Reproductive allocation in a gender dimorphic shrub: Anomalous female investment in Gynatrix pulchella?

Abstract

1 In gender dimorphic species, reproductive allocation (RA, the ratio of reproductive to vegetative biomass) is predicted to be greater in female plants than in male plants. Empirical research on dimorphic plant species supports this hypothesis. To date, of 44 dimorphic angiosperms for which RA has been reported in the literature, RA is greater in females than males in 40 species, is equal in four, and in no species is it greater in males. 2 In many instances where differential RA occurs, sexual dimorphism in morphological or physiological traits has been reported. This dimorphism is often attributed to the differing costs of reproduction or to selection to counteract such costs. 3 We investigated RA and other morphological and physiological characters in Gynatrix pulchella, a dimorphic species that we found ranges from dioecious to subdioecious or gynodioecious, depending on season and locality. Our results showed that contrary to our predictions functionally male plants allocated significantly more biomass to reproduction than female plants across three populations. Greater male RA was due to a combination of larger, more numerous flowers and lower leaf biomass per branch than females. 4 There were no detectable costs of greater RA in males in terms of decreased overall growth or increased mortality. Additionally, leaf nitrogen content was greater in males than in females and there were no between-sex differences in gas exchange. 5 The finding that male plants allocate significantly more resources to reproduction than females in G. pulchella is apparently a unique case. However, equivalent RA for the sexes does occur in gynodioecious species and we suggest that higher male RA in gynodioecious plants may be more common than predictions based on sexual systems involving pure males. 6 Our results challenge accepted theory that allocation to reproduction should be greater in female plants than in males. In light of these findings, predictions regarding which sex should allocate more to reproduction need to be revised to accommodate the diversity of plant sexual systems. © 2006 The Authors.