Is there specificity in a defensive mutualism against soil versus lab nematodes, Dictyostelium discoideum farmers and their bacteria?

Background: The social amoeba Dictyostelium discoideum is a soil-dwelling microbe, which lives most of its life cycle in the vegetative stage as a predator of bacteria and as prey for nematodes. When bacteria are sparse, amoebae aggregate into a multicellular fruiting body. Some clones of D. discoideum have agriculture (Brock et al., 2011). They carry bacteria through the social stage, eat them prudently, and use some bacteria as defence against non-farming D. discoideum competitors. Caenorhabditis elegans preys on D. discoideum in the laboratory but does not encounter it in nature because C. elegans lives on rotten fruit. The nematode Oscheius tipulae is abundant in the soil. Questions: Do the defensive bacteria that farmers carry also protect farmers from nematodes? Is this protection specific to nematodes that reside with D. discoideum? Hypotheses: Many organisms evolve defensive mutualisms against predators. The natural habitat of D. discoideum is populated with nematodes. Therefore, we hypothesize that farming D. discoideum clones use non-food bacteria for protection from nematodes. We predicted higher fitness of farmers than non-farmers in the presence of nematodes. We also predicted to see this change of fitness only in the presence of the soil nematode, O. tipulae. Organisms: Amoeba Dictyostelium discoideum, nematodes Caenorhabditis elegans and Oscheius tipulae, bacteria Klebsiella pneumoniae and Burkholderia xenovorans. Methods: We compared spore production of D. discoideum farmers and non-farmers with and without nematodes. We also looked at nematode proliferation in the presence of farmers, non-farmers, K. pneumonia, and B. xenovorans. Results: Overall, farmer D. discoideum produced fewer spores than non-farmers. There was a decrease in the spore counts in the presence of nematodes for both farmers and non-farmers. There was a significant decrease in the percentage change in spore production for the farmers in the presence of soil nematodes but not laboratory nematodes. Nematode proliferation with the laboratory nematode and soil nematode did not vary in the presence of farmers, non-farmers, K. pneumoniae or B. xenovorans. Conclusion: The non-food bacteria that farmers carry do not provide defence against nematodes. In fact, it was a disadvantage for farmers to carry bacteria, since the soil nematode decreased spore production for farmers compared with non-farmers. However, the differences Correspondence: B. Adu-Oppong, Department of Biology, CB 1137, Washington University in St. Louis, St. Louis, MO 63130, USA. e-mail: [email protected] Consult the copyright statement on the inside front cover for non-commercial copying policies. Evolutionary Ecology Research, 2014, 16: 351–361 © 2014 Boahemaa Adu-Oppong between the laboratory nematode and the soil nematode are marked enough to conclude that different species of nematodes respond differently to D. discoideum as a food source.