Nyman Tommi, Bokma Folmer, Kopelke Jens-Peter
Faculty of Biosciences, University of Joensuu, FI-80101 Joensuu, Finland.
BMC Biol. 2007 Nov 1;5:49. doi: 10.1186/1741-7007-5-49.
Plants, plant-feeding insects, and insect parasitoids form some of the most complex and species-rich food webs. According to the classic escape-and-radiate (EAR) hypothesis, these hyperdiverse communities result from coevolutionary arms races consisting of successive cycles of enemy escape, radiation, and colonization by new enemy lineages. It has also been suggested that "enemy-free space" provided by novel host plants could promote host shifts by herbivores, and that parasitoids could similarly drive diversification of gall form in insects that induce galls on plants. Because these central coevolutionary hypotheses have never been tested in a phylogenetic framework, we combined phylogenetic information on willow-galling sawflies with data on their host plants, gall types, and enemy communities.
We found that evolutionary shifts in host plant use and habitat have led to dramatic prunings of parasitoid communities, and that changes in gall phenotype can provide "enemy-free morphospace" for millions of years even in the absence of host plant shifts. Some parasites have nevertheless managed to colonize recently-evolved gall types, and this has apparently led to adaptive speciation in several enemy groups. However, having fewer enemies does not in itself increase speciation probabilities in individual sawfly lineages, partly because the high diversity of the enemy community facilitates compensatory attack by remaining parasite taxa.
Taken together, our results indicate that niche-dependent parasitism is a major force promoting ecological divergence in herbivorous insects, and that prey divergence can cause speciation in parasite lineages. However, the results also show that the EAR hypothesis is too simplistic for species-rich food webs: instead, diversification seems to be spurred by a continuous stepwise process, in which ecological and phenotypic shifts in prey lineages are followed by a lagged evolutionary response by some of the associated enemies.
植物、植食性昆虫和昆虫寄生蜂构成了一些最为复杂且物种丰富的食物网。根据经典的逃逸与辐射（EAR）假说，这些高度多样化的群落源于协同进化的军备竞赛，其由敌人逃逸、辐射以及新敌人谱系的定殖等连续循环组成。也有人提出，新宿主植物提供的“无天敌空间”可促进食草动物的宿主转移，并且寄生蜂同样可推动在植物上形成虫瘿的昆虫中虫瘿形态的多样化。由于这些核心的协同进化假说从未在系统发育框架中得到检验，我们将柳树瘿蜂的系统发育信息与它们的宿主植物、虫瘿类型和天敌群落数据相结合。
我们发现宿主植物利用和栖息地的进化转变导致了寄生蜂群落的大幅削减，并且虫瘿表型的变化即使在没有宿主植物转移的情况下也能在数百万年里提供“无天敌形态空间”。然而，一些寄生蜂成功定殖到了最近进化出的虫瘿类型中，这显然导致了几个天敌类群的适应性物种形成。不过，天敌较少本身并不会增加单个锯蜂谱系中的物种形成概率，部分原因是天敌群落的高度多样性便于剩余寄生分类群进行补偿性攻击。
综合来看，我们的结果表明生态位依赖的寄生现象是促进食草昆虫生态分化的主要力量，并且猎物分化可导致寄生蜂谱系中的物种形成。然而，结果也表明EAR假说对于物种丰富的食物网来说过于简单：相反，多样化似乎是由一个连续的逐步过程所推动，在这个过程中，猎物谱系的生态和表型转变之后会伴随着一些相关天敌的滞后进化响应。
