The K(a) /K(s) and π(a) /π(s) Ratios under Different Models of Gametophytic and Sporophytic Selection

Alternation of generations in plant life cycle provides a biological basis for natural selection occurring in either the gametophyte or the sporophyte phase or in both. Divergent biphasic selection could yield distinct evolutionary rates for phase-specific or pleiotropic genes. Here, we analyze models that deal with antagonistic and synergistic selection between alternative generations in terms of the ratio of nonsynonymous to synonymous divergence (K(a)/K(s)). Effects of biphasic selection are opposite under antagonistic selection but cumulative under synergistic selection for pleiotropic genes. Under the additive and comparable strengths of biphasic allelic selection, the absolute K(a)/K(s) for the gametophyte gene is equal to in outcrossing but smaller than, in a mixed mating system, that for the sporophyte gene under antagonistic selection. The same pattern is predicted for K(a)/K(s) under synergistic selection. Selfing reduces efficacy of gametophytic selection. Other processes, including pollen and seed flow and genetic drift, reduce selection efficacy. The polymorphism (π(a)) at a nonsynonymous site is affected by the joint effects of selfing with gametophytic or sporophytic selection. Likewise, the ratio of nonsynonymous to synonymous polymorphism (π(a)/π(s)) is also affected by the same joint effects. Gene flow and genetic drift have opposite effects on π(a) or π(a)/π(s) in interacting with gametophytic and sporophytic selection. We discuss implications of this theory for detecting natural selection in terms of K(a)/K(s) and for interpreting the evolutionary divergence among gametophyte-specific, sporophyte-specific, and pleiotropic genes.