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Plants, and all other living things, require nitrogenfor growth; it is an essential component of nucleicacids and proteins. Although air is mostly nitrogen,this gaseous form is inaccessible to plants and mustbe fixed into ammonium to render it biologicallyrelevant. Soil bacteria called rhizobia fix nitrogen, but to do this they must first take upresidence inside the roots of legumes like pea, alfalfa, clover, and soybean.
Soon after a legume begins to grow, rhizobia invade its root hairs and multiply, causing theplant to form specialized organs—nodules—that contain the proliferating bacteria. Thissymbiotic arrangement benefits both parties: legumes can thrive without nitrogen fertilizersonly if they have functional nitrogen-fixing nodules, while the bacteria receive the energyneeded to multiply and fix nitrogen from the plant. When the plant dies, the fixed nitrogen isreleased into the soil so other plants can use it. This process has significant implications foragriculture, as nitrogen is the most common nutrient deficient in the earth's soil and, thus,the one most commonly supplied by chemical fertilizers.
Rhizobia are a diverse group taxonomically, genetically, and metabolically. They can be foundin distant genera. Their symbiotic trait appears to have arisen independently multiple times byhorizontal transfer of genes. However, it is not thought that this horizontal gene transferis sufficient to confer symbiosis, or to explain the biodiversity of rhizobia. There must beselective pressures preventing or permitting the expression of the acquired symbiosis trait andadaptive mechanisms to deal with these pressures. But neither the pressures nor the measurestaken to circumvent them are known.
What does the passage indicate about the expression of the symbiosis trait inrhizobia?
(A) It occurs only in certain strains of rhizobia with metabolic similarities.
(B) It is not determined by environmental pressures.
(C) It is responsible for the biodiversity of rhizobia.
(D) It does not result from horizontal gene transfer alone.
重点单词讲解：
bacteria [bæk'tiəriə] n. (复数)细菌
gaseous ['gæsiəs] adj. 气体的，气态的
component [kəm'pəunənt] n. 元件，组件，成份
render ['rendə] vt. 使成为，提供，报答，着色; 执行，实施
critical ['kritikəl] adj. 批评的，决定性的，危险的，挑剔的
indicate ['indikeit] v. 显示，象征，指示
arrangement [ə'reindʒmənt] n. 安排，商议，整理，布置，商定，[音]改编，改编曲
adaptive [ə'dæptiv] adj. 适合的，适应的，能适应的
certain ['sə:tn] adj. 确定的，必然的，特定的
thrive [θraiv] vi. 兴旺，繁荣，茁壮成长
bacteria [bæk'tiəriə] n. (复数)细菌
答案：D
解析：
Choice D is the best answer. When discussing theexpression of the symbiosis trait in rhizobia, theauthor states that "it is not thoughtthat...horizontal gene transfer is sufficient toconfer symbiosis." In other words, horizontal gene transfer alone is not enough to cause theexpression of the symbiosis trait.