Parker Benjamin J, Hrček Jan, McLean Ailsa H C, Brisson Jennifer A, Godfray H Charles J
Department of Zoology, University of Oxford, Oxford, UK.
Department of Microbiology, University of Tennessee, Knoxville, TN, USA.
Mol Ecol. 2021 Mar;30(6):1559-1569. doi: 10.1111/mec.15821. Epub 2021 Feb 11.
Many insects host vertically transmitted microbes, which can confer benefits to their hosts but are costly to maintain and regulate. A key feature of these symbioses is variation: for example, symbiont density can vary among host and symbiont genotypes. However, the evolutionary forces maintaining this variation remain unclear. We studied variation in symbiont density using the pea aphid (Acyrthosiphon pisum) and the bacterium Regiella insecticola, a symbiont that can protect its host against fungal pathogens. We found that relative symbiont density varies both between two Regiella phylogenetic clades and among aphid "biotypes." Higher density symbiont infections are correlated with stronger survival costs, but variation in density has little effect on the protection Regiella provides against fungi. Instead, we found that in some aphid genotypes, a dramatic decline in symbiont density precedes the loss of a symbiont infection. Together, our data suggest that the optimal density of a symbiont infection is likely different from the perspective of aphid and microbial fitness. Regiella might prevent loss by maintaining high within-host densities, but hosts do not appear to benefit from higher symbiont numbers and may be advantaged by losing costly symbionts in certain environments. The standing variation in symbiont density observed in natural populations could therefore be maintained by antagonistic coevolutionary interactions between hosts and their symbiotic microbes.
许多昆虫携带垂直传播的微生物，这些微生物能为宿主带来益处，但维持和调控它们的成本很高。这些共生关系的一个关键特征是变异：例如，共生体密度在宿主和共生体基因型之间可能存在差异。然而，维持这种变异的进化力量仍不清楚。我们利用豌豆蚜（Acyrthosiphon pisum）和昆虫雷氏菌（Regiella insecticola）研究了共生体密度的变异，昆虫雷氏菌是一种能保护其宿主免受真菌病原体侵害的共生体。我们发现，相对共生体密度在雷氏菌的两个系统发育分支之间以及蚜虫“生物型”之间都存在差异。共生体感染密度较高与生存成本增加相关，但密度变化对雷氏菌提供的抗真菌保护作用影响不大。相反，我们发现，在一些蚜虫基因型中，共生体密度的急剧下降先于共生体感染的丧失。综合来看，我们的数据表明，从蚜虫和微生物适应性的角度来看，共生体感染的最佳密度可能不同。雷氏菌可能通过维持宿主体内的高密度来防止丧失，但宿主似乎并未从更高的共生体数量中受益，在某些环境中，失去成本高昂的共生体可能对宿主有利。因此，自然种群中观察到的共生体密度的持续变异可能是由宿主与其共生微生物之间的拮抗协同进化相互作用维持的。
