Chemical Defense Balanced by Sequestration and De Novo Biosynthesis in a Lepidopteran Specialist

The evolution of sequestration (uptake and accumulation) relative to de novo biosynthesis of chemical defense compounds is poorly understood, as is the interplay between these two strategies. The Burnet moth Zygaena filipendulae (Lepidoptera) and its food-plant Lotus corniculatus (Fabaceae) poses an exemplary case study of these questions, as Z. filipendulae belongs to the only insect family known to both de novo biosynthesize and sequester the same defense compounds directly from its food-plant. Z. filipendulae and L. corniculatus both contain the two cyanogenic glucosides linamarin and lotaustralin, which are defense compounds that can be hydrolyzed to liberate toxic hydrogen cyanide. The overall amounts and ratios of linamarin and lotaustralin in Z. filipendulae are tightly regulated, and only to a low extent reflect the ratio in the ingested food-plant. We demonstrate that Z. filipendulae adjusts the de novo biosynthesis of CNglcs by regulation at both the transcriptional and protein level depending on food plant composition. Ultimately this ensures that the larva saves energy and nitrogen while maintaining an effective defense system to fend off predators. By using in situ PCR and immunolocalization, the biosynthetic pathway was resolved to the larval fat body and integument, which infers rapid replenishment of defense compounds following an encounter with a predator. Our study supports the hypothesis that de novo biosynthesis of CNglcs in Z. filipendulae preceded the ability to sequester, and facilitated a food-plant switch to cyanogenic plants, after which sequestration could evolve. Preservation of de novo biosynthesis allows fine-tuning of the amount and composition of CNglcs in Z. filipendulae.人们对化学防御化合物的从头生物合成（de novo biosynthesis）与摄取储积（sequestration，即摄取与积累）之间的演化关系，以及这两种策略间的互作关系，仍知之甚少。斑蛾科的蕨斑蛾（Zygaena filipendulae，鳞翅目Lepidoptera）与其寄主植物百脉根（Lotus corniculatus，豆科Fabaceae）为上述问题提供了典范研究案例——Z. filipendulae是目前已知唯一兼具从头合成与直接从寄主植物摄取储积同种防御化合物能力的昆虫类群。Z. filipendulae与L. corniculatus均含有两种氰苷葡萄糖苷（cyanogenic glucosides）：亚麻苦苷（linamarin）与百脉根苷（lotaustralin），这类防御化合物经水解后可释放有毒的氰化氢。斑蛾体内亚麻苦苷与百脉根苷的总含量及比例受到严格调控，仅在极低程度上与取食的寄主植物中的两者比例相符。本研究证实，Z. filipendulae可根据寄主植物的组成，通过转录与蛋白质两个层面的调控，调整氰苷葡萄糖苷的从头生物合成过程。这最终确保幼虫在维持有效防御系统以抵御天敌的同时，节省了能量与氮源。本研究通过原位PCR（in situ PCR）与免疫定位（immunolocalization）技术，将氰苷葡萄糖苷的生物合成途径定位至幼虫的脂肪体（fat body）与体壁（integument），由此可推断幼虫在遭遇天敌后可快速补充防御化合物。本研究支持如下假说：Z. filipendulae的氰苷葡萄糖苷从头合成能力先于储积能力出现，并推动其寄主植物转向产氰植物，随后储积能力才得以演化。保留从头生物合成能力，使得Z. filipendulae能够对体内氰苷葡萄糖苷的含量与组成进行精细调控。