Insect Collections as an Untapped Source of Bioactive Compounds—Fireflies (Coleoptera: Lampyridae) and Cardiotonic Steroids as a Proof of Concept

SIMPLE SUMMARY: Natural history museums around the world possess extensive collections of dried insects (e.g., beetles, butterflies or flies). Traditionally, museum specimens were mostly studied anatomically for systematic (i.e., how different insects are related) and taxonomic purposes (e.g., description of new species). During the last decades, it has become a common practice to study insect systematics based on DNA extracted from dried museum specimens. Being a highly sought-after prey, many insects evolved powerful toxins to ward off predators. One example is seen in fireflies, a beetle family famous for their ability to emit light. A few species of mainly North American fireflies, however, are known to produce toxins that are called lucibufagins. In this study, we tested if lucibufagins also occur in European species of fireflies. Instead of sampling in the field, we chemically analyzed firefly specimens from museum collections by using a method that preserves the valuable museum specimens. In total, we found lucibufagins in 21 species of fireflies, including specimens that were older than 100 years. Our study emphasizes that insect collections provide a valuable archive of chemical information that can be used for the discovery of novel pharmacologically interesting compounds as well as for addressing ecological questions without destroying valuable specimens. ABSTRACT: Natural history collections provide an invaluable basis for systematics, ecology, and conservation. Besides being an important source of DNA, museum specimens may also contain a plethora of natural products. Especially, dried insect collections represent a global repository with billions of inventoried vouchers. Due to their vast diversity, insects possess a great variety of defensive compounds, which they either produce autogenously or derive from the environment. Here, we present a case study on fireflies (Coleoptera: Lampyridae), which produce bufadienolides as a defense against predators. These toxins belong to the cardiotonic steroids, which are used for the treatment of cardiac diseases and specifically inhibit the animal enzyme Na(+)/K(+)-ATPase. Bufadienolides have been reported from only seven out of approximately 2000 described firefly species. Using a non-destructive approach, we screened 72 dry coleopteran specimens for bufadienolides using HPLC-DAD and HPLC-MS. We found bufadienolides including five novel compounds in 21 species of the subfamily Lampyrinae. The absence of bufadienolides in the phylogenetically related net-winged beetles (Lycidae) and the lampyrid subfamilies Luciolinae and Lamprohizinae indicates a phylogenetic pattern of bufadienolide synthesis. Our results emphasize the value of natural history collections as an archive of chemical information for ecological and evolutionary basic research and as an untapped source for novel bioactive compounds.