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2016-12-08    À´Ô´:¾­¼ÃѧÈË    ¡¾´ó ÖРС¡¿      ÃÀ¹úÍâ½Ì ÔÚÏß¿ÚÓïÅàѵ
Science and technology: Evolution: Bee kind to viruses
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A strange tale of collaboration between plants, pathogens and insects.
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Cucumber mosaic virus is not restricted to its eponymous host.
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They could also ravage tomatoes—stunting them and causing them to produce contorted tendril-like leaves.
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Given this devastation, it is surprising susceptible plants continue to exist; natural selection should have produced resistance years ago.
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A paper in this week’s PLoS Pathogens, however, explains the apparent contradiction.
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The team that wrote it, led by John Carr of Cambridge University, found that the virus actually helps its host to reproduce.
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It does so using an unsuspecting accomplice: the bumble-bee.
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Like many discoveries, this one was accidental.
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Dr Carr had ordered some equipment to analyse the volatile chemicals emitted by infected tomato plants.
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While he was waiting for it to arrive, a colleague offered to lend him some bees, as these pollinators are known for their sense of smell.
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His team placed both healthy and infected plants in a greenhouse, covered them so that the bees could smell but not see them, and released the insects.
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The bees could indeed tell the difference—but to Dr Carr’s surprise they did not favour the healthy plants.
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Rather, they strongly preferred the infected ones.
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For some viral strains, they were four times more likely to visit an infected plant than a healthy one.
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Subsequent experiments, carried out after the analytical apparatus had arrived, showed that this difference in behaviour was, indeed, a response to differences in the chemicals that infected and uninfected plants give off.
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The question was how the bees’ preference played into the plants’ evolution.
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Tomatoes are able to fertilise themselves, without need of a pollinator, yet even this self-pollination can be assisted by a bee visiting a flower.
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Vibrations caused by the insect’s buzzing help release pollen from the flower’s anthers so that it can fall onto the stigma.
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Could this process, Dr Carr wondered, make up for the evolutionary disadvantage the virus otherwise inflicts?
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To investigate, he and his colleagues grew both healthy and infected tomatoes in a greenhouse with no bees, and a similar mixture in a greenhouse into which bees were released one at a time, and their movements tracked.
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They then analysed the seeds of the fruits of each plant as a proxy for evolutionary fitness.
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All of the seeds they looked at seemed viable.
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What differed was the number of seeds in each fruit.
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In the bee-free greenhouse, infected plants had only about 50 seeds per fruit, whereas healthy ones averaged 70.
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When bees were present, though, something strange happened.
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The seed count of fruits from healthy flowers visited by bees increased to 85—but that of the flowers of infected plants jumped much higher, to 115.
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Further investigation showed a possible reason.
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Not only did bees visit the infected plants more, but they also spent longer buzzing around infected flowers.
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The extra pollen thus displaced probably explains how the sickly plants outproduced their healthy confreres.
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Using these experimental data, the researchers modelled how the system might have evolved.
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They found that the pollinator’s preference for the smell of infected plants was big enough to stop the emergence of viral-resistance in tomatoes.
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That explains what is in it for the tomatoes and also for the virus.
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What the bees get out of it, though, remains mysterious.
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