美科学家发现地球最古老“僵尸蚂蚁”证据

          在九月即将发行的《The American Naturalist》杂志上，哈佛大学的研究人员David P. Hughes发现，若木蚁(carpenter ant)感染一种名为Ophiocordyceps unilateralis的真菌后，在短时间内就能导致木蚁死亡，而该真菌则能在木蚁体内合适的位置进行生长繁殖。在木蚁死亡几天之内，在木蚁脑内后部，真菌的子实体——底座(stroma)中长出孢子，一两星期之后，底座开始释放孢子落到地下。落下的每个孢子又开始等待其潜在寄主。

（Carpenter ant. The arrow points to the stroma, the fruiting body of the fungus. ）

         Hughes课题组在泰国的森林中发现，这种被感染死亡的木蚁总是紧紧钳在离地高约25厘米的树叶上，这些树叶位于植株的西北方向，而且该方向的温度、湿度以及光照对真菌的生长繁殖显然都是最佳位置。当研究人员将这些有木蚁的树叶挪到较高位置或放到地上，则该真菌将不能生长。

        但当研究人员对死亡的木蚁进行解剖后发现，即使木蚁在利于真菌生长的位置死亡，对真菌来说也只成功了一半。在死亡的木蚁体内，真菌将木蚁的部分尸体转化成为生长所需的糖类，但木蚁的肌肉部分留下以钳住树叶。木蚁的外壳形成真菌隔绝外部细菌和其他真菌的保护层。对木蚁来说，要避免这种可怕的感染只能选择远离这种真菌，并且避免在感染区域从事觅食活动。

        目前，这项研究发现能否能够应用到控制木蚁上还不得而知。

参阅文献：
1. Sandra B. Andersen, Sylvia Gerritsma, Kalsum M. Yusah, David Mayntz, Nigel L. Hywel%u2010Jones, Johan Billen, Jacobus J. Boomsma, and David P. Hughes. The Life of a Dead Ant: The Expression of an Adaptive Extended Phenotype. The American Naturalist, September 2009 DOI: 10.1086/603640

昆蟲大腦被真菌寄生(Ophiocordyceps Unilateralis)

2010-4-30

还没看过的请看这短片
For those who doesnt know, this is the video clip
http://vhead.blog.sina.com.cn/player/outer_player.swf?auto=0&vid=17577390&uid=1313444507

解说（中文版）
據美國《科學》雜誌線上新聞報道，研究人員發現，這些昆蟲的大腦被一種名為Ophiocordyceps unilateralis的真菌所寄生，後者會迫使在樹上棲息的螞蟻（Camponotus leonardi）來到森林的地面，並啃食一些小樹葉。
這種寄生的真菌會控制螞蟻停留在距地面25公分的地方。與位於距離地面20米高的蟻巢相比，地面的氣候更冷並且更潮濕，而這也正是真菌最喜愛的生活環境。儘管螞蟻很快就會死掉，但它的工作還沒有全部完成。這種寄生的真菌會在昆蟲的外殼中宿營，並長出一根莖來（如上圖），從而最終釋放孢子，以形成更多的螞蟻墳墓。研究人員在7月23日的《美國博物學家》上報告了這一發現。

English Explaination
There really are zombies roaming the earth. (They're just really, really small.)
Just in time for Halloween, this creepy tale of zombie slaves has all the elements of a good gross-out movie -- parasites, spores, brain control, exploding body parts -- and all of it is real.
Here's the storyline: Spores hijack the brain of an unwitting victim, force it to leave the safety of its home and travel to a spot that favors the parasite's needs.
There, the host is killed and its body is used to spread spores to more victims.
It sounds like an evil plot to take over the world, but it's one of the everyday dangers facing Camponotus leonardi, a tree-dwelling species of carpenter ant that lives high in the trees in Thailand's rainforests.
"It really does seem sinister," said David Hughes, a Harvard University organismal biologist.
One ill-fated foraging trip is all it takes for an unwary worker ant to become the zombie slave of a fungus called Ophiocordyceps unilateralis.
Researchers have long known that the fungus preys on this ant species. But it wasn't until recently that they found how it controls their minds.
Now, they know the fungus directs infected ants to tree leaves that are at the ideal height, temperature and humidity the fungus needs to grow.
That puts the fungus near the top of a class of creepy parasites that control their hosts' behavior.
Hughes is a member of an international team of researchers who studied how the fungus does its magic on the ant. Their work was published recently in the journal American Naturalist.
Hughes studies adaptive parasitic behavioral manipulation, in which parasites control their hosts' minds or decision-making abilities. (Wasn't this spelled out in a Star Trek movie?)
There are several examples of this in nature, including:
• The jewel wasp of southern Asia and Africa. This bug stings cockroaches in their brains and injects a venom that disables the roach's ability to decide where to walk.
The wasp grabs its zombie roach by an antenna and walks it to a nest where even more bad things happen. A wasp lays its egg on the roach, and its larvae eat the insect alive over several days.
• A hair worm that eats grasshoppers from the inside out. This worm controls grasshoppers and makes them jump into pools where they drown. This allows the worms to emerge and mate under water.
• A nematode called Myrmeconema neotropicum that turns ant abdomens bright red. Then this nematode forces the ant to stick its belly into the air, resembling delicious berries that birds are fond of.
Once the birds eat the ants, they spread the next generation of nematode eggs through their droppings.
Robert Dudley, a University of California, Berkeley, biologist, studies the nematode. He was among a team of researchers who discovered the parasite in 2005 in the jungles of Central America and the Amazon.
"What is obvious is the (hosts) hold their abdomens much more vertically than other ants. It's like they are advertising," Dudley said. "There is definitely some kind of neural effect. It's hijacking the brain."
Ants are often ideal targets for parasites because of their social structure, said Joan Herbers, an Ohio State University entomologist and ant expert. A centralized colony with thousands of closely packed individuals is an ideal place for a parasite to set up shop, she said.
It's creepy, for sure, but it's also natural.
"Parasites are part of what makes the world go 'round," Herbers said.
For Hughes, Orphiocordyceps is fascinating because of the control it exerts.
Think about it -- this fungus doesn't have a brain, yet it is able to make ants leave the treetops to find a leaf or twig about 10 inches from the ground.
Hughes thinks the fungus developed specialized chemicals or compounds that affect the ants' brain chemistry. He hopes to identify them in future studies.
"What compounds are necessary to change the ants' behavior in such a fine, detailed manner?" Hughes said. "It's a fabulous example of adaptation."
Discovering compounds that control ants' brains doesn't make Hughes an evil scientist. In fact, he foresees using these compounds to help exterminators keep destructive fire ants at bay.

Source here
http://www.dispatch.com/live/con ... -09_G3_IMFEJ9T.html

科学家发现地球最古老“僵尸蚂蚁”证据

2010年08月19日08:54新华网

这只木蚁（Camponotus leonardi）的身体已被“Ophiocordyceps unilateralis”真菌消耗一空。

据国外媒体报道，美国哈佛大学科学家近日发现了地球上最古老的“僵尸蚂蚁”的证据。一种生活于4800万年前的真菌能够通过释放化学物质改变和控制蚂蚁的行为，使其变成自己的傀儡，直到蚂蚁最终死亡。

这种真菌学名为“Ophiocordyceps unilateralis”，其恐怖之处的证据发现于德国达姆施塔特市附近梅塞尔化石坑的树叶化石上，这些树木大约生活于4800万年前。科学家们发现，这种寄生真菌早在远古时代就已进化出控制自己所寄生的动物的能力，甚至比喜马拉雅山脉的隆起时间还要早。

目前，这种真菌仍然存活于地球之上，通常寄生于木蚁身上。木蚁返回树冠蚁穴前，经过森林地被物时通常会感染这种寄生真菌，其行为从此就会受到真菌的控制。真菌在蚂蚁体内不断生长，并释放出化学物质影响蚂蚁的行为，使其成为“僵尸蚂蚁”。一些“僵尸蚂蚁”从此离开自己所在的蚁群，独自在外流浪，寻找新鲜的树叶。

被真菌控制的“僵尸蚂蚁”从此不再有自己的行为和生活，而它们生命的最后阶段也是最痛苦、最恐怖的。在生命的最后几小时内，“僵尸蚂蚁”会爬向自己所处树叶的下方，用下颚死死地咬住树叶的中央叶脉，从而将自己困死于树叶之上，同时寄生的真菌也被锁定于树叶之上。这片树叶就成为了“蚂蚁坟墓”。

美国哈佛大学科学家大卫-休斯介绍说，“全部都是如此。在一平方米的范围内，你或许能够找到20到30个‘蚂蚁坟墓’。它们通常都处于距离地面一定高度的树叶之上，在死亡前会紧紧地咬住树叶的主叶脉。”

在树冠上或在森林地被物上，这种真菌无法成长。但是，“僵尸蚂蚁”通常会死于两者之间的树叶上，也就是距离地面一定高度的树叶，但未达到树冠的高度。这种环境的湿度和温度最适宜真菌的生存。“僵尸蚂蚁”死亡后，寄生真菌就会从其头部长出萌芽，产生孢子并于夜间“发射”到森林地被物上，从而再感染其他的蚂蚁。

由休斯领导的科学家团队注意到，被感染的蚂蚁死亡前会用尽所有的力气死死咬住树叶，它们如此用力从而在这个世界上留下了最后的印迹。它们用自己的下颚在树叶上产生的小孔形成了一个十分特别的咬痕，呈清晰的哑铃形状。

科学家们的研究成果发表于近期出版的《生物学快报》杂志之上。在研究报告中，科学家们介绍了他们是如何从大量的记载被昆虫、真菌和其他生物体破坏的树叶的图片数据库中发现这些证据的。在研究过程中，他们发现了一幅4800万年前生活于梅塞尔地区的树叶的化石图片。图片显示，这种特殊的“死亡之握”印迹来自被感染的“僵尸蚂蚁”。在4800万年前，梅塞尔地区正处于亚热带森林地区。

休斯表示，“现在，我们把它看作是行为控制的首个案例，也许是能够被发现的唯一证据。因为在大多数情况下，这种行为控制虽然很强大，但却很短暂，一般不会留下永久的印迹。现在的问题是，这种寄生行为的动机是什么，它们为什么不只是杀死宿主，而是要先控制宿主的大脑和肌肉，然后再杀死它。在所有的寄生生物体中，只有很少一部分进化出操纵宿主行为的能力。”

科学家们目前还未弄清楚寄生真菌究竟是如何控制被感染蚂蚁的，但他们知道寄生真菌向蚂蚁体内释放了生物碱化学物质。