RachelM

http://www.plosone.org/article/info%3Adoi%2F10.1371%2Fjournal.pone.0008748

Researchers studying tripartite mutualism in ant species at the University of Wisconsin found in vitro antagonistic reactions between the actinobacteria and cultivar fungus while in vivo reactions were less understood. Fungus was paired with a specific strain of the bacteria and interactions were measured through biomass inhibition/accumulation. Although in vivo results yielded less conclusive data, Acromyrmex sp. preferentially chose and recognized their indigent Pseudonocardia.

Mutualistic symbiosis is an important ecological concept since the evolution of one organism is mutually dependent on the evolution of another. This may also be true in predator-prey relations as well. The tripartite relationship between ants, fungus, and bacteria was thought to have originated 50 million years ago. Ants cultivate the fungus for food and supply it with the necessary nutrients. This fungus, often specific to the ant colony, is hosts to a number of species of bacteria in the genus Escovopsis. Ants developed a way to combat the antifungal microbe by producing the bacteria Pseudonocardia. This bacteria, found on cuticle of the worker ants and the queen, produces secondary metabolites that exhibit antibiotic resistance to Escovopsis. Ants typically house a monoculture of the Pseudonocardia bacteria and a monoculture of the fungus. When new strains are introduced to the specific fungus (horizontal transmission), inhibitory growth responses may occur. Antagonism between the two species yields less biomass in some cases. Strains are typically introduced vertically from parents to offspring. The combination of the ant and the cultivar fungus could indeed impact the survival of the ant colony. This study observed the ant colonies in vitro through sub colonization. Sub colonies began collapsing after three weeks due to time rather than do to the secondary metabolites produced by the host’s microbial symbiont. The extent of the in vivo impacts was harder to determine and there was no direct negative impact. This result may have been do to a number of factors, such a the location separation between the cultivar and the bacteria. More work needs to be done on the in vivo results to draw a better conclusion.

What can this article tell us about the species definition in co-evolution? Different strains of bacteria are competing against each other and ants are associated typically with few or only one strain of bacteria and one cultivar. Does this make each strain a different species due to antagonism responses within what are considered mutual symbionts? Interestingly, in the article, the author mentions that the fungal cultivar “defends its monopoly by imprinting ant fecal droplets with incompatibility compounds that aid in the detection of non-native fungal strains” (Poulsen 2010).
The question “why do species definitions matter in light of your favorite microbe or microbiological issue?” interests me since we have yet to discuss the importance of the co-evolution of species. It seems that we were always asking, “what makes a species a species?” Physiologically, these species are similar and a few genetic mutations are the only difference between what wins out in the end. The line is harder to distinguish than eukaryotic organisms and at what point are you able to draw the line? Even the fungus cultivar behaves similarly and excludes related fungus from the nest.

It seems that trying to phylogenetically classify bacterial species might be a lost cause since there is too much grey. As humans, we attempt to classify everything to make order out of chaos. As sentient beings, we must philosophize and attempt to create a bridge from the known to the unknown but perhaps there is no way to fully classify certain organisms. Can we accept that the difference between a species and an ecotype is less cut and dry than we’d originally like it to be? It is easy to expect a dualism between all things but can we always make things so cut and dry?
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