Symbionts for pathogen defense in beewolves

Beewolves of the genus Philanthus (Hymenoptera, Crabronidae) are solitary digger wasps that engage in a highly specialized symbiotic association with high-GC gram-positive bacteria of the order Actinomycetales (‘Candidatus Streptomyces philanthi’). The bacteria are cultivated in specialized antennal gland reservoirs of female wasps and secreted into the subterranean brood cells. The wasp larva later actively transfers the symbionts to its cocoon, where the streptomycetes provide protection against a broad range of opportunistic fungi and bacteria by producing bioactive compounds. The adult female wasps apparently acquire the symbionts by probing the cocoon surface with the antennae during emergence. This procedure, however, results in a low number of bacterial cells in emerging females, and the symbiont population therefore suffers strong bottleneck effects during each vertical transmission event. We detected ‘Ca. S. philanthi’ in about 30 different beewolf species (genus Philanthus), and the symbionts form a well-defined monophyletic clade within the genus Streptomyces, suggesting a highly specific association between the wasps and ‘Ca. S. philanthi’.

We are interested in the chemical interactions between the symbiotic partners of the beewolf-Streptomyces symbiosis as well as the evolutionary history of the symbiosis and the genomic consequences for the bacteria. We are currently analyzing the bacterial compounds that are responsible for the protection of the beewolf offspring against pathogens and the substances that the beewolves provide for the growth of the bacteria in the antennal gland reservoirs. Furthermore, we are sequencing the whole genome of the symbionts. The genome sequence will not only allow us to identify potential antibiotic genes, it will also shed light on the genomic consequences of the symbiotic lifestyle for the bacteria. Intracellular symbionts of insects have been shown to suffer dramatic genome reduction, rearrangements and shifts in base composition as compared to free-living relatives. The genome sequence of ‘Ca. Streptomyces philanthi’ will reveal the effects of an extracellular mutualistic lifestyle on genome architecture of a high-GC Gram-positive bacterium and thereby allow us to draw more general conclusions on the evolutionary and genomic consequences of insect-bacteria symbioses.