ORIGINAL PAPER Social context-dependent immune gene expression in bumblebees (Bombus terrestris) Jeanny Richter & Sophie Helbing & Silvio Erler & H. Michael G. Lattorff Received: 30 September 2011 / Revised: 25 January 2012 / Accepted: 30 January 2012 / Published online: 10 February 2012 # Springer-Verlag 2012 Abstract Social insects are prone to attack by parasites as they provide numerous resources of food and brood, ho- meostatic nest conditions and a high density of individuals, enhancing the transmission of parasites. The defence of social insects might occur at different levels, the individual and the group. Individual defence occurs in part via the innate immune system resulting in the expression of antimi- crobial substances. Group level defences, summarised as ‘social immunity’, represent a suite of behavioural and organisational features. Here, all effects contributing to so- cial immunity except for the social context were removed from bumblebee (Bombus terrestris) workers, kept either in groups or solitarily. The gene expression of six effector molecules of the immune system was monitored in both groups and in controls from the same source colonies. The social treatment has a highly significant effect on immune gene expression, with groups exhibiting higher levels of two antimicrobial peptides (AMPs) and two lysozymes. Pheno- loxidase is affected at the regulatory level, with a strong upregulation of its suppressor Spn27A in groups suggesting a trade-off with antimicrobial activity. AMPs are strongly upregulated in groups, whereas lysozymes are strongly downregulated in solitary treatments suggesting another trade-off. Clearly, social immunity impacts elements of in- dividual immunity. Keywords Innate immune system . Social immunity . Quantitative real-time PCR . Antimicrobial peptide . Lysozyme . Phenoloxidase Introduction Social insects are characterised by large number of individ- uals living together in tight proximity interacting frequently with closely related colony members. This turns social insects into attractive targets for parasites and pathogens (Bailey and Ball 1991; Schmid-Hempel 1998, 2011). The storage of resources and brood as well as the homeostatic regulation of nest temperature and humidity generate excel- lent environments for parasites. Increasing density of indi- viduals increases the risk of disease transmission in social insect colonies (Wilson-Rich et al. 2009). Additionally, the high relatedness of colony members makes them susceptible to disease agents (Schmid-Hempel 1998; Stow et al. 2007). Due to the low genetic variation among individuals, often caused by monogyny and/or monandry, pathogens are able to spread easily between group members and might infect the whole colony (Cremer et al. 2007; Stow et al. 2007). In consequence of this increased exposure to parasites and pathogens accompanied by group living, social insects have developed efficient defence strategies to lower the risk of disease transmission. The insect immune system exhibits a highly conserved structure as well as many homologous components in com- parison to vertebrate immune systems (Evans et al. 2006; Hultmark 2003; Kurtz 2005). Social insect’ s innate immune response is mediated by four immune pathways: Toll, Imd, JAK/STAT and the JNK pathways (Evans et al. 2006; Hultmark 2003). However, the honey bee immune system shows orthologs for all of the pathways; its overall immune Communicated by D. Naug Electronic supplementary material The online version of this article (doi:10.1007/s00265-012-1327-2) contains supplementary material, which is available to authorized users. J. Richter : S. Helbing : S. Erler : H. M. G. Lattorff (*) Institut für Biologie, Molekulare Ökologie, Martin-Luther-Universität Halle-Wittenberg, Hoher Weg 4, 06099 Halle (Saale), Germany e-mail: lattorff@zoologie.uni-halle.de Behav Ecol Sociobiol (2012) 66:791–796 DOI 10.1007/s00265-012-1327-2