Data from: A free lunch? No cost for acquiring defensive plant pyrrolizidine alkaloids in a specialist arctiid moth (Utetheisa ornatrix)

Many herbivorous insects sequester defensive chemicals from their host plants. We tested sequestration fitness costs in the specialist moth Utetheisa ornatrix (Lepidoptera: Arctiidae). We added pyrrolizidine alkaloids (PAs) to an artificial diet at different concentrations. Of all the larval and adult fitness components measured, only development time was negatively affected by PA concentration. These results were repeated under stressful laboratory conditions. On the other hand, the amount of PAs sequestered greatly increased with the diet PA concentration. Absence of a detectable negative effect does not necessarily imply a lack of costs if all individuals express the biochemical machinery of detoxification and sequestration constitutively. Therefore, we used qPCR to show that expression of the gene used to detoxify PAs, pyrrolizidine-alkaloid-N-oxygenase (pno), increased 41-fold in our highest PA treatment. Nevertheless, fitness components were affected only slightly or not at all, suggesting that sequestration in this species does not incur a strong cost. The apparent lack of costs has important implications for our understanding of the evolution of ecological interactions; for example, it implies that selection by specialist herbivores may decrease the levels of certain chemical defense in plant populations.诸多植食性昆虫会从寄主植物中贮积防御性化学物质。本研究以专食性丽夜蛾（Utetheisa ornatrix，鳞翅目：灯蛾科）为对象，测试了其贮积防御化学物质的适合度代价。我们向人工饲料中添加了不同浓度的吡咯里西啶生物碱（pyrrolizidine alkaloids, PAs）。在所有测定的幼虫与成虫适合度组分中，仅发育时长会随PAs浓度升高而受到显著负向影响，且该结果在胁迫性实验室条件下可重复。与此同时，昆虫贮积的PAs总量会随饲料中PAs浓度的升高而显著增加。若所有个体均组成型表达解毒与贮积相关的生化机制，则未检测到显著负面影响并不一定意味着不存在适合度代价。为此，我们通过定量聚合酶链式反应（quantitative polymerase chain reaction, qPCR）证实，用于解毒PAs的基因——吡咯里西啶生物碱-N-加氧酶（pyrrolizidine-alkaloid-N-oxygenase, pno）——在最高PAs处理组中的表达量上调了41倍。尽管如此，适合度组分仅受到轻微影响或未受影响，这表明该物种的化学物质贮积并未带来显著的适合度代价。这种看似不存在适合度代价的现象，对我们理解生态相互作用的演化具有重要意义；例如，这意味着专食性植食性昆虫的选择压可能会降低植物种群中特定化学防御物质的水平。