Part 2

Read the text and answer the questions below.

Leaf-cutting ants and fungus

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 mould, somewhere else but in ant’s bacteria layer the mould is checked, a long made a discovery that would do credit to any pharmaceutical laboratory.

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 herbivores.

Fungus-growing ants 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 clades. The leaf-cutting ants are fresh vegetarians, the other groups cultivate in the lower attines because their nests are smaller and their techniques more primitive, feed their gardens with detritus like dead leaves, insects and faeces. In 1994 a team of biologists, Ulrich Mueller and Ted Schultz from Cornell University and Ignacio H Chapela and Stephen A Rehner from the United States Department of Agriculture, analysed the DNA of ant fungi. The common assumption that the fungus ants are all derived from a single fungus, they found, was only half true.

The leaf-cutters’ fungus was indeed descended from a single strain, propagated clonally, or 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 PhD student at the University of Toronto, seemed to McFadden to re-examine the associations of fungus and ants, that are very productive for a while and then succumb to some destructive pathogens, each of the fungus plant’s high-Mueller believes, for all the genetic diversity is required to change environmental structure, cutting ducks for pesticide. Mueller says that there had to be a parasite in the ant fungus garden. 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,’” McFadden said and his efforts to find a hidden interloper.

But after three years of sifting through ants and fungus, McFadden discovered they are free of five of six infectious ants’ moulds. The sixth species, the mould in the Proceedings of the National Academy of Science, he and two colleagues, Mueller and Floral Murrish, isolated several organisms, particularly a family of parasitic moulds 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 dimpling and then in a day or two the whole garden is consumed. “Other ants won’t go near it and the ants associated with the garden just starve to death,” Rehner said. “They just seem to give up, except for those that have rescued their larvae.”

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 fascinated at the role of parasites as driving force 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, occasionally domesticating new ones to stay one step ahead of the relentless Escovopsis.

Interestingly, Mr Currie found that the leaf-cutters had in general fewer fungal weeds in their gardens than the lower attines did, but they had more Escovopsis instead. 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 swellings suitable for the ants to eat.

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 that this is a cuticular wax. But Cameron was the first in a hundred years to go put these things under a microscope. He saw it was not wax at all.” As a microscope discovered a bacterium on the ants’ cuticle that’s a strain of bacteria, now well known to the pharmaceutical industry because of the ability of the bacteria and in medicine. From 22 species of ants were cultured. Mr Currie and colleagues isolated a species of the Streptomyces bacteria, which is reported in Nature in April. The Streptomyces does not harm the ant fungus. But it is as a partner parasite of Escovopsis, inhibiting its growth and spore formation because both the leaf-cutters and the lower attine ants use Streptomyces, the bacteria may have been part of their symbiosis for almost as long as the Escovopsis mould. If so, this resembles Alexander Fleming’s discovery of antibiotic moulds in years before people did. Even so, the ants are accomplishing this feat 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 provoking antibiotic resistance in the target pathogen.

Questions 14-19

Use the information in the passage to match the options with the activities or features of ants below. Write the appropriate letters 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 naturally keep a highly dangerous parasite under control 19 use special strategies to fight against Escovopsis

Questions 20-24

Reading Passage 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 ants 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 mould embedded 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