Modelling the maintenance of male-fertile cytoplasm in a gynodioecious population

Abstract

Gynodioecy is the co-occurrence of females and hermaphrodites in populations. It is usually due to the combined action of cytoplasmic male sterility (CMS) genes and nuclear genes that restore male fertility. According to previous theoretical studies, it is very difficult to explain the maintenance of gynodioecy with CMS and male-fertile cytotypes, although it has been observed in some species. However, only very specific situations have been investigated so far. We present a model to investigate the conditions that promote the maintenance of this breeding system in the case of an outcrossed species when CMS and male-fertile (non-CMS) cytotypes are present in an infinite panmictic population. We show that the type of cost of restoration strongly affects the conditions for stable maintenance of gynodioecy. Stable nuclear-cytoplasmic gynodioecy requires a female advantage, which is a classical condition for gynodioecy, but also a cost of CMS for female fitness, which had been rarely investigated. A cost of restoration is also needed, which could affect either pollen or seeds. Finally, we found that gynodioecy was attainable for a large set of parameter values, including low differences in fitness among genotypes and phenotypes. Our theoretical predictions are compared with previous theoretical work and with results of empirical studies on various gynodioecious species.

The effect of disease on the evolution of females and the genetic basis of sex in populations with cytoplasmic male sterility

“…To investigate the effect of disease on the evolution of mating systems, we first constructed a genetic model of cyto-nuclear gynodioecy that consisted of two maternally inherited cytotypes (one male fertile, one male sterile) and a single nuclear restorer locus with two alleles, similar to previous genetic models of gynodioecy [27,41]. We then introduced numerical dynamics and population regulation by incorporating a density-dependent birth rate and fixed mortality rate.…”

“…(a) Genetic model Our genetic model followed that of Dufay et al [41] for an outcrossing plant. Sex was determined by the interaction between cytotype and a single nuclear restorer locus (table 1).…”

“…The male sterile cytotype, CMS, caused individuals to become female due to a loss of male function, unless a restorer allele, R, was present. Previous theoretical work has shown that stable cyto-nuclear gynodioecy can only be maintained if a female fertility advantage and a cost of restoration are included [27,41,43]. Females were assumed to have a female fertility advantage, a, and a dominant, constitutive cost associated with the restorer allele, cr, was applied to the male component of fertility of individuals carrying at least one restorer allele ( [41]; table 1).…”

“…Previous theoretical work has shown that stable cyto-nuclear gynodioecy can only be maintained if a female fertility advantage and a cost of restoration are included [27,41,43]. Females were assumed to have a female fertility advantage, a, and a dominant, constitutive cost associated with the restorer allele, cr, was applied to the male component of fertility of individuals carrying at least one restorer allele ( [41]; table 1). We also included a cost, cc, associated with carrying a male sterile cytotype because previous work has shown that stable cyto-nuclear gynodioecy is not possible without cc , 1 [41].…”

“…Here we combine a cyto-nuclear genetic model of gynodioecy [41] with a classic susceptible-infected (SI) model of disease dynamics [42] to investigate how sex-specific resistance to disease affects the evolution of females and the genetic basis of sex. We first ask whether disease can drive the evolution of gynodioecy from hermaphroditism by providing a selective advantage for females.…”