Sexual mass allocation in species with inflorescences as pollination units: A comparison between Arum italicum and Arisaema (Araceae)

Sex allocation models predict that cosexuality is stabilized by high alloca tion to attractive structures in pollen-limited species or by high allocati on to shared structures that contribute to both genders. High investment in unilateral fixed costs favor the evolution of dioecy or gender change. Wit h these predictions in mind, I studied sexual mass allocation at flowering in the monoecious Arum italicum (Araceae) and compared it with information available for its sex labile relative Arisaema dracontium. In A. italicum, 68% of biomass was allocated to structures believed to be involved in polli nator attraction and capture. This allocation pattern contrasts with that o f Arisaema dracontium, in which 70% of biomass was allocated to scape, cons idered to be a unilateral fixed cost. The importance of attractive structur es in A. italicum was further supported by a disproportionate increased all ocation. in larger inflorescences, to the appendix (an attractive structure ) compared to fertile flowers. In addition, an increase in inflorescence ma ss involved a disproportionate increase in mass allocation to male, rather than female, flowers. This pattern also contrasts with a size-related gende r change from male to female in Arisaema species. These findings were consi stent with sex allocation model predictions and shed light on the evolution of sex liability in Arisaema species.