University of Liverpool, UK
An evolutionary biologist marvels at how species evolve to help each other out.
Mutualistic interactions between species underpin much of nature’s biodiversity. These associations range from prolonged and intimate, such as those between nitrogen-fixing bacteria and leguminous plants, to fleeting — visits of pollinating animals to flowers, for example. Despite the wide-ranging importance of mutualisms, there have been few experimental studies on their origins or evolution.
In a fascinating experiment, Kristina Hillesland and David Stahl at the University of Washington in Seattle watched a novel two-species interaction develop from teetering baby steps to a stable, robust mutualism over just 300 generations (K. L. Hillesland and D. A. Stahl Proc. Natl Acad. Sci. USA 107, 2124–2129; 2010).
Grown in the lab with lactate as the sole nutrient source, the bacterium Desulfovibrio vulgaris and the archaeon Methanococcus maripaludis, which never interact in nature, had to collaborate to survive. D. vulgaris fermented lactate to produce acetate, carbon dioxide and hydrogen — a reaction that sustains growth only if the hydrogen concentration is kept low. M. maripaludis fulfilled this requirement by consuming hydrogen to reduce carbon dioxide to methane.
Communities were initially poorly adapted to do this, and underwent drastic fluctuations in population size, with some even going extinct. In other communities, however, natural selection ensured that the two processes were optimized, thereby jointly increasing the fitness of both species. Co-evolved communities outperformed their evolutionary ancestors by growing 80% faster and producing 30% more biomass.
The work demonstrates the evolution of a stable multispecies mutualism, enhancing our understanding of how such interactions, so important for biodiversity, come about in nature.