Complexities of Inferring Symbiont Function: Symbiont Dynamics in Social Amoeba Populations and Their Impacts on the Amoeba Microbiota.

The relationship between the social amoeba Dictyostelium discoideum and its endosymbiotic bacteria provides a model system for studying the development of symbiotic relationships. Laboratory experiments have shown that any of three species of the symbiont allow D. discoideum food bacteria to persist through the amoeba life cycle and survive in amoeba spores rather than being fully digested. This phenomenon is termed "farming," as it potentially allows spores dispersed to food-poor locations to grow their own. The occurrence and impact of farming in natural populations, however, have been a challenge to measure. Here, we surveyed natural D. discoideum populations and found that only one of the three symbiont species, Paraburkholderia agricolaris, remained prevalent. We then explored the effect of on the amoeba microbiota, expecting that by facilitating bacterial food carriage, it would diversify the microbiota. Contrary to our expectations, tended to infectiously dominate the D. discoideum microbiota, in some cases decreasing diversity. Similarly, we found little evidence for facilitating the carriage of particular food bacteria. These findings highlight the complexities of inferring symbiont function in nature and suggest the possibility that could be playing multiple roles for its host. The functions of symbionts in natural populations can be difficult to completely discern. The three bacterial farming symbionts of the social amoeba Dictyostelium discoideum have been shown in the laboratory environment to allow the amoebas to carry, rather than fully digest, food bacteria. This potentially provides a fitness benefit to the amoebas upon dispersal to food-poor environments, as they could grow their food. We expected that meaningful food carriage would manifest as a more diverse microbiota. Surprisingly, we found that tended to infectiously dominate the D. discoideum microbiota rather than diversifying it. We determined that only one of the three symbionts has increased in prevalence in natural populations in the past 20 years, suggesting that this symbiont may be beneficial, however. These findings suggest that may have an alternative function for its host, which drives its prevalence in natural populations.