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Abstract: Most dioecious plants are perennial and subject to trade‐offs between sexual reproduction and vegetative performance. However, these broader life‐history trade‐offs have not usually been incorporated into theoretical analyses of the evolution of separate sexes. One such analysis has indicated that hermaphroditism is favoured over unisexuality when female and male sex functions involve the allocation of nonoverlapping types of resources to each sex function (e.g. allocations of carbon to female function vs. allocations of nitrogen to male function). However, some dioecious plants appear to conform to this pattern of resource allocation, with different resource types allocated to female vs. male sex functions. Using an evolutionarily stable strategy approach, we show that life‐history trade‐offs between sexual reproduction and vegetative performance enable the evolution of unisexual phenotypes even when there are no direct resource‐based trade‐offs between female and male sex functions. This result might help explain the preponderance of perennial life histories among dioecious plants and why many dioecious plants with annual life histories have indeterminate growth with ongoing trade‐offs between sexual reproduction and vegetative growth. [ABSTRACT FROM AUTHOR]

Background The ‘gynodioecy–dioecy pathway’ is considered to be one of the most important evolutionary routes from hermaphroditism to separate sexes (dioecy). Despite a large accumulation of evidence for female seed fertility advantages in gynodioecious species (females and hermaphrodites coexist) in support of the first step in the gynodioecy–dioecy pathway, we still have very little evidence for the second step, i.e. the transition from gynodioecy to dioecy. Scope We review the literature to evaluate whether basic predictions by theory are supported. To establish whether females' seed fertility advantage and frequencies are sufficient to favour the invasion of males, we review these for species along the gynodioecy–dioecy pathway published in the last 5 years. We then review the empirical evidence for predictions deriving from the second step, i.e. hermaphrodites' male fertility increases with female frequency, selection favours greater male fertility in hermaphrodites in gynodioecious species, and, where males and hermaphrodites coexist with females (subdioecy), males have greater male fertility than hermaphrodites. We review how genetic control and certain ecological features (pollen limitation, selfing, plasticity in sex expression and antagonists) influence the trajectory of a population along the gynodioecy–dioecy pathway. Conclusions Females tend to have greater seed fertility advantages over hermaphrodites where the two coexist, and this advantage is positively correlated with female frequency across species, as predicted by theory. A limited number of studies in subdioecious species have demonstrated that males have an advantage over hermaphrodites, as also predicted by theory. However, less evidence exists for phenotypic selection to increase male traits of hermaphrodites or for increasing male function of hermaphrodites in populations with high female frequency. A few key case studies underline the importance of examining multiple components of male fertility and the roles of pollen limitation, selfing and plasticity, when evaluating advantages. We conclude that we do not yet have a full understanding of the transition from gynodioecy to dioecy. [ABSTRACT FROM AUTHOR]

The maintenance of females in gender dimorphic populations requires that they have a fitness advantage to compensate for their loss of male reproductive function. We assess whether inbreeding avoidance provides this advantage in two subdioecious Wurmbea dioica populations by estimating seed production, outcrossing rates and inbreeding depression. Fruiting males produced less than half as many seeds as females, owing to low outcrossing rates and early acting inbreeding depression. Inbreeding coefficients of fruiting males demonstrated that progeny were more inbred than their parents, implying that few selfed progeny reach maturity, as confirmed by inbreeding depression estimates that exceeded 0.85. In a glasshouse experiment, open-pollinated females exhibited a fitness advantage of 3.7 relative to fruiting males, but when we increased fruiting male outcrossing rate, female advantage was only 1.4. This reduced advantage is insufficient to maintain females if nuclear genes control sex. Thus, inbreeding avoidance could maintain females at high frequencies, although this is contingent upon high frequencies of fruiting males, which can be altered by environmentally determined gender plasticity. [ABSTRACT FROM AUTHOR]