Selection on sporulation strategies in a metapopulation can lead to coexistence

In constant environments the coexistence of similar species or genotypes is generally limited. In a metapopulation context, however, types that utilize the same resource but are distributed along a competition-colonization trade-off, can coexist. Much thought in this area focuses on a generic trade-off between within-deme competitive ability and between-deme dispersal ability. We point out that the sporulation program in yeasts and other microbes can create a natural trade-off such that strains which initiate sporulation at higher rates suffer in terms of within-deme competition but benefit in terms of between deme dispersal. We develop metapopulation models where the within- deme behavior follows chemostat dynamics. We first show that the rate of sporulation determines the colonization ability of the strain, with colonization ability increasing with sporulation rate up to a point. Metapopulation stability of a single strain exists in a defined range of sporulation rates. We then use pairwise invasability plots to show that coexistence of strains with different sporulation rates generally occurs, but that the set of sporulation rates that can potentially coexist is smaller than the set that allows for stable metapopulations. We extend our pairwise results to show how a continuous set of strains can coexist and verify our conclusions with numerical calculations and stochastic simulations. Our results show that stable variation in sporulation rates is expected under a wide range of ecological conditions.