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Science and technology: Evolution: Bee kind to viruses
科學技術：進化：蜜蜂愛病毒
A strange tale of collaboration between plants, pathogens and insects.
植物、病毒和昆蟲的奇特合作。
Cucumber mosaic virus is not restricted to its eponymous host.
黃瓜花葉病毒能感染不是只有黃瓜。
They could also ravage tomatoes—stunting them and causing them to produce contorted tendril-like leaves.
這種病毒也荼毒番茄——病株會長出卷曲的病葉。
Given this devastation, it is surprising susceptible plants continue to exist; natural selection should have produced resistance years ago.
考慮到病毒的危害，至今還存在易感染植物著實讓人驚奇——自然選擇應該早就幫助它們形成了病毒抵抗力。
A paper in this week’s PLoS Pathogens, however, explains the apparent contradiction.
本周，PLoS Pathogens發表的一篇論文解釋了這一明顯矛盾。
The team that wrote it, led by John Carr of Cambridge University, found that the virus actually helps its host to reproduce.
發表這篇文章的團隊由劍橋大學的約翰·卡爾帶領，他們發現病毒實際上在幫助宿主繁衍。
It does so using an unsuspecting accomplice: the bumble-bee.
而達成這種目標借助的不是別的，正是大黃蜂。
Like many discoveries, this one was accidental.
像很多發現一樣，這個發現也純屬偶然。
Dr Carr had ordered some equipment to analyse the volatile chemicals emitted by infected tomato plants.
卡爾博士訂購了一些設備用來分析番茄病株釋放的易揮發化學物質。
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.
在等候交貨的這段時間，一位同事提出借給他一些蜜蜂，因為這些傳粉使者具有靈敏的嗅覺。
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.
于是團隊人員把正常植株和感染植株一同放到溫室，遮蓋它們確保蜜蜂只能嗅到而不能看到它們，然后釋放蜜蜂。
The bees could indeed tell the difference—but to Dr Carr’s surprise they did not favour the healthy plants.
蜜蜂確實可以嗅出植株的不同，但令卡爾博士感到驚訝的是，它們并不喜歡健康植株。
Rather, they strongly preferred the infected ones.
相反，它們對感染植株青睞有加。
For some viral strains, they were four times more likely to visit an infected plant than a healthy one.
有一些受感染的病株甚至能吸引比健康植株多三倍的蜜蜂。
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.
在分析設備到貨以后進行的后續實驗揭示了蜜蜂區別對待植株的原因——感染植株和健康植株釋放的化學物質不同。
The question was how the bees’ preference played into the plants’ evolution.
那么蜜蜂的偏愛在植物進化中起到了什么作用呢？
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.
番茄可以自我繁衍，不需要傳粉者，但是如果有蜜蜂來傳播花粉，這種自花授粉的植物也會得到幫助。
Vibrations caused by the insect’s buzzing help release pollen from the flower’s anthers so that it can fall onto the stigma.
昆蟲翅膀的震動幫助播撒取自花藥的花粉，并落到柱頭上。
Could this process, Dr Carr wondered, make up for the evolutionary disadvantage the virus otherwise inflicts?
卡爾博士疑惑這個過程能否彌補病毒在其他方面帶來的進化劣勢。
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.
為了進一步調查，卡爾博士和同事在一個溫室里種植了健康番茄和感染番茄，但是沒有蜜蜂，又在另一個溫室里進行同樣的種植，但是向這個溫室每次釋放一只蜜蜂，并追蹤它的軌跡。
They then analysed the seeds of the fruits of each plant as a proxy for evolutionary fitness.
之后他們選取每株番茄的種子作為進化適應性的代表進行分析。
All of the seeds they looked at seemed viable.
他們觀察的種子似乎都有望存活。
What differed was the number of seeds in each fruit.
不同點在于每個番茄果實內所含的種子數量。
In the bee-free greenhouse, infected plants had only about 50 seeds per fruit, whereas healthy ones averaged 70.
在沒有蜜蜂的溫室里，感染番茄平均每個果實僅含50枚種子，健康番茄70枚。
When bees were present, though, something strange happened.
在蜜蜂活動的溫室里，神奇的事情上演了。
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.
蜜蜂拜訪過的健康番茄果實內的種子增長至85枚，而感染番茄躍升至115枚。
Further investigation showed a possible reason.
更深入的調查提供了可能的解釋。
Not only did bees visit the infected plants more, but they also spent longer buzzing around infected flowers.
感染番茄不僅能吸引更多蜜蜂，還能讓它們環繞感染植物飛舞更久。
The extra pollen thus displaced probably explains how the sickly plants outproduced their healthy confreres.
充裕的花粉很可能是這些生病植株比它們健康的同伴更高產的原因。
Using these experimental data, the researchers modelled how the system might have evolved.
研究人員運用實驗數據模擬了這一系統可能的發展過程。
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.
他們發現，傳粉者對感染植株味道的青睞足以阻止番茄形成病毒抵抗力。
That explains what is in it for the tomatoes and also for the virus.
這同樣解釋了番茄和病毒的利害關系。
What the bees get out of it, though, remains mysterious.
但是蜜蜂能從中分一杯什么羹，還是個謎。
翻譯：Angela
