Venomous Maggots? A first exploration of the toxin arsenal of larval stages of the horse fly Tabanus autumnalis

As a rapidly evolving trait, animal venom exhibits compositional variation even at the intraspecific level between populations, ontogenetic stages and sexes. In extreme cases venom is used for different functional purposes across ontogenetic stages. This phenomenon occurs for example in case of horse flies (Tabanidae), which utilize venom for predation during the larval stage and for blood feeding in the adult stage. While the venom composition and activity of adult tabanids has been investigated for some species, nothing is known about the venom compounds of tabanid larvae. With the current study we provide first insights into the venom composition of larval stages of a tabanid. Besides a plethora of hydrolyzing enzymes, we find the venom of T. autumnalis larvae dominated by peptide toxins. Some of these peptide toxins could be annotated as putative neurotoxins and cytolytic peptides and these compounds likely are the main drivers of fast prey incapacitation. Previous studies identified the main functional constituents utilized by tabanid imagines for blood feeding to be protease inhibitors affecting vasoconstriction and thrombin to promote blood feeding. Interestingly we find highly similar putative protease inhibitors in the larval venom, though with relatively low expression levels. This provides a first indication for a shifted expression profile between the venom of tabanid life stages to fulfill both the predatory needs of the larvae and the blood lust of female imagines.