Social immunity in ants: The role of trophallaxis and Cathepsin D in colony-wide immunocompetence of Camponotus pennsylvanicus

Carpenter ants regularly perform trophallaxis, a social interaction whereby ants exchange nutritive droplets with hungry nestmates. Previous research indicated that, relative to naive nestmates, ants immunized with the bacterium Serratia marcescens produced droplets with higher antibiotic properties.  Moreover, the antimicrobial peptide Cathepsin D was implicated as one putative immune protein since it was consistently upregulated in the immunized and challenges ants.  To test whether the exchange of Cathepsin D- rich droplets amongst ants is responsible for attaining social immunity at the colony level, we carried in vivo experiments where immunized ants from seven different colonies were allowed to interact with hungry nestmates.  Subsequently, all ants were challenged and their survival recorded.   Our results indicate that trophallactic exchanges between immunized and naïve hungry recipients confer a survival benefit to the later relative to recipients of control ants, following a challenge. RNA interference techniques are currently underway to silence the gene coding for Cathepsin D. If Cathepsin D is involved in social immunity, we predict that the down regulation or the complete loss of function of this gene will a) reduce the antimicrobial activity of the droplets, b) change the droplet protein profile of immunized ants so as to resemble that of naïve or control nestmates (visualized in SDS-PAGE), and c) the benefits of trophallactic exchanges from immunized to the naïve nestmates will disappear. This approach would demonstrate the significance of Cathepsin D in achieving social immunity in C. pennsylvanicus.