Table_2_Mutation in the RNA-Dependent RNA Polymerase of a Symbiotic Virus Is Associated With the Adaptability of the Viral Host.DOCX

Host adaptation has the potential to cause rapid genetic variation in symbiotic microorganisms in insects. How mutations in symbiotic viruses favor viral fitness in hosts and even influence host adaptability to new environments remains elusive. Here, we explored the role of genetic divergence at one site of a symbiotic virus, Acyrthosiphon pisum virus (APV), in the host aphid’s adaptation to unfavorable plants. Based on the transcriptomes of the pea aphid Vicia faba colony and Vicia villosa colony, 46 single nucleotide polymorphism (SNP) sites were found in the APV genomes from the two aphid colonies. One SNP at site 5,990, G5990A, located at the RNA-dependent RNA polymerase (RdRp) domain, demonstrated a predominance from G to A when the host aphids were shifted from V. faba to the low-fitness plants V. villosa or Medicago sativa. This SNP resulted in a substitution from serine (S) to asparagine (N) at site 196 in RdRp. Although S196N was predicted to be located at a random coil far away from conserved functional motifs, the polymerase activity of the N196 type of RdRp was increased by 44.5% compared to that of the S196 type. The promoted enzymatic activity of RdRp was associated with a higher replication level of APV, which was beneficial for aphids as APV suppressed plant’s resistance reactions toward aphids. The findings showed a novel case in which mutations selected in a symbiotic virus may confer a favor on the host as the host adapts to new environmental conditions.

宿主适应可驱动昆虫体内共生微生物发生快速遗传变异。共生病毒的突变如何提升病毒在宿主内的适应性，乃至影响宿主对新环境的适应能力，这一问题迄今仍未明确。本研究针对共生病毒豌豆蚜病毒（Acyrthosiphon pisum virus, APV）的某一位点遗传分化，探讨其在宿主蚜虫适应不良寄主植物过程中的作用。本研究基于豌豆蚜蚕豆种群与毛苕子种群的转录组数据，在两个蚜虫种群对应的APV基因组中，共鉴定出46个单核苷酸多态性（single nucleotide polymorphism, SNP）位点。其中位于RNA依赖的RNA聚合酶（RNA-dependent RNA polymerase, RdRp）结构域的5990位点G5990A单核苷酸多态性，在蚜虫宿主从蚕豆转移至低适应性寄主毛苕子或紫花苜蓿时，呈现出G向A的优势突变趋势。该SNP导致RdRp第196位氨基酸发生丝氨酸（S）到天冬酰胺（N）的替换。尽管预测显示S196N突变位点位于远离保守功能基序的无规卷曲区域，但N196型RdRp的聚合酶活性较S196型提升了44.5%。RdRp酶活性的增强与APV更高的复制水平相关，而APV可通过抑制植物对蚜虫的抗性反应，进而对蚜虫产生有益影响。本研究发现了一个全新案例：当宿主适应新环境时，共生病毒中被选择的突变可赋予宿主生存优势。