# Defensive symbionts of the European beewolf face competition from brood cell microbiota during vertical transmission

## TL;DR

Beewolf wasps pass on protective bacteria to their larvae, but these bacteria face competition from other microbes during the transfer process.

## Contribution

The study reveals the transmission route and microbial competition dynamics of defensive symbionts in beewolf wasps.

## Key findings

- Symbionts are taken up into the larval gut and regurgitated onto the cocoon during spinning.
- The cocoon's bacterial community shifts from Streptomyces to Pseudomonadota after gut content excretion.
- Adult and diapausing beewolf larvae are dominated by Wolbachia, unlike feeding larvae.

## Abstract

Beewolf wasps rely on an ancient defensive symbiosis with Streptomyces bacteria that protect their larvae from fungal infection. Female beewolves apply the bacteria to the brood-cell ceiling, and larvae later transfer the symbionts onto the cocoon surface, where they produce antifungal metabolites. Here, we investigated the mechanism of symbiont transfer from the beewolf brood cell to the larval cocoon and characterized the microbial community dynamics across different beewolf life stages and during larval hibernation. Fluorescence in situ hybridization revealed that the symbionts are transiently taken up into the proximal midgut lumen and then regurgitated onto the cocoon during the spinning process. High-throughput sequencing showed that the bacterial community of beewolf feeding larvae resembles that of the honeybee prey, whereas that of adults and diapausing larvae is dominated by Wolbachia. Moreover, the cocoon bacterial community is initially dominated by the defensive Streptomyces philanthi symbiont, but when larvae excrete the gut content inside the cocoon, other bacterial taxa including Lactobacillus, Gilliamella, and Bartonella shift the community composition toward dominance by Pseudomonadota. Our findings provide new insights into the transmission route of an ancient extracellular symbiont and its potential competition with other bacteria in this long-term defensive symbiosis.

Defensive Streptomyces symbionts of beewolf wasps experience a complex vertical transmission route from adult antennae via larval guts to the cocoon, which entails interactions with the gut microbiota and environmental microbes.