Which phenotypic traits of Dictyostelium discoideum farmers are conferred by their bacterial symbionts?

The bacterial partners in symbiotic relationships with eukaryotes can have a powerful effect on the phenotypic traits of the host. Here we explore this issue using a simple model eukaryote, Dictyostelium discoideum, and its facultative bacterial symbionts. Some clones of the social amoeba D. discoideum, called farmers, maintain symbiotic relationships with certain species of bacteria while other clones, called non-farmers, do not. D. discoideum farmer clones that carry bacterial symbionts in the genus Burkholderia have four distinct traits associated with the farming symbiosis: i) short slug migration distances, ii) symbiont transport, iii) prudent harvesting of bacteria, and iv) resistance to toxicity of the farmer-associated bacteria. These traits, with their advantages and disadvantages, could be either conferred by bacterial symbionts, or intrinsic to the amoebae that have been colonized by bacteria. We compared five farmers and five non-farmers, both with and without farmer-associated bacteria, to disentangle the direct effects of bacteria from those intrinsic to the host. We found that short migration distance and symbiont transport are bacterially conferred traits. Prudent harvesting seems to represent a trait influenced by bacteria but not entirely controlled by them. Resistance to farmer-associated Burkholderia is present whether or not the farmer clone is currently carrying it or has been cured and newly exposed. Taken together, these data suggest that the association between host farmer and Burkholderia is not recent.