Data from: Limiting opportunities for cheating stabilizes virulence in insect parasitic nematodes

Cooperative secretion of virulence factors by pathogens can lead to social conflict when cheating mutants exploit collective secretion, but do not contribute to it. If cheats outcompete cooperators within hosts, this can cause loss of virulence. Insect parasitic nematodes are important biocontrol tools that secrete a range of significant virulence factors. Critically, effective nematodes are hard to maintain without live passage, which can lead to virulence attenuation. Using experimental evolution we tested whether social cheating might explain unstable virulence in the nematode Heterorhabditis floridensis by manipulating relatedness via multiplicity of infection (MOI), and the scale of competition. Passage at high MOI, which should reduce relatedness, led to loss of fitness: virulence and reproductive rate declined together and all eight independent lines suffered premature extinction. As theory predicts, relatedness treatments had more impact under stronger global competition. In contrast, low MOI passage led to more stable virulence and increased reproduction. Moreover, low MOI lineages showed a trade-off between virulence and reproduction, particularly for lines under stronger between-host competition. Overall, this study indicates that evolution of virulence theory is valuable for the culture of biocontrol agents: effective nematodes can be improved and maintained if passage methods mitigate possible social conflicts.病原体协同分泌毒力因子（virulence factors）时，若欺骗型突变株利用集体分泌的产物却不参与分泌过程，便会引发社会冲突。若欺骗型菌株在宿主体内胜过合作型菌株，则可能导致毒力丧失。昆虫寄生线虫是一类重要的生物防治工具，可分泌多种关键毒力因子。尤为关键的是，未经活体传代便难以维持高效的线虫种群，该过程可能引发毒力衰减。我们通过感染复数（multiplicity of infection, MOI）调控亲缘关系与竞争规模，借助实验演化技术探究社会欺骗是否可解释佛罗里达异小杆线虫（Heterorhabditis floridensis）毒力不稳定的现象。高感染复数传代本应降低亲缘关系，却导致适合度下降：毒力与繁殖率同步降低，且8个独立谱系全部提前灭绝。正如理论预测，亲缘关系调控在更强的全局竞争下影响更为显著。与之相反，低感染复数传代可维持更稳定的毒力并提升繁殖能力。此外，低感染复数谱系呈现出毒力与繁殖力之间的权衡关系，这一现象在面临更强宿主间竞争的谱系中尤为明显。综上，本研究表明毒力演化理论对生物防治制剂的培养具有指导价值：若传代方式可缓解潜在的社会冲突，便能优化并维持高效线虫种群的活性。