I. MUTUALISM

 

Are there conflicts between mutualists?

• Mutualisms—cooperative interactions among species—are considered enigmatic: why cooperate when an individual might gain greater benefits by cheating? Our research has shown this fitness conflict in mutualism is real: when plants are prevented from choosing among partners, natural selection favors rhizobium symbiont genotypes that cheat their host (ie. provide below average fitness benefit to the host, while amassing above average fitness for their own progeny) (Porter & Simms 2014). Despite the temptation to cheat however, cheaters appear to be rare in mutualisms (Jones et al 2015). We are interested in understanding why.

• While mutualists can face fitness conflicts over the resources they exchange, such as how much nitrogen a rhizobial symbiont fixes per unit of host carbon, they can also face conflicts over other joint phenotypes—traits whose expression is impacted by both host and symbiont genomes. For example, hosts and symbionts may both benefit from investing in shared vegetative structures, but if their offspring are not co-packaged, the onset of partner reproduction can lead to fitness conflicts (Porter & Simms, in prep).

How are cooperative interactions between hosts and symbionts reshaped during biological invasions?

• When plants and animals invade a new area, they often re-establish mutualisms that enhance their performance in the native range, by either co-invading with mutualists (Porter et al 2011) or forming novel mutualisms.

• As lead PI on NSF-DEB1355216, “Ecological and evolutionary forces reshaping mutualism during species introductions” (2014-2018), we are collaborating with co-PI Maren Friesen’s lab to examine cooperation in the legume-rhizobium symbiosis during biological invasion. We aim to understand how mutualists facilitate or constrain their partners’ range expansion by simultaneously evaluating shifts in cooperation phenotypes and genomic evolution for both hosts and symbionts. We examine specialization and the evolution of cooperation in both natural and experimentally evolved invasions, leveraging cutting edge genomic and transcriptomic tools to take new approaches to central questions linking ecological patterns with evolutionary processes.

• We are investigating patterns of colonization due to legume and rhizobium succession following major disturbance. Mount Saint Helens, WA is an active volcano that erupted catastrophically in 1980. Nitrogen-fixing plants such as legumes are important primary colonizers however, we know little about the successional dynamics of the rhizobium bacteria critical to nitrogen-fixation. We are examining how variation in rhizobium quality and abundance along gradients of succession impact the spread of colonizing legumes either directly via nitrogen provisioning or indirectly by altering herbivory.

 

II. ENVIRONMENTAL ADAPTATION

 

How do symbiontic interactions impact local adaptation to the environment?

Local adaptation can occur at various hierarchical levels in the interaction:

• Symbiont: We have found that free-living rhizobium populations have repeatedly adapted to local soil chemistry (Porter & Rice 2013).

•  Host: We have found that free-living plant populations have repeated adapted to exhibit locally adaptive color camouflage (Porter 2013).

• Host-symbiont interaction: invasive plant populations can rapidly differentiate to optimize benefits from symbionts under local soil conditions as they spread (Porter et al 2011).

We are integrating these findings to ask whether there are tradeoffs among adaptations expressed at different points in the interaction.

What explains the maintenance of the tremendous microbial diversity found in nature? Why doesn’t one “super-strain” take over?

• Diversity can be maintained if adaptation to one environment diminishes fitness in another. This fitness tradeoff can promote niche specialization and prevent competitive exclusion (Porter & Rice 2013). Yet many microbial adaptations show no detectable tradeoffs!

• We study the evolutionary ecology of microbial adaptive variants—genes, proteins, physiologies, or traits that underlie environmental adaptation—to understand the conditions under which tradeoffs arise in microbes and the impacts these tradeoffs have on diversity.

• Tradeoffs in microbes are fascinating because microbes are genomically fluid—adaptive genes can be horizontally transmitted between otherwise unrelated strains. This can allow adaptive variants to assort across environmental gradients without the constraints that limit both allelic substitution and exploration of the adaptive landscape in macroorganisms.

• Our research links the genomic, physiological, and ecological bases of local adaptation to naturally high-nickel serpentine soils in several
clades of wild Mesorhizobium bacteria (Porter et al 2016). Our goal is to identify and characterize the molecular and physiological determinants of Ni tolerance to link fundamental processes in microbial adaptation, diversification, and ecological specialization.