Population dynamics of a gametophytic factor controlling selective fertilization

The population behavior of a gametophytic factor (Ga) which involves gametic selection due to failure ofga pollen onGa Ga orGa ga styles in competition withGa pollen, was investigated by computer simulation. A constant versus randomly varying gametic selection parameter (k) and four different schemes of zygotic selection were introduced in this model for analyzing conditions favorable for the maintenance of locusGa polymorphic in a large, mixed selfing and random mating population. Stable polymorphism was obtained only with rather substantial heterozygote advantage at locusGa whereas the opposing pressures of gametic and zygotic selection yielded fixation of alleleGa orga around a critical value of k instead of a range of k-values allowing nontrivial equilibria. With weak selection and stochastic k, however, very slow rates of change in the genotypic proportions allowed transient polymorphism. In these cases, the rate of outcrossing (t) and the initial frequency ofGa were critical in determining the rate of allelic substitution. Moreover, low values of t allowed the replacement of alleleGa byga even with rather weak zygotic selection. These findings on the balance between gametic and zygotic selection and a markedly frequency-dependent process are briefly discussed in relation to the dynamics of similar factors involving the mating system.