Did aculeate silk evolve as an antifouling material?

Abstract

Many of the challenges we currently face as an advanced society have been solved in unique ways by biological systems. One such challenge is developing strategies to avoid microbial infection. Social aculeates (wasps, bees and ants) mitigate the risk of infection to their colonies using a wide range of adaptations and mechanisms. These adaptations and mechanisms are reliant on intricate social structures and are energetically costly for the colony. It seems likely that these species must have had alternative and simpler mechanisms in place to ensure the maintenance of hygienic domicile conditions prior to the evolution of these complex behaviours. Features of the aculeate coiled-coil silk proteins are reminiscent of those of naturally occurring α-helical antimicrobial peptides (AMPs). In this study, we demonstrate that peptides derived from the aculeate silk proteins have antimicrobial activity. We reconstruct the predicted ancestral silk sequences of an aculeate ancestor that pre-dates the evolution of sociality and demonstrate that these ancestral sequences also contained peptides with antimicrobial properties. It is possible that the silks evolved as an antifouling material and facilitated the evolution of sociality. These materials serve as model materials for consideration in future biomaterial development.

Fig 1. Testing the antimicrobial activity of peptides from the honeybee (Apis mellifera) silk protein AmelF3.

A. Schematic showing where the peptides are located within the silk protein. Colors indicate strength of antimicrobial activity and a bracket identifies which peptides led to flocculation of the bacteria. The predicted coiled-coil region of the protein is indicated. B. Comparative antimicrobial activity of the various peptides expressed as the time delay (min) for an E. coli culture to reach an optical density at 600 nm (OD600) of 0.2 units. Dotted lines show the delay associated with 94 and 98% kill of the population. Error bars show the standard error of the mean calculated from four separate growth cultures.

Fig 2. Testing the antimicrobial activity of the KIKT peptide from ancestral and extant sequences homologous to AmelF3 from honeybees.

A. Phylogenetic tree of bee species within the Hymenopteran suborder Aculeata with black typeface indicating extant and ancestral sequences used in the analysis. Full protein sequences can be found in S2 Table. B. Comparative antimicrobial activity of the various peptides expressed as the time delay (min) for an E. coli culture to reach an optical density at 600 nm (OD600) of 0.2 units. NB. The peptides generated from ancestral sequences contained a higher proportion of hydrophobic residues and precipitated from solution during the assay period. Error bars show the standard error of the mean calculated from four separate growth cultures.