmentally friendly because they allow farmers to use fewer and less noxious chemicals for crop production. For example, a 21 percent reduction in the use of insecticide has been reported on Bt cotton (transgenic cotton that produces Bt toxin). In addition, when glyphosate is used to control weeds, other, more persistent herbicides do not need to be applied.

    On the negative side, opponents of transgenic crops suggest that there are many questions that need to be answered before transgenic crops are grown on a large scale. One question deals with the effects that Bt plants have on nontarget organisms such as beneficial insects, worms, and birds that consume the genetically engineered crop. For example, monarch caterpillars feeding on milkweed plants near Bt cornfields will eat some corn pollen that has fallen on the milkweed leaves. Laboratory studies indicate that caterpillars can die from eating Bt pollen. However, field tests indicate that Bt corn is not likely to harm monarchs. Furthermore, the application of pesticides (the alternative to growing Bt plants) has been demonstrated to cause widespread harm to nontarget insects.

    Another unanswered question is whether herbicide-resistant genes will move into the populations of weeds. Crop plants are sometimes grown in areas where weedy relatives also live. If the crop plants hybridize and reproduce with weedy relatives, then this herbicide-resistant gene will be perpetuated in the offspring. In this way, the resistant gene can make its way into the weed population. If this happens, a farmer can no longer use glyphosate, for example, to kill those weeds. This scenario is not likely to occur in many instances because there are no weedy relatives growing near the crop plant. However, in some cases, it may become a serious problem. For example, canola readily hybridizes with mustard weed species and could transfer its herbicide-resistant genes to those weeds.

    We know that evolution will occur when transgenic plants are grown on a large scale over a period of time. Of special concern is the development of insect populations resistant to the Bt toxin. This pesticide has been applied to plants for decades without the development of insect-resistant populations. However, transgenic Bt plants express the toxin in all tissues throughout the growing season. Therefore, all insects carrying genes that make them susceptible to the toxin will die. That leaves only the genetically resistant insects alive to perpetuate the population. When these resistant insects mate, they will produce a high proportion of offspring capable of surviving in the presence of the Bt toxin. Farmers are attempting to slow the development of insect resistance in Bt crops by, for example, planting nontransgenic border rows to provide a refuge for susceptible insects. These insects may allow Bt susceptibility to remain in the population.

    Perhaps the most serious concern about the transgenic crop plants currently in use is that they encourage farmers to move farther away from sustainable agricultural farming practices, meaning ones that allow natural resources to continually regenerate over the long run. Transgenics, at least superficially, simplify farming by reducing the choices made by the manager. Planting a glyphosate-resistant crop commits a farmer to using that herbicide for the season, probably to the exclusion of all other herbicides and other weed-control practices. Farmers who use Bt transgenics may not feel that they need to follow through with integrated pest-management practices that use beneficial insects and timely applications of pesticides to control insect pests. A more sustainable approach would be to plant nontransgenic corn, monitor the fields throughout the growing season, and then apply a pesticide only if and when needed.

  • 现在几乎任何有机体,小到病毒大到人体的基因,都可以被移植到植物中,转基因植物就这样产生了。 这项技术现如今被运用于农业之中,世界上大约有109,000,000英亩的转基因农作物,其中68%在美国。 最常见的转基因作物有大豆、玉米、棉花和油菜。 通常情况下,这些植物含有使其能够抵抗除草剂草甘膦的基因,或者含有能够抗虫的基因,这种基因会产生一种叫做芽孢杆菌毒蛋白的抗虫蛋白。

    从积极的方面来看,转基因农作物的支持者认为,因为使用的化学品更少并且毒性更小了,转基因农作物是更加环保的。 比如,抗虫棉花使得杀虫剂的使用减少了21%。 此外,当草甘膦被用来除草的时候,其他更加持久的除草剂就无需使用了。

    从消极的方面来看,转基因作物的反对者认为在转基因农作物大面积耕种之前,很多问题都要先弄清楚。 其中一个问题是,抗虫作物对于非目标生物体,例如吃了转基因作物的有益的昆虫、蠕虫和鸟类会产生什么影响。 比如,抗虫玉米地附近生长的斑蝶毛毛虫以马利筋草为食,有时他们吃的马利筋草上会有转基因玉米的花粉。 实验室研究表明,毛毛虫可能会因为吃了抗虫花粉而死亡。 但是,田间试验表明抗虫玉米不会对帝王斑蝶造成危害。 而且,杀虫剂的使用(抗虫作物的替代品)已被证实会对非目标昆虫造成大范围的危害。

    另一个尚未得到解答的问题是,抗除草剂的基因是否会进入草里。 有时候,农作物生长的地方同样也长有草。 如果农作物和草杂交,那么抗除草剂的基因会延续到杂交后代的基因中去。 这样一来,抗除草剂基因也会存在于草的基因里。 如果这种事情发生了,农民将不能再使用草甘膦除草了。 这种情节在许多情况下是不会发生的,因为农作物的附近没有杂草。 但是,在一些情况下,这有可能成为一个严重的问题。 例如,油菜已经和芥末杂草杂交,并且有可能将抗除草剂基因转移到这些杂草中。

    转基因植物经过一段时间的大规模种植之后会发生进化。 尤其值得关注的是,昆虫会不会对抗虫毒蛋白产生耐药性。 数十年来,杀虫剂用于农作物之上并没有使农作物产生抗虫性。 但是,转基因抗虫作物在生长期内将抗虫毒素传递到所有的细胞组织内。 因此,所有携带对抗虫毒素过敏的基因的虫子都会被毒死。 含有耐药基因的虫子则活下来负责延续物种。 当这些抗药虫子交配之后,它们会生出比例很高的能够抵抗抗虫毒蛋白的后代。 农民们试图减缓转基因作物的抗虫性,例如通过在农田边界种植非转基因作物,以此来向对抗虫毒蛋白敏感的昆虫提供避难场所。 这些昆虫会使得对抗虫毒蛋白的敏感性延续在虫子们之中。

    也许关于目前使用中的转基因农作物的最重要的担忧在于,它们使得农民脱离了可持续的农业耕种实践,可持续农业耕种意味着农业资源可以长期不断再生。 至少从表面上看来,转基因简化了农作物种植,因为它简化了管理方面的选择。 种植耐草甘膦的作物使得农民可以施用草甘膦,不用再考虑使用其他除草剂和其他除草方法。 种植抗虫转基因作物的农民不用自始至终为完善的害虫管理措施而担心,包括利用有益昆虫、及时喷洒农药。 更为可持续的做法是种植非转基因作物,在其生长期内及时监控,并且仅在需要的时候才喷洒农药。
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    原文定位:If the crop plants hybridize and reproduce with weedy relatives, then this herbicide-resistant gene will be perpetuated in the offspring.