Season-specific dominance broadly stabilizes polymorphism under symmetric and asymmetric multivoltinism

Seasonality causes intraannual fitness changes in multivoltine populations (defined as having multiple generations per year). While it is well-known that seasonally balanced polymorphism can be established by overdominance in geometric mean fitness, an unsettled aspect of the deterministic theory is the relative contribution of various season-specific dominance mechanisms to the potential for polymorphism. In particular, the relative importance of seasonal reversals in allelic dominance, where the alleles at a locus alternate in recessivity of their deleterious effects, merits clarification. Here, I analyze the parameter space for the discrete generation two-season multivoltine model and find that biallelic polymorphism is easily maintained owing to an abundance of stabilizing dominance schemes, and moreover, a substantial fraction of these schemes are nonreversing with the season (∼25-50%). In addition, I derive the approximate equilibrium allele frequency cycle under bivoltinism and find that the amplitude of allelic oscillation is maximized by nonreversing dominance if the homozygous fitnesses (per annum) are roughly symmetric. Lastly, I derive conditions for the intralocus evolution of dominance. These predict a long-term trend toward maximally beneficial reversal. Overall, the results counter the disproportionate emphasis placed on dominance reversal as a stabilizing mechanism and clarify that nonreversing dominance is expected to frequently characterize seasonally fluctuating alleles under both weak and strong selection, especially in their early history.