Abstract
Symbiotic associations impact and are impacted by their surrounding ecosystem. The association between Burkholderia bacteria and the soil amoeba Dictyostelium discoideum is a tractable model to unravel the biology underlying symbiont-endowed phenotypes and their impacts. Several Burkholderia species stably associate with D. discoideum and typically reduce host fitness in food-rich environments while increasing fitness in food-scarce environments. Burkholderia symbionts are themselves inedible to their hosts but induce co-infections with secondary bacteria that can serve as a food source. Thus, Burkholderia hosts are “farmers” that carry food bacteria to new environments, providing a benefit when food is scarce. We examined the ability of specific Burkholderia genotypes to induce secondary co-infections and assessed host fitness under a range of co-infection conditions and environmental contexts. Although all Burkholderia symbionts intracellularly infected Dictyostelium, we found that co-infections are predominantly extracellular, suggesting that farming benefits are derived from extracellular infection of host structures. Furthermore, levels of secondary infection are linked to conditional host fitness; B. agricolaris infected hosts have the highest level of co-infection and have the highest fitness in food-scarce environments. This study illuminates the phenomenon of co-infection induction across Dictyostelium associated Burkholderia species and exemplifies the contextual complexity of these associations.
Highlights
Symbiotic interactions can alter the fitness and evolutionary trajectory of both partners [1,2,3,4]
We began with three Burkholderia strains: Ba.70, Bh.11, and Bb.859, each representing one D. discoideum symbiont species B. agricolaris, B. hayleyella, and B. bonniea (Supplementary Fig. 1) [36]
Secondary bacteria consisted of a Klebsiella pneumoniae strain, soil isolated Rhizobium and Serratia, and lab Agrobacterium tumefaciens and Pseudomonas aeruginosa strains
Summary
Symbiotic interactions can alter the fitness and evolutionary trajectory of both partners [1,2,3,4]. Light mediates pathogenicity of a fungal plant endosymbiont [10], temperature affects reproductive fitness of aphids hosting Buchnera [11], and parasitoid pressure determines whether Hamiltonella defensa is beneficial to host aphids [12]. These examples demonstrate that even canonically beneficial or detrimental associations may produce alternative effects in alternative contexts [4, 13,14,15,16,17]
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