Foster carers influence brood pathogen resistance in ants

Queen presence mediates the relationship between collective behaviour and disease susceptibility in ant colonies

The success of social living can be explained, in part, by a group's ability to execute collective behaviours unachievable by solitary individuals. However, groups vary in their ability to execute these complex behaviours, often because they vary in their phenotypic composition. Group membership changes over time due to mortality or emigration, potentially leaving groups vulnerable to ecological challenges in times of flux. In some societies, the loss of important individuals (e.g. leaders, elites and queens) may have an especially detrimental effect on groups' ability to deal with these challenges. Here, we test whether the removal of queens in colonies of the acorn ant Temnothorax curvispinosus alters their ability to execute important collective behaviours and survive outbreaks of a generalist entomopathogen. We employed a split-colony design where one half of a colony was maintained with its queen, while the other half was separated from the queen. We then tested these subcolonies' performance in a series of collective behaviour assays and finally exposed colonies to the entomopathogenic fungus Metarhizium robertsii by exposing two individuals from the colony and then sealing them back into the nest. We found that queenright subcolonies outperformed their queenless counterparts in nearly all collective behaviours. Queenless subcolonies were also more vulnerable to mortality from disease. However, queenless groups that displayed more interactions with brood experienced greater survivorship, a trend not present in queenright subcolonies. Queenless subcolonies that engage in more brood interactions may have had more resources available to cope with two physiological challenges (ovarian development after queen loss and immune activation after pathogen exposure). Our results indicate that queen presence can play an integral role in colony behaviour, survivorship and their relationship. They also suggest that interactions between workers and brood are integral to colonies survival. Overall, a social group's history of social reorganization may have strong consequences on their collective behaviours and their vulnerability to disease outbreaks.

Scrutinizing the immune defence inventory of Camponotus floridanus applying total transcriptome sequencing

BACKGROUND

Defence mechanisms of organisms are shaped by their lifestyle, environment and pathogen pressure. Carpenter ants are social insects which live in huge colonies comprising genetically closely related individuals in high densities within nests. This lifestyle potentially facilitates the rapid spread of pathogens between individuals. In concert with their innate immune system, social insects may apply external immune defences to manipulate the microbial community among individuals and within nests. Additionally, carpenter ants carry a mutualistic intracellular and obligate endosymbiotic bacterium, possibly maintained and regulated by the innate immune system. Thus, different selective forces could shape internal immune defences of Camponotus floridanus.

RESULTS

The immune gene repertoire of C. floridanus was investigated by re-evaluating its genome sequence combined with a full transcriptome analysis of immune challenged and control animals using Illumina sequencing. The genome was re-annotated by mapping transcriptome reads and masking repeats. A total of 978 protein sequences were characterised further by annotating functional domains, leading to a change in their original annotation regarding function and domain composition in about 8% of all proteins. Based on homology analysis with key components of major immune pathways of insects, the C. floridanus immune-related genes were compared to those of Drosophila melanogaster, Apis mellifera, and other hymenoptera. This analysis revealed that overall the immune system of carpenter ants comprises many components found in these insects. In addition, several C. floridanus specific genes of yet unknown functions but which are strongly induced after immune challenge were discovered. In contrast to solitary insects like Drosophila or the hymenopteran Nasonia vitripennis, the number of genes encoding pattern recognition receptors specific for bacterial peptidoglycan (PGN) and a variety of known antimicrobial peptide (AMP) genes is lower in C. floridanus. The comparative analysis of gene expression post immune-challenge in different developmental stages of C. floridanus suggests a stronger induction of immune gene expression in larvae in comparison to adults.

CONCLUSIONS

The comparison of the immune system of C. floridanus with that of other insects revealed the presence of a broad immune repertoire. However, the relatively low number of PGN recognition proteins and AMPs, the identification of Camponotus specific putative immune genes, and stage specific differences in immune gene regulation reflects Camponotus specific evolution including adaptations to its lifestyle.

Emergency measures: Adaptive response to pathogen intrusion in the ant nest

Ants have developed prophylactic and hygienic behaviours in order to limit risks of pathogenic outbreaks inside their nest, which are often called social immunity. Here, we test whether ants can adapt the "social immune response" to the level of pathogenic risk in the colony. We challenged Myrmica rubra colonies with dead nestmates that had either died from being frozen or from infection by the fungus Metarhizium anisopliae. Ant survival was compromised by the presence of the fungus-bearing corpses: workers died faster with a significantly lower survival from the 4th day compared to workers challenged with freeze-killed corpses. When faced with fungus-bearing corpses, workers responded quickly by increasing hygienic behaviours: they spent more time cleaning the nest, moving the corpses, and self-grooming. Ants in fungus-threatened colonies also decreased contact rates with other workers, and moved corpses further in the corners of the nest than in colonies in contact with non-infected corpses. These results show that ant colonies are able to assess the risk level associated with the presence of corpses in the nest, and adjust their investment in terms of hygienic behaviour.