Only helpful when required: a longevity cost of harbouring defensive symbionts

Maternally transmitted symbionts can spread in host populations if they provide a fitness benefit to their hosts. Hamiltonella defensa, a bacterial endosymbiont of aphids, protects hosts against parasitoids but only occurs at moderate frequencies in most aphid populations. This suggests that harbouring this symbiont is also associated with costs, yet the nature of these costs has remained elusive. Here we demonstrate an important and clearly defined cost: reduced longevity. Experimental infections with six different isolates of H. defensa caused strongly reduced lifespans in two different clones of the black bean aphid, Aphis fabae, resulting in a significantly lower lifetime reproduction. However, the two aphid clones were unequally affected by the presence of H. defensa, and the magnitude of the longevity cost was further determined by genotype × genotype interactions between host and symbiont, which has important consequences for their coevolution.母源传播的共生体若能为宿主提供适合度益处，便可在宿主种群中得以扩散。Hamiltonella defensa（汉密尔顿氏菌）是蚜虫的细菌内共生体，可保护宿主抵御寄生蜂，但在多数蚜虫种群中仅以中等频率存在。这提示携带该共生体同时会伴随适合度代价，然而此类代价的本质迄今尚未明确。本研究揭示了一类明确且关键的代价：寿命缩短。我们利用6株不同分离株的H. defensa开展实验感染，结果显示其可显著缩短两个蚕豆蚜（Aphis fabae）克隆品系的寿命，进而导致宿主终生繁殖力显著下降。不过，这两个蚜虫克隆品系受H. defensa感染的影响程度并不一致，且寿命代价的强弱进一步由宿主与共生体间的基因型×基因型互作所决定，该发现对二者的协同进化具有重要意义。