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寄生虫传播的神奇策略

来源:The American Naturalist 2008-01-23 15:28

图片来源:Steve Yanoviak/University of Arkansas 

那些采摘浆果的人要小心了。科学家日前发现了一种能够传染给大滑行蚁(Cephalotes atratus)的寄生虫。这种寄生虫能够使蚂蚁的尾部变得又红又亮,看起来就像一颗熟透了的浆果(如上图)。

据美国《科学》杂志在线报道,这些寄生虫还会产下数百枚卵,并使得蚂蚁的尾部在空气中摇摆。这种方法会引诱小鸟吃下这些“浆果”,从而使寄生虫随着小鸟的粪便传播。接下来,蚂蚁又会搜集鸟儿的粪便,并用这些粪便给后代喂食,从而又开始了一个新的循环。这是寄生虫利用模拟浆果进行传播的第一个例证。研究小组将在4月份出版的《美国博物学家》(The American Naturalist) 杂志上报告这一研究成果。(科学时报 群芳/编译)

(《美国博物学家》(The American Naturalist),Vol 171:April 2008,S. P. Yanoviak, R. Dudley, G. Poinar, Jr.)

英文原文:

Parasite-induced fruit mimicry in a tropical canopy ant

S. P. Yanoviak (Florida Medical Entomology Laboratory), M. Kaspari (University of Oklahoma), R. Dudley (Smithsonian Tropical Research and University of California, Berkeley), and G. Poinar, Jr. (Oregon State University)

(Vol 171:April)

Moving from one host to another—a critical stage in many parasite life cycles—presents an enormous logistical obstacle for many parasitic species. One common strategy to overcome this problem is for the parasite to change the appearance or behavior of the intermediate host so that it is more attractive to predatory terminal hosts. A team of scientists working high in the canopy of tropical rain forests of Panama and Peru recently discovered an amazing example of this phenomenon: a new species of parasitic nematode, Myrmeconema neotropicum, modifies its host ants such that they resemble ripe fruits. Research conducted by Steve Yanoviak (University of Arkansas at Little Rock), Michael Kaspari (University of Oklahoma), Robert Dudley (University of California, Berkeley), and George Poinar, Jr. (Oregon State University) suggests that the infected ants are attractive to and are consumed by frugivorous birds, which subsequently transmit the nematodes to new colonies via their feces.

The unfortunate ants in this case are workers of the Neotropical canopy species Cephalotes atratus, which recently became famous for their aerial gliding ability. Whereas normal C. atratus workers are completely black, parasitized ants have bright red gasters, the globular rear ends of ants. The red gasters of infected ants contain hundreds of nematode eggs and are much easier to pluck from the ant's body than the gasters of normal ants. Moreover, infected ants walk with their gasters in a conspicuous elevated position, and ants become sluggish and less defensive in the latter stages of infection. Thus, a duped frugivorous bird could easily pluck the gaster from a parasitized ant. Given that bird feces are commonly collected by foraging C. atratus workers, birds provide a reliable mechanism for transmitting the parasite to new ant colonies. Once inside a colony, the parasitic worms develop inside the ant larvae.

Red-gastered C. atratus were mistakenly described as a separate taxonomic variety over a century ago, and they have since appeared in many museum collections. "Like other ant biologists, I initially thought this was another species of Cephalotes," says Kaspari. "Robert didn’t think so and we made a bet over beers. Then Steve opened one up under the scope and–Wow! I lost the bet," Kaspari admits.

This is the first report of parasite-mediated fruit mimicry in an arthropod, and provides an amazing example of the complexity of host-parasite interactions. It also shows how finely-tuned such interactions may become. In this case, the reddening of the ant gaster is very closely linked to both the development of the parasite and to normal changes in the ant's behavior over time: the infected ant most closely resembles a ripe berry only when the parasites are fully developed, which corresponds to the period in the ant's life when it is working as a forager, and therefore spending large amounts of time outside the nest.

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