Résumé
We study modification of sex types in gynodioecious populations by using a simple model in which femaleness is due to simple cytoplasmic inheritance. Males and hermaphrodites carry a non-sterility cytoplasm and differ in their genotype at a single locus which does not affect the females. In this case, changes in frequency of an allele producing males depend only on its effects in the non-sterility cytoplasm. An important consequence is that the male investment of hermaphrodites is unaffected by the presence of females in the system. We also study a more realistic model in which there is joint nucleo-cytoplasmic inheritance of male-sterility. This model can be studied only by computer calculations. We find conditions for modification of the hermaphrodites' allocation patterns, and show that there is selection for increase maleness and that this leads to loss of the non-sterility cytoplasm. We also investigate the conditions for males to invade. This requires large increases in male function, but spread of alleles causing complete maleness can be faster than with nuclear gynodioecy. Final populations often contain hermaphrodites as well as males and females. Dioecy or subdioecy may therefore evolve from populations with nucleo-cytoplasmic gynodiecy.