Fig. 1.

Diagram illustrating the basic stages of the gynodioecy–dioecy (G-D) pathway. The first transition involves the invasion of a male-sterile individual (female) into an hermaphroditic (‘H’) population. If females successfully establish, the population can be said to be gynodioecious (‘GD’) (step 1). The presence of females places selective pressure on hermaphrodites to invest in male function at the cost of female function, and males invade the population (step 2). Males may invade either via gradual reductions in female fertility and increases in male fertility (2a) or via major mutations influencing male and female fertility (2b). These populations are subdioecious (‘SD’). In the final transition, pure males supplant hermaphrodites such that the system is completely dioecious (‘D’) (step 3) (based on Charlesworth and Charlesworth, 1978; Charlesworth & Gutman, 1999).