The evolution of eukaryotic organisms is often strongly influenced by microbial symbionts that confer novel traits to their hosts. Here we describe the intracellular Enterobacteriaceae symbiont of the invasive ant Cardiocondyla obscurior, 'Candidatus Westeberhardia cardiocondylae'. Upon metamorphosis, Westeberhardia is found in gut-associated bacteriomes that deteriorate following eclosion. Only queens maintain Westeberhardia in the ovarian nurse cells from where the symbionts are transmitted to late-stage oocytes during nurse cell depletion. Functional analyses of the streamlined genome of Westeberhardia (533 kb, 23.41% GC content) indicate that neither vitamins nor essential amino acids are provided for the host. However, the genome encodes for an almost complete shikimate pathway leading to 4-hydroxyphenylpyruvate, which could be converted into tyrosine by the host. Taken together with increasing titers of Westeberhardia during pupal stage, this suggests a contribution of Westeberhardia to cuticle formation. Despite a widespread occurrence of Westeberhardia across host populations, one ant lineage was found to be naturally symbiont-free, pointing to the loss of an otherwise prevalent endosymbiont. This study yields insights into a novel intracellular mutualist that could play a role in the invasive success of C. obscurior.

SummaryCommunication in social insects usually serves the good of the whole society and thus increases the inclusive fitness of all individuals. Hence, cheating and dishonesty are not expected when nestmates are to be alarmed or recruited to food sources. However, kin selection predicts a conflict of interest among individuals about the partitioning of reproduction. Dishonest communication may then be advantageous. Workers usually do not lay eggs in the presence of a fertile queen, but in many species they do so when the queen is removed. This effect has been explained by manipulative, i.e. dishonest, queen control or honest fertility signalling. Numerous studies have documented qualitative and quantitative differences in the pheromone blends of reproductives and non-reproductives. We examine these data for signs of honest signalling, conflict and manipulation.

In group-living animals, mutual policing to suppress reproduction is an important mechanism in the resolution of conflict between selfish group members and the group as a whole. In societies of bees, ants, and wasps, policing against the production of males by other workers is expected when egg laying by workers decreases the average inclusive fitness of individual group members. This may result (i) from the relatedness of workers being lower to worker-than to queen-derived males or (ii) from a lowered overall colony efficiency. Whereas good evidence exists for policing behavior caused by genetic conflicts, policing caused by efficiency factors has not been demonstrated. We investigated the regulation of reproduction in the ant Platythyrea punctata, a species in which colonies are clones because workers are capable of producing female offspring by thelytokous parthenogenesis. Reproductive conflicts resulting from differences in genetic relatedness are therefore not expected, but uncontrolled reproduction by all workers could lead to the destruction of sociality. Here we show that worker policing by aggressive attacks against additionally reproducing workers keeps the number of reproducing workers low. Furthermore, through experimental manipulation of the number of brood items per colony, we show that worker policing can enhance group efficiency.