Making sense of virus size and the tradeoffs shaping viral fitness

Viruses span an impressive size range, with genome length varying a
thousandfold and virion volume nearly a millionfold. For cellular
organisms the scaling of traits with size is a pervasive influence on
ecological processes, but whether size plays a central role in viral
ecology is unknown. Here we focus on viruses of aquatic unicellular
organisms, which exhibit the greatest known range of virus size. We
outline hypotheses within a quantitative framework, and analyze data where
available, to consider how size affects the primary components of viral
fitness. We argue that larger viruses have fewer offspring per infection
and slower contact rates with host cells, but a larger genome tends to
increase infection efficiency, broaden host range, and potentially
increase attachment success and decrease decay rate. These countervailing
selective pressures may explain why a breadth of sizes exist and even
coexist when infecting the same host populations. Oligotrophic ecosystems
may be enriched in “giant” viruses, because environments with
resource-limited phagotrophs at low concentrations may select for broader
host range, better control of host metabolism, lower decay rate, and a
physical size that mimics bacterial prey. Finally, we describe where
further research is needed to understand the ecology and evolution of
viral size diversity.

病毒的尺寸跨度令人惊叹，其基因组长度差异可达千倍，病毒粒子（virion）体积差异更是接近百万倍。对于细胞生物而言，性状随尺寸的缩放规律是影响生态过程的普遍因素，但尺寸是否在病毒生态学中占据核心地位，目前仍不明确。本研究聚焦于侵染水生单细胞生物的病毒——这类病毒拥有目前已知最广泛的尺寸分布范围。我们在定量研究框架内提出相关假说，并结合现有可获取的数据展开分析，以探讨尺寸如何影响病毒适合度的核心构成要素。我们认为，尺寸更大的病毒，单次感染产生的子代病毒粒数量更少，与宿主细胞的接触速率也更慢；但更大的基因组往往能提升感染效率、拓宽宿主范围，还可能提高附着成功率并降低衰变速率。这些相互拮抗的选择压力，或许可以解释为何病毒尺寸呈现多样性，甚至在侵染同一宿主种群时也存在多种尺寸共存的现象。贫营养生态系统中可能富集“巨型”病毒，因为在这类环境中，资源受限且浓度较低的吞噬营养生物，会倾向于选择拥有更宽泛宿主范围、更高效宿主代谢调控能力、更低衰变速率，以及模拟细菌猎物物理尺寸的病毒。最后，我们梳理了未来亟需开展的研究方向，以进一步解析病毒尺寸多样性的生态学与进化机制。