Relevance of bacterial symbiont strains during development in a defensive symbiosis

Symbiosis with microbes are common among insects and can serve key functions for the host. While nutritional symbioses often involve tight associations with obligate partners, defensive symbioses can be more dynamic with multiple facultative symbionts colonizing the host, sometimes even multiple symbiont strains within a single host. The phenomenon of coexisting strains within one individuum has been rarely studied and is thought to result in increased competition between strains, potential relaxed selection of symbiotic traits or on the other hand in functional complementation. In Lagria beetles, a community of symbionts inhabit specialized organs in the females and are transmitted vertically onto the egg, from where they colonize three dorsal cuticular invaginations in the larvae. It was shown that several strains of Burkholderia gladioli can coinfect and defend the eggs of the beetle L. villosa against fungal pathogens by producing antibiotic compounds. However, one strain dominates the symbiotic community across host populations and life stages, while other protective strains only occur occasionally. Here, we show that eggs of the congeneric species L. hirta are also protected by their symbionts leading to an increased hatching rate, when exposed to a pathogenic fungus. Unlike in L. villosa, the beetles harbor two related B. gladioli strains (Lh-StH & Lh-StG), which both colonize the symbiotic structures but show different levels of fungal inhibition on the eggs. Interestingly both strains coexist consistently, whereas Lh-StG is culturable and less host-dependent, while Lh-StH seems to be tighter associated to its host due to its smaller genome and unculturability.