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

Taking below-ground processes seriously: plant coexistence and soil depth

Some of the earliest ecologists, like Eugen Warming and Christen Raunkiaer, were enthralled with the minutia of the differences in plant life forms and how these differences determined where plants lived. They realized that differences in plant growth forms corresponded to how different plants made their way in the world. Since this early era, understanding the mechanisms of plant competition is one of the most widely-studied aspects of ecology. This is such an important aspect of ecology because understanding plant coexistence allows us to understand what controls productivity in the basal trophic level for most terrestrial food webs. There are a plethora of plausible mechanisms for how plants are able to coexist, and most involve above-ground partitioning strategies (such as different leaf shapes) or phenological differences (such as germination or bolting timing). Yet, below-ground interactions among plants as a way to understand competition and coexistence have been making a strong resurgence in the literature lately. This resurgence has been driven by new hypotheses and technologies.In what is probably the best hypothesis test of the role for below-ground niche partitioning, Mathew Dornbush and Brian Wilsey reveal how soil depth can affect coexistence. They seeded 36 tallgrass prairie species into plot that were either shallow, medium or deep soiled, and asked if species richness and diversity were affected after 3 years. They found that species richness significantly increased with increased soil depth, revealing that deeper soils likely had greater niche opportunities for species. Not only did deeper soils harbor greater richness, but compositions were non-random subsets. The species inhabiting shallow soils were a subset of medium soils, and medium a subset of deep. This means that increasing depth opened new niche opportunities, unique from the ones for shallow soils.

This study is the first field-based experiment of soil depth and coexistence, that I know of and the results are compelling. Plant species are segregating below-ground niches, and perhaps we look for other partitioning strategies for species that inhabit the same soil depth.

Dornbush, M., & Wilsey, B. (2009). Experimental manipulation of soil depth alters species richness and co-occurrence in restored tallgrass prairie Journal of Ecology DOI: 10.1111/j.1365-2745.2009.01605.x

Other notable recent papers on below-ground processes:

Bartelheimer, M., Gowing, D., & Silvertown, J. (2009). Explaining hydrological niches: the decisive role of below-ground competition in two closely related species Journal of Ecology DOI: 10.1111/j.1365-2745.2009.01598.x

Cramer, M., van Cauter, A., & Bond, W. (2009). Growth of N-fixing African savanna species is constrained by below-ground competition with grass Journal of Ecology DOI: 10.1111/j.1365-2745.2009.01594.x

Meier, C., Keyserling, K., & Bowman, W. (2009). Fine root inputs to soil reduce growth of a neighbouring plant via distinct mechanisms dependent on root carbon chemistry Journal of Ecology, 97 (5), 941-949 DOI: 10.1111/j.1365-2745.2009.01537.x