Department of Biology, Stanford University, Stanford, California.
Rocky Mountain Biological Laboratory, Crested Butte, Colorado.
Mol Ecol. 2019 Apr;28(8):2100-2117. doi: 10.1111/mec.15057. Epub 2019 Apr 25.
Gut microbes are believed to play a critical role in most animal life, yet fitness effects and cost-benefit trade-offs incurred by the host are poorly understood. Unlike most hosts studied to date, butterflies largely acquire their nutrients from larval feeding, leaving relatively little opportunity for nutritive contributions by the adult's microbiota. This provides an opportunity to measure whether hosting gut microbiota comes at a net nutritional price. Because host and bacteria may compete for sugars, we hypothesized that gut flora would be nutritionally neutral to adult butterflies with plentiful food, but detrimental to semistarved hosts, especially when at high density. We held field-caught adult Speyeria mormonia under abundant or restricted food conditions. Because antibiotic treatments did not generate consistent variation in their gut microbiota, we used interindividual variability in bacterial loads and operational taxonomic unit abundances to examine correlations between host fitness and the abdominal microbiota present upon natural death. We detected strikingly few relationships between microbial flora and host fitness. Neither total bacterial load nor the abundances of dominant bacterial taxa were related to butterfly fecundity, egg mass or egg chemical content. Increased abundance of a Commensalibacter species did correlate with longer host life span, while increased abundance of a Rhodococcus species correlated with shorter life span. Contrary to our expectations, these relationships were unchanged by food availability to the host and were unrelated to reproductive output. Our results suggest the butterfly microbiota comprises parasitic, commensal and beneficial taxa that together do not impose a net reproductive cost, even under caloric stress.
肠道微生物被认为在大多数动物的生命中起着至关重要的作用，但宿主承担的适应度效应和成本效益权衡仍知之甚少。与迄今为止研究的大多数宿主不同，蝴蝶主要从幼虫取食中获取营养，这使得成年期肠道微生物群提供营养的机会相对较少。这为衡量宿主是否因肠道微生物群而付出净营养代价提供了机会。由于宿主和细菌可能会争夺糖分，我们假设在食物充足的情况下，肠道菌群对成年蝴蝶来说在营养上是中性的，但对半饥饿的宿主不利，尤其是在高密度时。我们在丰富或有限的食物条件下饲养野外捕获的成年 Speyeria mormonia。由于抗生素处理并未在其肠道微生物群中产生一致的变化，因此我们利用细菌负荷和操作分类单元丰度的个体间变异性来检查宿主适应性与自然死亡时存在的腹部微生物群之间的关系。我们发现微生物群与宿主适应性之间的关系非常少。细菌总负荷或主要细菌类群的丰度均与蝴蝶的繁殖力、卵质量或卵的化学成分无关。一种 Commensalibacter 物种丰度的增加与宿主寿命的延长有关，而一种 Rhodococcus 物种丰度的增加与寿命的缩短有关。与我们的预期相反，这些关系不受宿主食物供应的影响，与繁殖力无关。我们的研究结果表明，蝴蝶微生物群包含寄生、共生和有益的分类群，即使在热量应激下，它们也不会对繁殖产生净成本。
