# Symbiotic T6SS affects horizontal transmission of Paraburkholderia bonniea among Dictyostelium discoideum amoeba hosts

## TL;DR

A type VI secretion system in Paraburkholderia bonniea helps it spread between amoeba hosts, supporting long-term symbiosis.

## Contribution

The study reveals the T6SS's role in horizontal transmission, not host fitness, in amoeba-Paraburkholderia symbiosis.

## Key findings

- Wildtype P. bonniea transmits more efficiently than the ∆tssH mutant.
- T6SS is crucial for long-term symbiotic relationships between amoebae and Paraburkholderia.
- Host gene expression is unaffected by T6SS presence, suggesting no direct fitness impact.

## Abstract

Three species of Paraburkholderia are able to form facultative symbiotic relationships with the amoeba, Dictyostelium discoideum. These symbiotic Paraburkholderia share a type VI secretion system (T6SS) that is absent in other close relatives. We tested the phenotypic and transcriptional effect of tssH ATPase gene disruption in P. bonniea on its symbiosis with D. discoideum. We hypothesized that the ∆tssH mutant would have a significantly reduced ability to affect host fitness or transmit itself from host to host. We found that the T6SS does not directly affect host fitness. Instead, wildtype P. bonniea had significantly higher rates of horizontal transmission compared to ∆tssH. In addition, we observed significant differences in the range of infection prevalence achieved by wildtype vs. ∆tssH symbionts over multiple host social stages in the absence of opportunities for environmental symbiont acquisition. Successful symbiont transmission significantly contributes to sustained symbiotic association. Therefore, the shared T6SS appears necessary for a long-term evolutionary relationship between D. discoideum and its Paraburkholderia symbionts. The lack of difference in host fitness outcomes was confirmed by indistinguishable host gene expression patterns between hosts infected by wildtype or ∆tssH P. bonniea in an RNA-seq time series. These data also provided insight into how Paraburkholderia symbionts may evade phagocytosis by its amoeba host. Most significantly, cellular oxidant detoxification and lysosomal hydrolase delivery appear to be subject to the push and pull of host-symbiont crosstalk.