Refining the Roots of the Beewolf-Streptomyces Symbiosis: Antennal Symbionts in the Rare Genus Philanthinus (Hymenoptera, Crabronidae)
ABSTRACTInsects engage in symbiotic associations with a large diversity of beneficial microorganisms. While the majority of well-studied symbioses have a nutritional basis, several cases are known in which bacteria protect their host from pathogen infestation. Solitary wasps of the generaPhilanthusandTrachypus(beewolves; Hymenoptera, Crabronidae) cultivate the actinomycete “CandidatusStreptomyces philanthi” in specialized antennal gland reservoirs. The symbionts are transferred to the larval cocoon, where they provide protection against pathogenic fungi by producing at least nine different antibiotics. Here we investigated the closest relatives ofPhilanthusandTrachypus, the rare genusPhilanthinus, for the presence of antennal gland reservoirs and symbiotic streptomycetes. Molecular analyses identified “Ca. Streptomyces philanthi” in reservoirs ofPhilanthinus quattuordecimpunctatus. Phylogenies based on the 16S rRNA gene suggest thatP. quattuordecimpunctatusmay have acquired “Ca. Streptomyces philanthi” by horizontal transfer from other beewolf species. In histological sections and three-dimensional reconstructions, the antennal gland reservoirs were found to occupy six antennal segments (as opposed to only five inPhilanthusandTrachypus) and to be structurally less complex than those of the evolutionarily more derived genera of beewolves. The presence of “Ca. Streptomyces philanthi” in antennal glands ofPhilanthinusindicates that the symbiosis between beewolves andStreptomycesbacteria is much older than previously thought. It probably evolved along the branch leading to the monophyletic tribe Philanthini, as it seems to be confined to the generaPhilanthus,Trachypus, andPhilanthinus, which together comprise 172 described species of solitary wasps.