Sequestration of phytochemical defenses during feeding is an important adaptive strategy for many insect herbivores as a protection against natural enemies. Extensive research has been conducted investigating this complicated relationship, particularly for herbivorous lepidopteran larvae and their host plants. As a result, several sequestered defensive compounds have been identified, isolated, and analyzed using tandem liquid chromatography-mass spectrometry (LC-MS/MS), including terpenic, phenolic, and nitrogen-containing metabolites. Yet, the chemical similarity of these compounds across sequestering Lepidoptera remains unexplored. This project presents a novel use of a valuable tool in the mass spectrometry community - the Global Natural Products Social Molecular Networking (GNPS) database. GNPS currently offers the most extensive open-source repository of identified allelochemicals paired with MS/MS data, as well as analysis capabilities that allow compounds to be compared structurally to one another and generate a molecular network based on structural similarity. This project uses MS/MS spectral data mined from existing literature, to create a molecular network for 15 sequestered chemical defense compounds associated with 17 lepidopteran families. The phylogeny of these families is then compared with a defense dendrogram based on molecular network associations to assess potential phylogenetic patterns of defense sequestration.
Results/Conclusions
Results show expected clustering of terpenic, phenolic, and nitrogen-containing metabolites within the molecular network. The minimum network relatedness, however, was higher than anticipated, indicating that all compounds in the network are somewhat similar. A comparison of the lepidopteran phylogeny to the defense dendrogram shows little to no evidence of a phylogenetic pattern for defense sequestration. The focal Lepidoptera families are also equally represented in the chemical space provided by the molecular network, with no apparent correlation between chemical distance and phylogenetic distance.
The overall structural similarity of the compounds in our network provides some evidence for structural constraints wherein lepidopteran herbivores can only maintain metabolic pathways to sequester relatively structurally similar compounds. Alternatively, high overall network similarity may be due to MS/MS fragmentation of related R groups from the central molecular structure, causing spectra from two molecules with similar fragment masses to be given a higher cosine score. Lack of phylogenetic signal for sequestered defenses provides further evidence for potential structural constraints, either related to herbivore metabolism, or similar physiology of lepidopteran predators, where defenses which disrupt key predator functions are selected for. Further research with larger sample sizes is needed to further elucidate these trends/mechanisms.