夕汩

北京时间1月4日消息，据英国《新科学家》杂志报道，丹麦研究人员日前发现，一种外表美丽的蝴蝶给自己的幼虫伪装了蚂蚁自身特有的气味，诱骗蚂蚁为其养育幼虫。并且，令研究人员颇感兴趣的是，蝴蝶和蚂蚁还展开了一场进化“军备竞赛”，力图压制对方入侵，这反倒促进了它们的进化。

　　研究人员发现，这种名为alcon blue butterfly的蝴蝶，其毛虫能长出一层拥有特殊化学信号的外衣，令蚂蚁误以为这些幼虫是它们自己的孩子，将其带回蚁穴精心养育。据丹麦哥本哈根大学教授戴维-纳什领导的一个研究小组介绍，蚂蚁似乎已经“意识到”自己上当受骗，其中一个种群正在同蝴蝶展开进化“军备竞赛”。

　　Maculinea alcon蝴蝶与丹麦两种Myrmica蚂蚁存在寄生关系。这种蝴蝶将卵产在植物上，长大到幼虫后从树上跌落至地上，等着恰好经过此处的蚂蚁将其拾起来，带回蚁穴。一直以来有一种理论认为，蚂蚁误将蝴蝶幼虫当成了它们自己的孩子，因为这些幼虫像蜡一般的防水外衣具有相似的化学物质。蚂蚁先用触须“品尝”蝴蝶幼虫的漂亮外衣，虽经过反复侦察，但最终还是误将它们当作自己的孩子。

　　纳什最近的研究证实了这一理论的正确性。通过比较蚂蚁和蝴蝶幼虫像蜡一般的外衣构成，他们发现，毛虫模仿蚂蚁越像，它们的“养父母”就越快将其带回蚁穴。蚂蚁将毛虫带回家的时间显得至关重要，因为一旦毛虫离开它们的食物——植物，在融入蚂蚁家族之前，它们吃不到一口饭。进入蚁穴之后，蚂蚁会向毛虫提供多于它们自己吃的食物。纳什解释说：“这也许是因为它们样子更大一些吧。蚂蚁可能认为，食物对正处于生长关键期的孩子来说至关重要，宁愿自己省下来让孩子多吃一些。”

　　纳什及同事还发现了另外一个令他们颇感兴趣的迹象：蚂蚁和蝴蝶正在进行一场进化“军备竞赛”。早在2000年，研究人员就发现，当他们将alcon蝴蝶毛虫放入到它们通常情况下不寄生的蚂蚁群时，这些毛虫存活率反而更高。这种情况表明，经常受到诱骗的蚂蚁已经想出了一个对付蝴蝶阴谋诡计的防御策略。

　　纳什在接受《新科学家》杂志采访时表示：“我们从寄生种群和alcon蝴蝶从来不寄居的其他种群中找出一些蚂蚁幼虫。”结果他们发现，没有受到蝴蝶骚扰的蚂蚁，它们像蜡一般的外衣拥有同蝴蝶非常相似的化学信号。

　　相比之下，来自它们常常寄生之所的蚂蚁，它们的化学信号经过进化，反而拥有更为广泛的多样性。这表明，蝴蝶和蚂蚁这两个物种在这种尔虞我诈的较量中，不断进化新的化学信号，一方试图施展骗术渗透，另一方则在寻找相应的应对之策，避免遭到渗透。在这场较量当中，似乎还是蝴蝶棋高一着，最终将侵入它们领地的蚂蚁消灭掉。

　　研究人员认为，Myrmica蚂蚁中一个名为Myrmica rubra种群的灭绝，可能就源于蝴蝶在较量中的胜利。种种迹象表明，作为应对之策，蝴蝶正开始在另外一个蚂蚁种群开辟寄生之地，而三年前它们还没有向这个种群展开渗透。纳什解释说：“一旦某个宿主物种数量变得十分稀少，蝴蝶就会转移寄生目标，向另外一个物种入侵，将这个物种当作宿主，直至第一个宿主物种的适应性丧失。”

　　目前就确定地说这种情况正发生在丹麦还为时尚早，纳什及同事已经启动了一个长期的监控实验，研究这种转变是否会在未来几年发生。芬兰约恩苏大大学昆虫寄生专家托米-尼曼表示，令人感兴趣之处是，蝴蝶迫使蚂蚁向着多样性的方向进化发展。他说：“一切胜利都是短暂的，稍纵即逝，因为，接下来蝴蝶幼虫就可以、也必须适应新的化学特征。”(引自新浪)

The Alcon blue is a 'brood parasite' - the insect world's equivalent of the cuckoo. David Nash and European colleagues found that its caterpillars are coated in chemicals that smell very similar to those used by the two species it uses as hosts. To ants, these chemicals are badges of identity and so similar are the caterpillars that the ants adopt them and raise them as their own. The more exacting the caterpillar’s chemicals, the higher its chances of being adopted.

The alien larvae are bad news for the colony, for the ants fawn over them at the expense of their own young, which risk starvation. If a small nest takes in even a few caterpillars, it has more than a 50% chance of having no brood of its own. That puts pressure on the ants to fight back and Nash realised that the two species provide a marvellous case study for studying evolutionary arms races.

Theory predicts that if the parasites are common enough, they should be caught in an ongoing battle with their host, evolving to become more sophisticated mimics, while the ants evolve to become more discriminating carers. The two species make a particularly good model for this because their geographical ranges overlap in a fractured mosaic.

Alcon blues lay their eggs on the rare marsh gentian plant and it’s there that they first grow before being adopted by a foraging ant. Both gentians and butterflies are rare but the ants are common, meaning that only a small proportion of colonies are ever parasitized. These form evolutionary hotspots where the two species wage adaptive war against each other in contrast to the many coldspots where colonies never encounter the deceptive butterflies.

Nash found evidence of evolutionary hotspots when he looked at one of the butterfly's host species, M.rubra. The chemical coats of populations that were victimised by Alcon blues were significantly different from each other, while those of uninfected colonies were much the same. Those that had encountered the parasitic moochers were adapting by shifting their own smells to distinguish their own from the caterpillars.

But when Nash looked at the second host species, M.ruginodis, he found no traces of evolutionary arms races. All the populations smelled very similar regardless of whether they were parasitized or not.

The difference lies in the mating patterns of the two ant species. M.rubra queens breed with local males, from colonies in the same gentian patches that are likely to also have a measure of resistance against Alcon blues. M.ruginodis queens disperse more widely and frequently mate with males from distant populations that do not encounter the parasites. Any new mutations for parasite resistance are quickly diluted by the flow of genes from these non-resistant neighbours.

Nash thinks that the butterfly uses M.ruginodis as a back-up host, a safe chump to manipulate when M.rubra evolves a strong enough resistance to its lies. To back this up, he found at least one site where M.rubra populations smelled very different but had no contact with Alcon blues. He thinks that these are populations that won their arms race - they once had to contend with the parasite but adapted so well that it abandoned them as hosts.

(Reference: Nash, D.R., Als, T.D., Maile, R., Jones, G.R., Boomsma, J.J. (2008). A Mosaic of Chemical Coevolution in a Large Blue Butterfly. Science, 319(5859), 88-90. DOI: 10.1126/science.1149180)