Shifts along parasite-mutualist continuum are opposed by fundamental trade-offs

Theory suggests that symbionts can readily evolve more parasitic or mutualistic strategies with respect to hosts. However, many symbionts have stable interactions with hosts that improve nutrient assimilation or confer protection from pathogens. We explored the potential for evolution of increased parasitism or decreased parasitism and mutualism in a natural gut symbiosis between larvae of Plutella xylostella and the microbe Enterobacter cloacae. We focused on interactions with the pathogen, Bacillus thuringiensis: selecting for parasitism in terms of facilitating pathogen infection, or increased mutualism in terms of host protection. Selection for parasitism led to symbionts increasing pathogen-induced mortality, but reduced their competitive ability with pathogens and their in vitro growth rates. Symbionts did not evolve to confer protection from pathogens. However, several lineages evolved reduced parasitism, primarily in terms of moderating impacts on host growth, potentially because prudence pays dividends through increased host size. Overall, evolution of increased parasitism was achievable but was opposed by trade-offs likely to reduce fitness. The evolution of protection may not have occurred because suppressing growth of B. thuringiensis in the gut might provide only weak protection or because evolution towards protective interactions was opposed by loss of competitive fitness in symbionts.

理论表明，共生体（symbionts）可针对宿主快速演化出更偏向寄生或互利共生的生存策略。然而，诸多共生体与宿主间却形成了稳定的互作关系，这类互作可提升宿主的营养吸收能力，或帮助宿主抵御病原体侵染。本研究以小菜蛾（Plutella xylostella）幼虫与阴沟肠杆菌（Enterobacter cloacae）构成的天然肠道共生体系为研究对象，探究了共生体向增强寄生、减弱寄生或演化为互利共生模式的演化潜力。研究聚焦于共生体与苏云金芽孢杆菌（Bacillus thuringiensis）的互作：分别设置了促进病原体侵染的寄生选择方向，以及提升宿主防御能力的互利共生选择方向。针对寄生方向的演化筛选使得共生体提升了病原体诱导的宿主死亡率，但同时降低了其与病原体的竞争能力与体外生长速率。共生体并未演化出帮助宿主抵御病原体的性状。不过，部分共生谱系演化出了减弱的寄生性，主要体现为对宿主生长的抑制作用得到缓解，这可能是因为审慎的共生策略可通过增大宿主体型带来收益。总体而言，增强寄生性的演化虽可实现，但会受到可能降低共生体进化适合度的权衡效应的制约。而防御性共生模式未发生演化，原因或有两点：一是在肠道中抑制苏云金芽孢杆菌的生长仅能提供微弱的防御效果，二是向防御性互作演化的过程会伴随共生体竞争适合度的损失。