Equilibrium populations of heterostylous plants

A general relation is derived which must be satisfied by the equilibrium frequencies of the incompatibility types occurring in populations of heterostylous plants. Specific assumptions are made about the underlying genetic mechanism, but these are fulfilled in practically all relevant cases. With usual mating systems corresponding to pollen or zygote elimination and no fitness differences, it follows that all incompatibility types must be equally frequent. Given different fitness values, the general relation leads to more complex equations determining the frequencies. These equations are used for estimating fitness values of the Long, Mid and Short types in Lythrum salicaria. Expressions for the equilibrium frequencies are also found for a special mating system in Oxalis alpina.