Increased genetic diversity from colony merging in termites does not improve survival against a fungal pathogen

Abstract

In some species of social insects the increased genetic diversity from having multiple breeders in a colony has been shown to improve pathogen resistance. Termite species typically found colonies from single mated pairs and therefore may lack the flexibility to buffer pathogen pressure with increased genetic diversity by varying the initial number of reproductives. However, they can later increase group diversity through colony merging, resulting in a genetically diverse, yet cohesive, workforce. In this study, we investigate whether the increased group diversity from colony fusion benefits social immunity in the subterranean termite Reticulitermes flavipes. We confirm previous findings that colonies of R. flavipes will readily merge and we show that workers will equally groom nestmates and non-nestmates after merging. Despite this, the survival of these merged colonies was not improved after exposure to a fungal pathogen, but instead leveled to that of the more susceptible or the more resistant colony. Our study brings little support to the hypothesis that colony fusion may improve immunity through an increase of genetic diversity in R. flavipes. Instead, we find that following exposure to a lethal pathogen, one colony is heavily influential to the entire group's survival after merging.

Figure 1

(a) The proportion of behaviors observed in each type of pairing in the agonism assay. The asterisks indicates that aggressive behavior was highly associated with interspecific pairings between R. flavipes and R. virginicus (χ²-test, p < 0.0005). (b) The pathogen assay arena with examples of mutual grooming between dyed and undyed termites, originally from different colonies. (c) Colonies exposed to a pathogen treatment spent more time grooming (Nested ANOVA, p < 0.05), but there was no difference in grooming between single (N = 6) and merged colonies (N = 6) (Nested ANOVA, p = 0.8849). (d) There was no difference in the time that termites from merged colonies spent grooming nestmates or non-nestmates (Nested ANOVA, p = 0.90711, N = 6). All analyses were performed in the statistical software R 3.5.0 (https://www.r-project.org/).

Figure 2

Kaplan-Meier survival distributions of all single (N = 6) and merged colony (N = 6) groups that were exposed to either a control or pathogen solution. Termites exposed to a pathogen had significantly lower survival than termites which received a control solution (p < 0.001). The asterisk indicates that merged colonies were found to have slightly lower survival than single colonies (p < 0.05). All analyses were performed in the statistical software R 3.5.0 (https://www.r-project.org/).

Figure 3

Kaplan-Meier survival distributions of each merged colony pair (Red, N = 1) are plotted with the survival distributions of their corresponding single colonies (Light and dark blue, N = 2). Bolded letters (a–f) correspond to each of the six groupings of merged and single colonies that were tested. Within each plot, letters denote significant groupings between pathogen treatment groups. The survival distributions of control groups are depicted in the plots, but are not included in groupings. Significance was determined by pairwise comparisons using a log-rank test (p < 0.05; Supplementary Information T1). All analyses were performed in the statistical software R 3.5.0 (https://www.r-project.org/).