Equality between the sexes in plants for costs of reproduction; evidence from the dioecious Cape genus Leucadendron (Proteaceae)

Abstract

It has been argued that sexual allocation is greater for female function than male function in plants in general and specifically for the large dioecious Cape genus Leucadendron. Here, we use new interpretations of published information to support the hypothesis of equality between sexes in this genus. The explanations are based on the fire ecology of the Cape that results in reproductive synchrony, reproductive doubling and competitive symmetry. Firstly, strict post-fire seedling establishment of the reseeder life-history in the Cape results in single-aged populations. Consequently, the reproductive and vegetative schedules of males must synchronously track that of females. This implies equal allocation to sex. Secondly, after fires, dioecious females have double the seedling to adult ratio of co-occurring hermaphrodites. This indicates that being liberated from male function allows females access to resources that double their fitness compared to hermaphrodites. Therefore, male and female costs of reproduction are equal in hermaphrodites. Thirdly, competitive symmetry must occur because males and female plants will frequently encounter each other as close near neighbours. Competitive asymmetry would both reduce mating opportunities and skew local sex ratios. The evidence to date is for 1:1 sex ratios in small plots and this indicates competitive symmetry and a lack of dimorphic niches. Finally, vegetatively dimorphic species must also allocate equally to sex, or else sexual asynchrony, lack of reproductive doubling or competitive asymmetry will occur.