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雅思9分达人TEST 4 READING PASSAGE 2真题!

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  READING PASSAGE 2

  You should spend about 20 minutes on Questions 14-26, which are based on

  Reading Passage 2 below.

  Leaf-Cutting Ants and Fungus

  A The ants and their agriculture have been extensively studied over the years, but the recent

  research has uncovered intriguing new findings about the fungus they cultivate, how they

  domesticated it and how they cultivate it and preserve it from pathogens. For example, the

  fungus farms, which the ants were thought to keep free of pathogens, turn out to be vulnerable to a devastating mold, found nowhere else but in ants’ nests. To keep the mold in check,

  the ants long ago made a discovery that would do credit to any pharmaceutical laboratory.

  B Leaf-cutting ants and their fungus farms are a marvel of nature and perhaps the best known

  example of symbiosis, the mutual dependence of two species. The ants’ achievement is

  remarkable — the biologist Edward O. Wilson has called it “one of the major breakthroughs

  in animal evolution” — because it allows them to eat, courtesy of their mushroom’s digestive

  powers, the otherwise poisoned harvest of tropical forests whose leaves are laden with terpenoids, alkaloids and other chemicals designed to sicken browsers.

  C Fungus growing seems to have originated only once in evolution, because all gardening ants

  belong to a single tribe, the descendants of the first fungus farmer. There are more than 200

  known species of the attine ant tribe, divided into 12 groups, or genera. The leaf-cutters use

  fresh vegetation; the other groups, known as the lower attines because their nests are smaller

  and their techniques more primitive, feed their gardens with detritus like dead leaves, insects

  and feces. In 1994 a team of four biologists, Ulrich G. Mueller and Ted R. Schultz from

  Cornell University and Ignacio H. Chapela and Stephen A. Rehner from the United States

  Department of Agriculture, analyzed the DNA of ant funguses. The common assumption

  that the funguses are all derived from a single strain, they found, was only half true.

  D The leaf-cutters’ fungus was indeed descended from a single strain, propagated clonally, or

  just by budding, for at least 23 million years. But the lower attine ants used different varieties of the fungus, and in one case a quite separate species, the four biologists discovered.

  Cameron R. Currie, a Ph.D. student in the University of Toronto, it seemed to Mr. Currie,

  resembled the monocultures of various human crops, that are very productive for a while and

  then succumb to some disastrous pathogen, such as the Irish potato blight. Monocultures,

  which lack the genetic diversity to respond to changing environmental threats, are sitting

  ducks for parasites. Mr. Currie felt there had to be a parasite in the ant-fungus system. But

  a century of ant research offered no support for the idea. Textbooks describe how leaf-cutter

  ants scrupulously weed their gardens of all foreign organisms. “People kept telling me, ‘You

  know the ants keep their gardens free of parasites, don’t you?’” Mr. Currie said of his efforts

  49

  TEST 4

  to find a hidden interloper.

  E But after three years of sifting through attine ant gardens, Mr. Currie discovered they are far

  from free of infections. In last month’s issue of the Proceedings of the National Academy of

  Sciences, he and two colleagues, Dr. Mueller and David Mairoch, isolated several alien organisms, particularly a family of parasitic molds called Escovopsis. Escovopsis turns out to be a

  highly virulent pathogen that can devastate a fungus garden in a couple of days. It blooms like a

  white cloud, with the garden dimly visible underneath. In a day or two the whole garden is enveloped. “Other ants won’t go near it and the ants associated with the garden just starve to death,’’

  Dr. Rehner said. “They just seem to give up, except for those that have rescued their larvae.”

  F Evidently the ants usually manage to keep Escovopsis and other parasites under control. But

  with any lapse in control, or if the ants are removed, Escovopsis will quickly burst forth.

  Although new leaf-cutter gardens start off free of Escovopsis, within two years some 60

  percent become infected. The discovery of Escovopsis’s role brings a new level of understanding to the evolution of the attine ants. “In the last decade, evolutionary biologists have been

  increasingly aware of the role of parasites as driving forces in evolution,” Dr. Schultz said.

  There is now a possible reason to explain why the lower attine species keep changing the

  variety of fungus in their mushroom gardens, and occasionally domesticating new ones — to

  stay one step ahead of the relentless Escovopsis.

  G Interestingly, Mr. Currie found that the leaf-cutters had in general fewer alien molds in their

  gardens than the lower attines, yet they had more Escovopsis infections. It seems that the

  price they pay for cultivating a pure variety of fungus is a higher risk from Escovopsis. But

  the leaf-cutters may have little alternative: they cultivate a special variety of fungus which,

  unlike those grown by the lower attines, produces nutritious swollen tips for the ants to eat.

  H Discovery of a third partner in the ant-fungus symbiosis raises the question of how the attine

  ants, especially the leaf-cutters, keep this dangerous interloper under control. Amazingly

  enough, Mr. Currie has again provided the answer. “People have known for a hundred years

  that ants have a whitish growth on the cuticle,” said Dr. Mueller, referring to the insects’

  body surface. “People would say this is like a cuticular wax. But Cameron was the first one

  in a hundred years to put these things under a microscope. He saw it was not inert wax. It is

  alive.” Mr. Currie discovered a specialised patch on the ants’ cuticle that harbours a particular

  kind of bacterium, one well known to the pharmaceutical industry, because it is the source

  of half the antibiotics used in medicine. From each of 22 species of attine ant studied, Mr.

  Cameron and colleagues isolated a species of Streptomyces bacterium, they reported in Nature

  in April. The Streptomyces does not have much effect on ordinary laboratory funguses. But

  it is a potent poisoner of Escovopsis, inhibiting its growth and suppressing spore formation.

  Because both the leaf-cutters and the lower attines use Streptomyces, the bacterium may have

  been part of their symbiosis for almost as long as the Escovopsis mold. If so, some Alexander

  Fleming of an ant discovered antibiotics millions of years before people did. Even now, the

  ants are accomplishing two feats beyond the powers of human technology. The leaf-cutters

  are growing a monocultural crop year after year without disaster, and they are using an antibiotic apparently so wisely and prudently that, unlike people, they are not provoking antibiotic resistance in the target pathogen.

  Questions 14-19

  Use the information in the passage to match the options (listed A-C) with the

  activities or features of ants below.

  Write the appropriate letters, A-C, in boxes 14-19 on your answer sheet.

  NB You may use any letter more than once.

  A Leaf-cutting ants

  B Lower attine ants

  C Both leaf-cutting ants and lower attine ants

  14 can use toxic leaves to feed fungus

  15 build small nests and live with different foreign fungus

  16 use dead vegetation to feed fungus

  17 raise a single fungus which do not live with other variety of foreigners

  18 normally keep a highly dangerous parasite under control

  19 use special strategies to fight against Escovopsis

  Questions 20-24

  Reading Passage 2 has eight paragraphs, A-H.

  Which paragraph contains the following information?

  Write the correct letter, A-H, in boxes 20-24 on your answer sheet.

  20 Dangerous outcome of Escovopsis

  21 Risk of growing single fungus

  22 Comparison of the features of two different nests for feeding gardens

  23 Discovery of significant achievements made by ants earlier than human

  24 Advantages of growing a new breed of fungus in the ant farm

  Questions 25-26

  Choose the correct letter, A, B, C or D.

  Write your answers in boxes 25-26 on your answer sheet.

  25 What does the author think of Currie’s opinion on the saying “ants keep their

  gardens free of parasites”?

  A His viewpoint was verified later.

  B His earlier study has sufficient evidence immediately.

  C There is no detail mentioned in the article.

  D His opinion was proved to be wrong later on.

  26 What did scientists find on the skin of ants under microscope?

  A some white cloud mold embed in their skin

  B that wax is all over their skin

  C a substance which is useful to humans

  D a substance which suppresses growth of all fungus

  词汇详解

  extensively adv. 广阔地 ; 广大地(近义词有 : widely, vastly。 extensively 常用于修饰学

  术研究的范围, 我们常说“ extensively studied”, 意为“ 广泛地研究”。)

  domesticate v. 驯养 ; 教化 ; 引进(短语“ domesticate tame” 强调“ 驯养” ; 近义词有 :

  introduce, teach。)

  pharmaceutical adj. 制药(学)的 n. 药物(相关短语 :pharmaceutical chemistry 药物化学 ;

  pharmaceutical care 药学服务。)

  marvel n. 奇迹 v. 对……感到惊异(常见用法 : marvel at 对……感到惊奇。)

  digestive adj. 消化的 ; 助消化的 n. 助消化药(作名词时, 近义词有 : pepsine, bromo。)

  tropical adj. 热带的 ; 热情的 ; 酷热的(常见短语 : tropical forest 表示“ 热带森林”。 近

  义词有 : intense, warm, hot, enthusiastic, passionate。)

  descendant adj. 下降的 ; 祖传的 n. 后裔 ; 子孙(作形容词时, 近义词有 : falling, downward ; 作名词时, 近义词有 : child, seed。)

  detritus n. 碎石(近义词有 : rubble, crushed stone。)

  budding adj. 萌芽的 ; 发育期的 n. 发芽(词根 : bud 意为“ 使发芽”。 衍生词 : budless 无

  芽的。)

  interloper n. 闯入者(动词形式为 interlope。)

  infection n. 感染 ; 传染 ; 影响 ; 传染病(常见表达 : virus infection 病毒感染。 近义词有 :

  impression, affection, effect, influence, incidence。 词根 : infect 表示“ 感染” 之意。 相

  关词汇有 : infectious 传染的, 传染性的, 易传染的 ; infected 被感染的 ; infective 有传染

  性的, 感染别人的。)

  underneath prep. 在……的下面 ; 在……的支配下 adv. 在下面 n. 下面 ; 底部 adj. 下面的 ;

  底层的(作副词时, 近义词有 : hereunder, down below ; 作名词时, 近义词有 : basis,

  bottom, base。)

  envelop v. 包围 ; 包封 ; 遮盖

  relentless adj. 无情的 ; 残酷的 ; 不间断的(近义词有 : cruel, iron, ongoing, ruthless。)

  alternative adj. 供选择的 ; 选择性的 ; 交替的 n. 二中择一 ; 供替代的选择(近义词有 :

  selective, vicissitudinary。)

  bacterium n. 细菌(近义词 : microbe, bacilus。)

  prudently adv. 谨慎地 ; 慎重地(近义词 : deliberately, cautiously。)

  provoke v. 激怒 ; 挑拨 ; 诱发(常见短语 : thought-provoking question 启发思维的问题。)

  resistance n. 阻力 ; 电阻 ; 抵抗 ; 反抗 ; 抵抗力(近义词 : resisting force, rebellion。)

  target n. 目标 ; 靶子(近义词 : goal, object, end, cause, aim。)

  题目详解

  Questions 14-19

  解 答

  14. 利用题目细节信息“ toxic leaves” 和“ feed fungus” 定位于原文段落 B 末句“ because

  it allows them to eat, courtesy of their mushroom’s digestive powers, the otherwise poi-soned harvest of tropical forests whose leaves are laden with terpenoids, alkaloids and other

  chemicals designed to sicken browsers”。 这道题的难点在于找到题目中“ feed fungus”

  所 对 应 的 原 文。 文 中 说 到“ the otherwise poisoned harvest of tropical forests whose

  leaves are laden with terpenoids, alkaloids and other chemicals designed to sicken browsers”, 既然热带雨林中的叶子有毒, 切叶蚁为什么还要收集而且不会中毒呢? 那是因

  为真菌的消化作用(mushroom’s digestive powers) 。 此外, 文章 C 段还提到“ The

  leaf-cutters use fresh vegetation(切叶蚁使用新鲜的植被养活自己的真菌园) ”, 所以切

  叶蚁收集有毒的树叶是为了饲养真菌。 因此, 正确答案为 A。

  15. 利用题目细节信息“ small nests”、“ different” 和“ foreign” 定位于原文 C 段第 3 句

  与 D 段第 2 句。 C 段第 3 句“ the other groups, known as the lower attines because their

  nests are smaller and their techniques more primitive”, 这里讲的是“ 它们的巢较小且种

  植技术更原始,被称为 lower attines”。 原文中的 “ nests are smaller”对应题目中的 “ small

  nests”。 但本段未出现题目中的后半部分信息, 需要接着往下看。 D 段第 2 句“ But

  the lower attine ants used different varieties of the fungus”, 这里说“ lower attine 蚂蚁使

  用不同种类的真菌”。 原文中的 “ different varieties of the fungus”对应题目中的 “ different

  foreign fungus”。 结合原文内容, 此处讲的都是 lower attines, 因此正确答案为 B。

  16. 利用题目细节信息“ dead vegetation” 定位于原文 C 段第 3 句“ known as the lower

  attines because their nests are smaller and their techniques more primitive, feed their

  gardens with detritus like dead leaves, insects and feces”。 这句话讲到“ 它们用枯叶、

  昆虫和粪便的碎屑来养活自己的园子”。 原文中的“ dead leaves” 与题目中的“ dead

  vegetation” 为同义替换, 所对应的是 lower attines。 因此, 正确答案为 B。

  17. 利用题目细节信息“ a single fungus” 定位于 D 段首句“ The leaf-cutters’ fungus was

  indeed descended from a single strain, propagated clonally, or just by budding, for at least

  23 million years.” 这里说“ 切叶蚁所种的菌种是单一菌株”, 而不像 lower attine ants

  那样 used different varieties of the fungus。 因此, 正确答案为 A。

  18. 利 用 顺 序 原 则 定 位 于 原 文 E 段 第 3 句“ Escovopsis turns out to be a highly virulent

  pathogen...” 和 F 段 首 句“ Evidently the ants usually manage to keep Escovopsis and

  other parasites under control.” 这说明蚂蚁通常能够控制住 Escovopsis 这种高危险的寄

  生物, 这与题目相对应。 此外, H 段首句提到“ Discovery of a third partner in the antfungus symbiosis raises the question of how the attine ants, especially the leaf-cutters, keep

  this dangerous interloper under control.” 这说明“ 果蚁, 尤其是切叶蚁能有效控制这种

  寄生物”, 对应 leaf-cutters。 因此, 正确答案为 A。

  19. 利 用 顺 序 原 则 先 定 位 于 原 文 F 段, 该 段 后 半 部 分 提 到“ lower attine species keep

  changing the variety of fungus in their mushroom gardens, and occasionally domesticating

  new ones — to stay one step ahead of the relentless Escovopsis”。 这说明低等果蚁不断改变蘑菇园中的真菌品种, 偶尔还培育新的真菌品种, 目的是为了在和无情的 Escovopsis 霉菌的对抗中占据上风。 然后再定位到原文 H 段首句“ Discovery of a third

  partner in the ant-fungus symbiosis raises the question of how the attine ants, especially the

  leaf-cutters, keep this dangerous interloper under control.” 这说明切叶蚁也能够通过“ 第

  三方” 来对抗 Escovopsis 霉菌。 因此, 这两种类型的蚂蚁都有自己的“ special strategies” 可以“ fight against Escovopsis”, 与题目相对应, 故正确答案为 C。

  Questions 20-24

  解 答

  20. 该信息出现在原文 E 段第 3 句“ Escovopsis turns out to be a highly virulent pathogen

  that can devastate a fungus garden in a couple of days.” 这里讲到“ Escovopsis 霉菌是剧

  毒性病原体,它可以在几天之内毁灭一个真菌园”。 原文中的“ turns out to be” 表示“ 结

  果”, 对应题目中的“ outcome”, 文中“ devastate a fungus garden in a couple of days”

  与题目中的“ Dangerous outcome of Escovopsis” 相对应。 因此, 正确答案为 E。

  21. 该信息出现在原文 D 段第 5 句话“ Monocultures, which lack the genetic diversity to

  respond to changing environmental threats, are sitting ducks for parasites.” 这里讲到“ 单

  一栽培往往因为缺乏基因的多样性来应对不断变化的环境威胁, 从而容易成为寄生

  虫攻击的目标”, 对应题目中的“ Risk of growing single fungus(单一种植的风险)”。

  因此, 正确答案为 D。

  22. 该信息出现在原文 C 段第 3 句“ The leaf-cutters use fresh vegetation; the other groups,

  known as the lower attines because their nests are smaller and their techniques more primitive, feed their gardens with detritus like dead leaves, insects and feces.” 这里提及“ leafcutters” 使用新鲜的植被养活自己的园子, 而后面紧接着对比说到“ lower attines” 由

  于巢穴较小, 技术也更加原始, 因此使用枯叶、 昆虫和粪便等碎屑来养活自己的园子,

  前后两者对比, 正好与题目相对应。 因此, 正确答案为 C。

  23. 该信息出现在原文 H 段倒数第 2、 3 句“ If so, some Alexander Fleming of an ant discovered antibiotics millions of years before people did. Even now, the ants are accomplishing two feats beyond the powers of human technology.” 原文讲到蚂蚁先于人类发现抗生

  素, ���且即使到现在, 蚂蚁仍然有两项成果超越人类。 原文中的“ two feats” 与题目

  中的 “ two achievements”为同义替换,而 “ millions of years before people did”和 “ beyond

  the powers of human technology” 与题目中的“ earlier than human” 相对应。 因此, 正

  确答案为 H。

  24. 该信息出现在原文 F 段末句“ ...and occasionally domesticating new ones — to stay one

  step ahead of the relentless Escovopsis”。 这里提到“ 培育新的真菌品种是为了在和无情的 Escovopsis 霉菌的对抗中占据上风”。 原文中的“ domesticating new ones” 与题

  目中的“ growing a new breed of fungus” 为同义替换, 其中“ ones” 指代的是原文前

  面提到的“ fungus”。 因此, 正确答案为 F。

  Questions 25-26

  解 答

  25. 该信息出现在原文 D 段与 E 段, 首先, 看到 D 段“ Mr. Currie felt there had to be a

  parasite in the ant-fungus system. But a century of ant research offered no support for the

  idea.” 这里提到“ 柯里先生认为在蚂蚁与真菌的体系中一定有寄生虫存在。 但是这

  个想法在过去一个世纪对蚂蚁的研究中并未得到任何证据支持”。 但 E 段接着写道

  “ In last month’s issue of the Proceedings of the National Academy of Sciences, he and two

  colleagues, Dr. Mueller and David Mairoch, isolated several alien organisms, particularly

  a family of parasitic molds called Escovopsis.” 这里说的是“ 柯里先生与他的同事分离

  出了几个外来生物的菌种, 特别是一种叫作 Escovopsis 的寄生霉菌族”。 这就说明

  Currie 之前的猜想得到了证实, 选项 A(他的观点后来得到了证实) 符合原文内容。

  因此, 正确答案为 A。

  26. 利用题目细节信息“ under microscope” 定位于原文 H 段“ People would say this is

  like a cuticular wax. But Cameron was the first one in a hundred years to put these things

  under a microscope. He saw it was not inert wax. It is alive.” 这里提到“ 卡梅隆把这些东

  西放在显微镜下观察,发现蚂蚁表皮上的不是惰性蜡,而是活性的”。 紧接又提到“ Mr.

  Currie discovered a specialised patch on the ants’ cuticle that harbours a particular kind of

  bacterium, one well known to the pharmaceutical industry, because it is the source of half

  the antibiotics used in medicine.” 这说明“ 蚂蚁的表皮有一种特殊的斑点, 其中隐藏着

  一种特殊的细菌。 这种细菌对于制药行业来说是非常有名的, 因为医学上使用的抗

  生素多半来源于此”。 选项 A“ 一些白色的云状霉菌嵌在它们的表皮上”, 但原文中

  并没有说是“ white cloud mold”,因此排除选项 A。 选项 B“ 蜡遍布在它们的皮肤上”,

  但原文中仅仅说的是“ He saw it was not inert wax. It is alive.(表皮上的不是惰性蜡,

  而是活性的。)” 因此排除 B 选项。 D 选项“ 一种能够抑制所有真菌生长的物质” 在

  文中没有提及,因此排除 D 选项。 选项 C“ 一种对人类有用的物质”,题目中的“ useful

  to humans” 与原文中的“ one well known to the pharmaceutical industry” 相对应。 因此,

  正确答案为 C。

  参考译文

  Leaf-cutting Ants and Fungus

  切叶蚁和真菌

  A The ants and their agriculture have been extensively studied over the years, but the recent

  research has uncovered intriguing new findings about the fungus they cultivate, how they

  domesticated it and how they cultivate it and preserve it from pathogens. For example, the

  fungus farms, which the ants were thought to keep free of pathogens, turn out to be vulnerable to a devastating mold, found nowhere else but in ants’ nests. To keep the mold in check,

  the ants long ago made a discovery that would do credit to any pharmaceutical laboratory.

  A 近年来, 人们对蚂蚁以及它们的“ 农业活动” 进行了广泛的研究。 但最近的研究揭

  示了一些关于它们所培育的真菌的有趣新发现, 包括它们是如何种植、 培育真菌以

  及防止真菌受到病原体感染的。 例如, 过去认为没有病原体的蚂蚁真菌农场原来非

  常容易感染一种毁坏性的霉菌, 而且这种霉菌仅存在于蚂蚁的巢穴中。 为了控制住

  这种霉菌, 蚂蚁很久以前就取得了一项能给所有药学实验室增添荣誉的发现。

  B Leaf-cutting ants and their fungus farms are a marvel of nature and perhaps the best known

  example of symbiosis, the mutual dependence of two species. The ants’ achievement is

  remarkable — the biologist Edward O. Wilson has called it “one of the major breakthroughs

  in animal evolution” — because it allows them to eat, courtesy of their mushroom’s digestive powers, the otherwise poisoned harvest of tropical forests whose leaves are laden with

  terpenoids, alkaloids and other chemicals designed to sicken browsers.

  B 切叶蚁和它们的真菌园是自然界的一个奇迹, 这也许还是两个物种相互依赖共生的

  最著名的案例。 生物学家爱德华 ·O· 威尔逊(Edward O. Wilson) 认为, 蚂蚁的这项

  成就是非凡的, 堪称“ 动物进化史上的重大突破之一”。 正是得益于真菌的消化能力,

  它们能够“ 食用” 热带雨林中一些有毒的叶子, 而这些叶子含有萜类化合物、 生物碱,

  以及其他能侵害食草动物的化学物质。

  C Fungus growing seems to have originated only once in evolution, because all gardening

  ants belong to a single tribe, the descendants of the first fungus farmer. There are more

  than 200 known species of the attine ant tribe, divided into 12 groups, or genera. The leafcutters use fresh vegetation; the other groups, known as the lower attines because their nests

  are smaller and their techniques more primitive, feed their gardens with detritus like dead

  leaves, insects and feces. In 1994 a team of four biologists, Ulrich G. Mueller and Ted R.

  Schultz from Cornell University and Ignacio H. Chapela and Stephen A. Rehner from theUnited States Department of Agriculture, analyzed the DNA of ant funguses. The common

  assumption that the funguses are all derived from a single strain, they found, was only half

  true.

  C 真菌种植的起源在进化史上似乎只有一次, 因为所有的园艺蚂蚁都属于一个种群,

  并且都是第一个真菌种植者的后代。 这种果蚁族有超过 200 个已知的种类, 它们被

  分成 12 组或 12 类。 其中, 切叶蚁用新鲜的植被来养活自己的真菌园 ; 另外一种被

  称为低等果蚁的蚂蚁则因为巢穴较小, 种植技术更原始, 只能用枯叶、 昆虫和粪便

  等碎屑来养活自己的真菌园。 1994 年, 一个由四位生物学家组成的团队对蚂蚁真菌

  的 DNA 进行了分析。 团队成员分别是来自康奈尔大学的乌尔里希 ·G· 穆勒 (Ulrich G.

  Mueller)和泰德 ·R· 舒尔茨 (Ted R. Schultz),以及来自美国农业部的伊格纳西奥 ·H· 查

  佩拉(Ignacio H. Chapela) 和史蒂芬 ·A· 雷纳(Stephen A. Rehner)。 他们发现, 真菌

  都是来源于单一菌株的这一设想并不完全正确。

  D The leaf-cutters’ fungus was indeed descended from a single strain, propagated clonally,

  or just by budding, for at least 23 million years. But the lower attine ants used different

  varieties of the fungus, and in one case a quite separate species, the four biologists discovered. Cameron R. Currie, a Ph.D. student in the University of Toronto, it seemed to Mr.

  Currie, resembled the monocultures of various human crops, that are very productive for a

  while and then succumb to some disastrous pathogen, such as the Irish potato blight. Monocultures, which lack the genetic diversity to respond to changing environmental threats,

  are sitting ducks for parasites. Mr. Currie felt there had to be a parasite in the ant-fungus

  system. But a century of ant research offered no support for the idea. Textbooks describe

  how leaf-cutter ants scrupulously weed their gardens of all foreign organisms. “People kept

  telling me, ‘You know the ants keep their gardens free of parasites, don’t you?’” Mr. Currie

  said of his efforts to find a hidden interloper.

  D 切叶蚁所种植的菌种确实来源于单一的菌株, 并且通过无性繁殖或出芽生殖的方式

  延续了至少 2300 万年之久。 但四位生物学家发现, 低等果蚁是一个例外, 它们培育

  出了不同种类的真菌,而且有别于单一菌株的真菌。 多伦多大学的博士生卡梅隆 ·R· 柯

  里 (Cameron R. Currie)认为,切叶蚁只种植一种真菌的做法相当于人类的单一作物制,

  在一段时间内产量非常高, 但随后会死于一些灾难性的病原体, 就如爱尔兰的马铃

  薯晚疫病。 单一栽培往往会因为缺乏基因的多样性来应对不断变化的环境威胁, 从

  而容易成为寄生虫攻击的目标。 柯里先生认为在蚂蚁与真菌的体系中一定有寄生虫

  存在。 但是这个想法在过去一个世纪对该蚂蚁的研究中并未得到任何证据支持。 教

  科书描述了切叶蚁如何小心翼翼地铲除花园里所有的外来生物。“ 人们不断地告诉我 :

  ‘ 你难道不知道蚂蚁会让寄生虫远离它们的园子吗? ’” 柯里先生说他正在努力找出隐藏的闯入者。

  E But after three years of sifting through attine ant gardens, Mr. Currie discovered they are far

  from free of infections. In last month’s issue of the Proceedings of the National Academy

  of Sciences, he and two colleagues, Dr. Mueller and David Mairoch, isolated several alien

  organisms, particularly a family of parasitic molds called Escovopsis. Escovopsis turns out

  to be a highly virulent pathogen that can devastate a fungus garden in a couple of days. It

  blooms like a white cloud, with the garden dimly visible underneath. In a day or two the

  whole garden is enveloped. “Other ants won’t go near it and the ants associated with the

  garden just starve to death,” Dr. Rehner said. “They just seem to give up, except for those

  that have rescued their larvae.”

  E 但经过三年对果蚁真菌园的仔细筛选, 柯里先生发现蚂蚁的真菌园并不能避免被

  感染。 在上个月出版的《美国国家科学院学报》 中, 他与两位同事米勒博士(Dr.

  Mueller) 和戴维 · 马里奇(David Mairoch) 分离出几个外来生物的菌种, 特别是一

  种叫作 Escovopsis 的寄生霉菌族。 结果证明 Escovopsis 霉菌是一种剧毒性病原体,

  它可以在几天之内毁灭一个真菌园。 Escovopsis 霉菌爆发时像白色的云, 园子在其笼

  罩下隐约可见, 一两天内便会笼罩全园。“ 其他蚂蚁不会去靠近它, 而这个园子内的

  蚂蚁会全部饿死,” 雷纳(Rehner)博士说 : “ 除了那些已经把幼虫拯救出来的蚂蚁外,

  其他蚂蚁似乎都放弃了(求生)。”

  F Evidently the ants usually manage to keep Escovopsis and other parasites under control.

  But with any lapse in control, or if the ants are removed, Escovopsis will quickly burst

  forth. Although new leaf-cutter gardens start off free of Escovopsis, within two years some

  60 percent become infected. The discovery of Escovopsis’ role brings a new level of understanding to the evolution of the attine ants. “In the last decade, evolutionary biologists have

  been increasingly aware of the role of parasites as driving forces in evolution,” Dr. Schultz

  said. There is now a possible reason to explain why the lower attine species keep changing

  the variety of fungus in their mushroom gardens, and occasionally domesticating new ones

  — to stay one step ahead of the relentless Escovopsis.

  F 显然, 蚂蚁通常能够将 Escovopsis 霉菌和其他寄生生物控制住。 但一旦在控制过程

  中出现任何失误或者蚂蚁被撤离, Escovopsis 霉菌将迅速爆发。 即使新建的切叶蚁真

  菌园开始时不会出现 Escovopsis 霉菌, 但两年内 60[%] 的真菌园又会被感染。 对 Escovopsis 霉菌作用的发现使得科学家们对果蚁的进化有了新的理解。“ 在过去的十年中,

  进化生物学家已经越来越意识到寄生虫在进化中的驱动力作用,” 舒尔茨博士说。 现

  在有一个似乎合理的理由可以解释为什么低等果蚁会不断改变蘑菇园中的真菌品种,偶尔还会培育出新的真菌品种——这是为了在与无情的 Escovopsis 霉菌的对抗中占

  据上风。

  G Interestingly, Mr. Currie found that the leaf-cutters had in general fewer alien molds in their

  gardens than the lower attines, yet they had more Escovopsis infections. It seems that the

  price they pay for cultivating a pure variety of fungus is a higher risk from Escovopsis. But

  the leaf-cutters may have little alternative: they cultivate a special variety of fungus which,

  unlike those grown by the lower attines, produces nutritious swollen tips for the ants to eat.

  G 有趣的是, 柯里先生发现切叶蚁真菌园里的外来霉菌总体上比低等果蚁的少, 但是

  它们更容易受到 Escovopsis 霉菌的感染。 似乎, 它们为培养出纯种真菌所付出的代

  价就是更容易受到来自 Escovopsis 霉菌的威胁。 但切叶蚁可能别无选择 : 它们培育

  出的这种特殊真菌跟低等果蚁所培育出的不同, 这种特殊真菌能够产出有营养的鼓

  包供蚂蚁食用。

  H Discovery of a third partner in the ant-fungus symbiosis raises the question of how the

  attine ants, especially the leaf-cutters, keep this dangerous interloper under control. Amazingly enough, Mr. Currie has again provided the answer. “People have known for a hundred

  years that ants have a whitish growth on the cuticle,” said Dr. Mueller referring to the

  insects’ body surface. “People would say this is like a cuticular wax. But Cameron was the

  first one in a hundred years to put these things under a microscope. He saw it was not inert

  wax. It is alive.” Mr. Currie discovered a specialised patch on the ants’ cuticle that harbours

  a particular kind of bacterium, one well known to the pharmaceutical industry, because

  it is the source of half the antibiotics used in medicine. From each of 22 species of attine

  ant studied, Mr. Cameron and colleagues isolated a species of Streptomyces bacterium,

  they reported in Nature in April. The Streptomyces does not have much effect on ordinary

  laboratory funguses. But it is a potent poisoner of Escovopsis, inhibiting its growth and suppressing spore formation. Because both the leaf-cutters and the lower attines use Streptomyces, the bacterium may have been part of their symbiosis for almost as long as the Escovopsis mold. If so, some Alexander Fleming of an ant discovered antibiotics millions of years

  before people did. Even now, the ants are accomplishing two feats beyond the powers of

  human technology. The leaf-cutters are growing a monocultural crop year after year without

  disaster, and they are using an antibiotic apparently so wisely and prudently that, unlike

  people, they are not provoking antibiotic resistance in the target pathogen.

  H 在蚂蚁真菌共生体中存在第三方的发现引发了对果蚁, 尤其是对切叶蚁如何控制危

  险的外来者的疑问。 令人惊奇的是, 柯里先生再次给出了答案。“ 一百年前人们就已经知道蚂蚁表皮有白色的物质,” 米勒博士这样说道, 他指的是昆虫的表皮。“ 人

  们会说这种物质看上去就像一层表皮蜡。 但作为这一百年来第一位把这种物质放在

  显微镜下观察的人, 卡梅隆看到这不是惰性蜡, 而是活性的。” 柯里先生发现, 蚂蚁

  的表皮上有一种特殊的斑点, 其中隐藏着一种特殊的细菌。 这种细菌对于制药行业

  来说是非常有名的, 因为医学上使用的抗生素多半来源于此。 卡梅隆先生和他的同

  事从他们所研究的 22 种果蚁身上分离出了一种链霉菌, 并将此成果发表于 4 月份的

  《自然》 杂志上。 链霉菌对普通的实验室真菌并没有太大影响, 但它却是 Escovopsis

  霉菌的强力毒杀者, 能够有效抑制 Escovopsis 霉菌的生长和孢子的生成。 由于切叶

  蚁和低等果蚁都使用链霉菌, 这种细菌可能是它们共生的一部分, 并且几乎与 Escovopsis 霉菌共生的时间一样长。 如果是这样, 那蚂蚁界的亚历山大 · 弗莱明就先于人

  类数百万年发现了抗生素。 即便到现在, 蚂蚁也正在完成两项超越人类技术水平的

  成果。 切叶蚁年复一年地培育着单一菌类, 并且无灾无难。 此外, 它们使用抗生素

  的方式显然比人类更为明智和谨慎。 因为, 与人类不同的是, 它们没有在目标病原

  体内诱发抗生素耐药性。


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