Abstract
Predatory robber flies (Diptera, Asilidae) have been suspected to be venomous due to their ability to overpower well-defended prey. However, details of their venom composition and toxin arsenal remained unknown. Here, we provide a detailed characterization of the venom system of robber flies through the application of comparative transcriptomics, proteomics and functional morphology. Our results reveal asilid venoms to be dominated by peptides and non-enzymatic proteins, and that the majority of components in the crude venom is represented by just ten toxin families, which we have named Asilidin1–10. Contrary to what might be expected for a liquid-feeding predator, the venoms of robber flies appear to be rich in novel peptides, rather than enzymes with a putative pre-digestive role. The novelty of these peptides suggests that the robber fly venom system evolved independently from hematophagous dipterans and other pancrustaceans. Indeed, six Asilidins match no other venom proteins, while three represent known examples of peptide scaffolds convergently recruited to a toxic function. Of these, members of Asilidin1 closely resemble cysteine inhibitor knot peptides (ICK), of which neurotoxic variants occur in cone snails, assassin bugs, scorpions and spiders. Synthesis of one of these putative ICKs, U-Asilidin1-Mar1a, followed by toxicity assays against an ecologically relevant prey model revealed that one of these likely plays a role as a neurotoxin involved in the immobilization of prey. Our results are fundamental to address these insights further and to understand processes that drive venom evolution in dipterans as well as other arthropods.
Highlights
Venoms are key adaptations that have evolved on numerous occasions in animal lineages to serve a range of ecological roles including defense, predation, communication and competition [1,2,3].Venoms constitute complex cocktails of proteins, peptides, salts and different organic molecules, collectively referred to as toxins [1,4]
Our results reveal that asilid venoms are not the protease-dominated venoms expected from liquid-feeding lineages, but that they are instead peptide-rich cocktails originating in the thoracic glands and expelled through an elaborate high-speed venom delivery system
This study provides a detailed description of the venom system of robber flies (Asilidae)
Summary
Venoms constitute complex cocktails of proteins, peptides, salts and different organic molecules, collectively referred to as toxins [1,4]. These toxins are expressed in venom glands and delivered via venom ducts to structures that inject the composed venom from the venom delivery system via a wound into the prey. The majority of all known toxins from animal venoms can be classified into a limited number of structural classes [9] This extreme level of convergence of venoms implies that toxins can provide insights into fundamental processes of protein functional evolution and biochemical adaptations
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