Allogrooming May Induce Immune Gene Expression as a Component of Social Immunity in the Western Honey Bee (Apis mellifera)

Many social animals will allogroom, or clean, their groupmates (1). Allogrooming can incur costs, such as the risk of the groomer becoming ill if the recipient of the allogrooming is infected with a pathogen (2). Several adaptive hypotheses have been proposed to explain why this behavior occurs so frequently in the presence of these costs, such as reciprocal altruism (3). Direct benefits to the allogroomer are often at play as well. In social ants, allogrooming of infected nestmates immunizes the groomer when later challenged with that pathogen (4). We will test another potential direct benefit of allogrooming an infected groupmate: that performing the behavior alters the expression of immune-related genes in the groomer and/or the recipient. Prosocial behaviors have been demonstrated to alter gene expression for both the actor and recipient. Rat pups who receive allogrooming from their mothers have altered expression of stress hormone receptors in the brain (5). Human actors of prosocial behavior demonstrate a decrease in the transcription of genes associated with adversity in immune cells (6). We will examine if performing a risky prosocial behavior, allogrooming an infected groupmate, alters the expression of immune genes that could alleviate the effects of infection for the groomer or the recipient. We will perform this experiment with the Western honey bee, Apis mellifera. Honey bees have been shown to increase allogrooming towards a nestmate who is experiencing immune challenge (7) Additionally, honey bees that are aggressive both more frequently allogroom sick nestmates and show improved response to immune challenge (7). We will collect returning forager honey bees into small cages of 10 individuals and uniquely paint-mark all occupants (20 total cages for N=200 individuals—we are pooling funds from multiple grants to reach our intended sample size). Half of the individuals from each cage will receive an injection of bacterial lipopolysaccharides (LPS), which provoke an immune response similar to natural infection (8). We will then observe the cages. Any time a group member allogrooms a nestmate, we will remove the groomer and the recipient and flash-freeze them in liquid nitrogen, recording the treatment types of the individuals (LPS vs control) and the direction of the behavior. At the same time, we will remove and freeze another set of individuals of the same treatment type(s) that had not been allogrooming as a control. We will continue this way until either one hour has passed or all individuals of a treatment type have been removed from the cage. Later, we will dissect tissues and measure expression levels of seven genes using RT-qPCR. We will measure three immune genes in the immune-related fat body (defensin-1, hymenoptaecin, vitellogenin) and two genes that predict prosocial responsiveness in the brain (ftz-f1, Nup98-96) relative to two housekeeping control genes (GAPDH, tropomyosin) (7, 9, 7). We anticipate that we will see an increase in expression of both immune genes and prosocial genes in the groomers and recipients relative to control individuals. This experiment could help elucidate another fitness benefit to a seemingly-costly behavior. Additionally, these results could help enhance our understanding of social immunity in highly social organisms.