A synthetic review of the theory of gynodioecy.

Cytoplasmic male sterility alleles (CMS) and corresponding nuclear restorer alleles (R) determine gender expression in gynodioecious populations. In this article, we combine cytonuclear epistasis theory, multilevel selection theory, and resource allocation theory to partition the many selective forces acting in gynodioecious species. This description of selective forces reveals that the notion that genomic conflict leads to the spread of restorers is erroneous. By elucidating the effect of each selective force alone and in combination with the others, our approach shows why change in a single parameter, such as the degree of restorer dominance or the cost of restoration, can result in a myriad of opposing selective effects, making it difficult to test theoretical predictions in experimental systems. In particular, our approach allows us to identify the harmonic mean resource allocation between pollen and ovules and the attendant Fisherian selection as playing a critical role in the evolution of the restorer allele and gender polymorphism. We use this conceptual framework to propose empirical methods that emphasize the role of Fisherian selection acting in gynodioecious populations. Because our approach increases the number and specificity of theoretical predictions, we argue that it provides a better framework for empirical testing.