Little Tom J, Watt Kathryn, Ebert Dieter
Institute of Evolutionary Biology, School of Biology, University of Edinburgh, Scotland.
Evolution. 2006 Jan;60(1):31-8.
Specificity in parasitic interactions can be defined by host genotypes that are resistant to only a subset of parasite genotypes and parasite genotypes that are infective on a subset of host genotypes. It is not always clear if specificity is determined by the genotypes of the interactors, or if phenotypic plasticity (sometimes called acclimation) plays a larger role. Coevolutionary outcomes critically depend on the pervasiveness of genetic interactions. We studied specificity using the bacterial parasite Pasteuria ramosa and its crustacean host Daphnia magna. First, we tested for short-term adaptation of P. ramosa lines that had been rapidly shifted among different host genotypes. Adaptation at this time-scale would demonstrate the contribution of phenotypic plasticity to specificity. We found that infectivity was stable across lines irrespective of recent passage history, indicating that in the short term infection outcomes are fixed by genetic backgrounds. Second, we studied longer-term evolution with two host clones and two parasite lines. In this experiment, P. ramosa lines had the possibility to evolve adaptations to the host genotype (clone) in which they were serially passaged, which allowed us to test for a genetic component to specificity. Substantial differences arose in the two passaged lines: one parasite line gained infectivity on the host clone it was grown on, but it lost infectivity on the other host genotype (this line evolved specificity), while the other parasite line evolved higher infectivity on both host clones. We crossed the two host genotypes used in the serial passage experiment and found evidence that the number of host genes that underlies resistance variation is small. In sum, our results show that P. ramosa specificity is a stably inherited trait, it can evolve rapidly, and it is controlled by few genes in the host. These findings are consistent with the idea of a rapid, ongoing arms race between the bacterium and its host.
寄生相互作用中的特异性可由仅对一部分寄生虫基因型具有抗性的宿主基因型以及能感染一部分宿主基因型的寄生虫基因型来定义。特异性是由相互作用者的基因型决定，还是表型可塑性（有时称为适应性）起更大作用，情况并不总是很清楚。共同进化的结果严重依赖于基因相互作用的普遍性。我们使用细菌寄生虫枝状巴氏杆菌及其甲壳类宿主大型溞来研究特异性。首先，我们测试了在不同宿主基因型之间快速转移的枝状巴氏杆菌品系的短期适应性。在这个时间尺度上的适应性将证明表型可塑性对特异性的贡献。我们发现，无论近期的传代历史如何，各品系的感染性都是稳定的，这表明在短期内感染结果由遗传背景决定。其次，我们用两个宿主克隆和两个寄生虫品系研究了长期进化。在这个实验中，枝状巴氏杆菌品系有可能进化出对其连续传代的宿主基因型（克隆）的适应性，这使我们能够测试特异性的遗传成分。两个传代品系出现了显著差异：一个寄生虫品系在其生长的宿主克隆上获得了感染性，但在另一种宿主基因型上失去了感染性（该品系进化出了特异性），而另一个寄生虫品系在两个宿主克隆上都进化出了更高的感染性。我们对连续传代实验中使用的两种宿主基因型进行杂交，发现有证据表明，导致抗性变异的宿主基因数量很少。总之，我们的结果表明，枝状巴氏杆菌的特异性是一种稳定遗传的性状，它可以快速进化，并且由宿主中的少数基因控制。这些发现与细菌与其宿主之间正在进行的快速军备竞赛的观点一致。
