Chemical Diversity of Millipede Defensive Alkaloids Provides Insights into their Evolutionary History

The recent explosion in the described chemistry from Colobognatha, a millipede subterclass, has rekindled interest in the defensive secretions from these ancient animals. Colobognatha are the only millipedes that produce terpenoid alkaloids, and prior to 2020, studies into these defensive secretions were limited to a single order, Polyzoniida. However, it has become clear that numerous species of the order Platydesmida also produce structurally diverse terpenoid alkaloids with potent biological activity. Platydesmida defensive secretions encompass multiple natural product scaffolds with greater chemical complexity compared to previously reported millipede-derived alkaloids. Here we report the analysis of the defensive secretions of Andrognathus corticarius, a millipede found across the Appalachian region. A. corticarius is evolutionary sister to all other Platydesmida and is the presumed oldest genus within the order. Analysis of the defensive secretions revealed that A. corticarius produces an arsenal of alkaloids that are dissimilar to all previously reported metabolites. Complete structural elucidation was accomplished using 2D NMR, HRMS, chemical derivatization, DFT and ECD, which revealed the presence of two distinct scaffolds: a 5,6-fused heterocycle named the andrognathines and a 6,6,6,5-bridged heterocycle containing seven continuous stereogenic centers named the andrognathanols. Each scaffold is decorated with diverse fatty acids, which leads to the extraordinary number of unique metabolites detected within the secretions. The alkaloids likely serve to defend the millipedes from predation as they are actively secreted upon physical disturbance. This discovery also provides support for a plausible biosynthetic pathway that appears to be evolving simplicity across the evolutionary history of the Platydesmida.