Mixed Wolbachia infections resolve rapidly during in vitro evolution

The intracellular symbiont Wolbachia pipientis evolved after the divergence of arthropods and nematodes, but it reached high prevalence in many of these taxa through its abilities to infect new hosts and their germlines. Some strains exhibit long-term patterns of co-evolution with their hosts, while other strains are capable of switching hosts. This makes strain selection an important factor in symbiont-based biological control. However, little is known about the ecological and evolutionary interactions that occur when a promiscuous strain colonizes an infected host. Here, we study what occurs when two strains come into contact in host cells following horizontal transmission and infection. We focus on the faithful w Mel strain from Drosophila melanogaster and the promiscuous w Ri strain from Drosophila simulans using an in vitro cell culture system with multiple host cell types and combinatorial infection states. Mixing D. melanogaster cell lines stably infected with w Mel and w Ri revealed that wMel outcompetes w Ri quickly and reproducibly. Furthermore, w Mel was able to competitively exclude w Ri even from minuscule starting quantities, indicating that this is a nearly deterministic outcome, independent of the starting infection frequency. This competitive advantage was not exclusive to wM el’s native D. melanogaster cell background, as w Mel also outgrew w Ri in D. simulans cells.

Overall, w Ri is less adept at i n vitro growth and survival than w Mel and its in vivo state, revealing differences between cellular and humoral regulation. These attributes may underlie the observed low rate of mixed infections in nature and the relatively rare rate of host-switching in most strains. Our in vitro experimental framework for estimating cellular growth dynamics of Wolbachia strains in different host species, tissues, and cell types provides the first strategy for parameterizing endosymbiont and host cell biology at high resolution. This toolset will be crucial to our application of these bacteria as biological control agents in novel hosts and ecosystems.

Author Summary

Wolbachia pipientis is one of the most common bacterial endosymbionts due to its ability to manipulate host reproduction, and it has become a useful biological control tool for mosquito populations. Wolbachia is passed from mother to offspring, however the bacterium can also “jump” to new hosts via horizontal transmission.

When a Wolbachia strain successfully infects a new host, it often encounters a resident strain that it must either replace or co-exist with as a superinfection. Here, we use a Drosophila melanogaster cell culture system to study the dynamics of mixed Wolbachia infections consisting of the high-fidelity w Mel and promiscuous w Ri strains. The w Mel strain consistently outcompetes the w Ri strain, regardless of w Mel’s initial frequency in D. melanogaster cells. This competitive advantage is independent of host species. While both strains significantly impede host cell division, only the w Mel strain is able to rapidly expand into uninfected cells. Our results suggest that the w Ri strain is pathogenic in nature and a poor cellular symbiont, and it is retained in natural infections because cell lineages are not expendable or replaceable in development. These findings provide insights into mixed infection outcomes, which are crucial for the use of the bacteria in biological control.