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.   --