Evolution of phenotypic polymorphism in symbiont-pairing in plant-fungal symbiosis

Temporal environmental variation presents a challenge to organisms because the optimal phenotype may vary with time. This is particularly problematic for populations in spatially uniform environments, because spatial variation is generally required to maintain genotypic polymorphism. One solution is to produce offspring that share a genotype but have developmental randomness that causes them to vary in phenotype-i.e., to be phenotypically polymorphic. This is known as adaptive coin-flipping or (in some cases) diversifying bet-hedging. Here, we investigate how the environment and trait expression affect the initial evolution of polyphenism, as well as the optimal polyphenic traits and frequency of trait expression. We focus on the case of plants allocating resources to mycorrhizal partners. We find that the optimal traits to express polyphenically are close to, but not the same as, the best single trait in each environmental state alone. Similarly, the optimal frequency of expressing each trait is close, but not identical, to the frequency of the environmental states. This reflects the benefit of being somewhat conservative in trait values, to reduce the mismatch between trait and environment, and somewhat risk-taking in trait expression frequency, to capture the benefit of expressing the "right" trait in the more frequent environment.