Symbiotic Streptomyces Provide Antifungal Defense in Solitary Wasps

Abstract

Mutualistic microorganisms play a key role in the ecology and evolution of many animals. A group of solitary digger wasps, the so-called "beewolves," engage in a unique protective symbiosis with the actinobacterium Streptomyces philanthi that they cultivate in specialized glands in their antennae and transfer to the larval cocoon. Within the first two weeks after cocoon spinning by the beewolf larva, the symbionts produce a complex cocktail of antimicrobial compounds, thereby efficiently protecting the larva against fungal and bacterial pathogens during the long period of hibernation in the cocoon. Phylogenetic analyses of hosts and symbionts suggest that the symbiosis evolved in the late Cretaceous. Although the bacteria can be exchanged among host species, beewolves ensure specificity in the association by controlling symbiont transmission, which resulted in the stable and long-term symbiosis with a single clade of symbionts. Despite their close phylogenetic relatedness and seemingly similar ecological niche in beewolf antennae, however, cultivation of symbionts from different host species revealed a surprising diversity in metabolic capabilities, indicating that the host-provided environment plays a crucial role in shaping the divergent evolutionary fates of the bacterial symbionts. The beewolf-Streptomyces association not only constitutes an excellent model system for studying partner choice in an ancient symbiotic interaction, it also provides a case study of an animal employing a strategy comparable to the combination treatment used in human medicine to successfully fend off a broad range of detrimental microorganisms.