Toxic Peptides Occur Frequently in Pergid and Argid Sawfly Larvae

Toxic peptides containing D-amino acids are reported from the larvae of sawfly species. The compounds are suspected to constitute environmental contaminants, as they have killed livestock grazing in areas with congregations of such larvae, and related larval extracts are deleterious to ants. Previously, two octapeptides (both called lophyrotomin) and three heptapeptides (pergidin, 4-valinepergidin and dephosphorylated pergidin) were identified from three species in the family Pergidae and one in Argidae. Here, the hypothesis of widespread occurrence of these peptides among sawflies was tested by LC-MS analyses of single larvae from eight pergid and 28 argid species, plus nine outgroup species. At least two of the five peptides were detected in most sawfly species, whereas none in any outgroup taxon. Wherever peptides were detected, they were present in each examined specimen of the respective species. Some species show high peptide concentrations, reaching up to 0.6% fresh weight of 4-valinepergidin (1.75 mg/larva) in the pergid Pterygophorus nr turneri. All analyzed pergids in the subfamily Pterygophorinae contained pergidin and 4-valinepergidin, all argids in Arginae contained pergidin and one of the two lophyrotomins, whereas none of the peptides was detected in any Perginae pergid or Sterictiphorinae argid (except in Schizocerella pilicornis, which contained pergidin). Three of the four sawfly species that were previously known to contain toxins were reanalyzed here, resulting in several, often strong, quantitative and qualitative differences in the chemical profiles. The most probable ecological role of the peptides is defense against natural enemies; the poisoning of livestock is an epiphenomenon.

已有研究报道，叶蜂（sawfly）幼虫体内存在含有D-氨基酸（D-amino acids）的毒性肽类物质。这类化合物被推测可能构成环境污染物：在这类幼虫大量聚集的区域放牧的家畜曾出现中毒死亡案例，且相关幼虫提取物对蚂蚁具有毒害作用。此前，研究人员已从广背叶蜂科（Pergidae）的3个物种与锤角叶蜂科（Argidae）的1个物种中，鉴定出2种八肽（octapeptides），二者均被命名为lophyrotomin，同时还鉴定出3种七肽（heptapeptides）：pergidin、4-缬氨酸pergidin（4-valinepergidin）与脱磷酸化pergidin（dephosphorylated pergidin）。本研究通过液相色谱-质谱联用（LC-MS）技术，对8种广背叶蜂科、28种锤角叶蜂科昆虫的单头幼虫以及9种外类群（outgroup）物种的幼虫开展分析，以此验证这类肽类在叶蜂类昆虫中广泛分布的假说。多数被测叶蜂物种中均可检出5种肽类中的至少2种，而所有外类群物种均未检出此类肽类。但凡检出肽类的物种，其所有被测个体均含有该类肽类。部分物种的肽类含量极高：例如广背叶蜂科的Pterygophorus nr turneri，其体内4-缬氨酸pergidin的含量可达鲜重的0.6%（单头幼虫可达1.75 mg）。所有被测的广背叶蜂亚科（Pterygophorinae）广背叶蜂物种均含有pergidin与4-缬氨酸pergidin；所有被测的锤角叶蜂亚科（Arginae）锤角叶蜂物种均含有pergidin与两种lophyrotomin中的一种。而广背叶蜂亚科（Perginae）的广背叶蜂物种以及锤角叶蜂亚科（Sterictiphorinae）的锤角叶蜂物种均未检出此类肽类，仅Schizocerella pilicornis例外，其体内含有pergidin。本研究对此前已知产毒的4种叶蜂物种中的3种进行了重新分析，结果发现其化学图谱存在多处显著的定量与定性差异。这类肽类最可能的生态学功能是抵御天敌，而家畜中毒仅为该功能的副现象（epiphenomenon）。