Effective Population Sizes for Cytoplasmic and Nuclear Genes in a Gynodioecious Species: The Role of the Sex Determination System

Equations are derived for the effective sizes of gynodioecious populations with respect to both nuclear and cytoplasmic genes (Nec and Nen, respectively). Compared to hermaphroditism, gynodioecy generally reduces effective population sizes for both kinds of loci to an extent depending on the frequency of females, the sex determination system, and the selfing rate of hermaphrodites. This reduction is due to fitness differences between the sexes and is highly influenced by the mode of inheritance of this fitness. In absence of selfing, nuclear gynodioecy results in a reduction of Nec that depends strongly on the dominance of male sterility alleles, while Nen remains equal to the census number (N). With cytonuclear gynodioecy, both cytoplasmic and nuclear effective sizes are reduced, and at the extreme, dioecy results in the minimum Nec values and either minimum or maximum Nen values (for low or high frequency of females, respectively). When selfing occurs, gynodioecy either increases or decreases Nen as compared to hermaphroditism with the same selfing rate of hermaphrodites. Unexpectedly, Nec also varies with the selfing rate. Thus the genetic sex-determination system appears as a major factor for the nuclear and cytoplasmic genetic diversities of gynodioecious species.