Sex expression and sexual dimorphism is subdioecious 'Wurmbea dioica' (Colchicaceae)

Abstract

Subdioecy is a sexual system with male, female, and hermaphrodite phenotypes and is often considered a transitory stage in the evolution of full dioecy. Here, we examine sex ratios, sex expression, and sexual dimorphism in three subdioecious populations of 'Wurmbea dioica' to gain insight into the stability of this sexual system. Sex ratios in the field were slightly male biased: 50% males, 43% females, and 7% hermaphrodites. Sex expression of females was constant for 3 yr under favorable glasshouse conditions. Male sex expression was labile, and 71% of males became hermaphrodites. Hermaphrodites remained unchanged, probably in response to the favorable conditions. In the field, estimates of femaleness of hermaphrodites tended toward maleness (Gp0.31). Females produced three times more seeds per plant than did hermaphrodites. This increased fecundity resulted from females producing more ovuliferous flowers per plant and more ovules per flower but fewer aborted seeds (21% vs. 51%). Hermaphrodites were self-compatible, and their lower seed set may reflect inbreeding depression and/or differences in allocation patterns compared with females. Males produced 1.3 times more pollen per flower that was 7% more viable than did hermaphrodites, indicating resource-based trade-offs between male and female functions when males produced functional pistils. Males produced flowers that were slightly larger in diameter than those of hermaphrodites, and flowers of both were 1.3 times larger than those of females. Males and hermaphrodites produced similar numbers of flowers, both producing more than females. Fecundity differences between females and hermaphrodites are consistent with models for the maintenance of constant females. Male sex lability may be advantageous if male fitness gain curves decelerate in response to increased resource availability and seed production augments plant fitness. This is consistent with resource-based threshold models of male gender modification.