The evolution of a symbiont

The evolution of negative interactions seems like a logical consequence of natural selection. Organisms compete for resources or view one another as a resource, thus finding ways to more efficiently find and consume prey. However, to me, the natural selection of symbiotic or mutualistic interactions has never seemed as straight forward (expect maybe the case where one species provides protection for the other, such as in ant-plant mutualisms). A specific example is the rise of nitrogen-fixing plants, who supply nutrients to bacteria called rhizobia capable of converting atmospheric nitrogen into forms, such as ammonia, usable to the plant host. Not only has this symbiosis evolved, but has seemed to evolve in very evolutionarily distinct lineages. The question is, what are the mechanisms allowing for this?

In a recent paper, Marchetti and colleagues answer part of the question. They experimentally manipulate a pathogenic bacteria and observe it turning into a symbiont. They transferred a plasmid from the symbiotic nitrogen fixing Cupriavidus taiwanensis into Ralstonia solanacearum and infected Mimosa roots with it. Plasmid transfer among distinct bacteria species is common and referred to horizontal genetic transfer (as opposed to vertical, which is the transfer to daughter cells). The presence of the plasmid caused R. solanacearum to quickly evolve into a root-nodulating symbiont. Two regulatory genes lost function, and this caused R. solanacearum to form nodules and to impregnate Mimosa root cells.

This extremely novel experiment reveals how horizontal gene transfer can supply the impetus for rapid evolution from being a pathogen to a symbiont. More importantly it reveals that sometimes just a few steps are required for this transition and how distantly-related bacterial species can acquire symbiotic behaviors.

Marchetti, M., Capela, D., Glew, M., Cruveiller, S., Chane-Woon-Ming, B., Gris, C., Timmers, T., Poinsot, V., Gilbert, L., Heeb, P., Médigue, C., Batut, J., & Masson-Boivin, C. (2010). Experimental Evolution of a Plant Pathogen into a Legume Symbiont PLoS Biology, 8 (1) DOI: 10.1371/journal.pbio.1000280

How to plan an experiment that could last 99 years

For a number of reasons, including the fact that most grants only allow for research over a time span of 1-3 years, ecologists and evolutionary biologists usually plan experiments that last few years (with notable exceptions, such as the LTER). A usual approach to study long term phenomena is to take advantage of “natural” experiments. This allows us to understand about processes over long time periods, but usually with limited control on the initial conditions.

In a recent paper by Thomas Bruns and collaborators I learned about another way. They study spores viability of an important genus of ectomycorrhizal fungi, symbiont of Pinaceae: Rhizopogon. Pinaceae (the family of pines and other conifers) need ectomycorrhizal fungi to survive and usually spores and seeds are dispersed independently. It was not known how long their spores can last, which has very important implications, for example for colonization of areas not previously colonized by Pinaceae, or colonization after large scale disturbances, since if seeds cannot find mycorrhizae they have really few chances of survival. Now we know, based on this research that spore banks can be build and last probably decades.

What they did is really interesting, and was inspired on a previous study on seeds. They planted known number of spores of several species of Rhizopogon in terracotta pots, that were later planted into the ground (to mimic natural conditions). They planted 16 replicates, and they plan to open and analyze them later in the century based on the spore viability (for example, if in a few years most spores seem to be not viable that may reduce the expected length of the experiment to increase resolution). This paper found that after 4 years the inoculum potential of these spores seems to be increasing with time. I found the approach used in this experiment really fascinating and I look forward to see what happens in the next years!

Thomas D. Bruns, Kabir G. Peay, Primrose J. Boynton, Lisa C. Grubisha, Nicole A. Hynson, Nhu H. Nguyen, Nicholas P. Rosenstock (2009). Inoculum potential of
spores increases with time over the first 4 yr of a 99-yr spore burial experiment
New Phytologist, 181 (2), 463-470 DOI: 10.1111/j.1469-8137.2008.02652.x