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 allocation to attractive structures in pollen-limited species or by high allocation to shared structures that contribute to both genders. High investment in unilateral fixed costs favor the evolution of dioecy or gender change. With 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 pollinator attraction and capture. This allocation pattern contrasts with that of Arisaema dracontium, in which 70% of biomass was allocated to scape, considered to be a unilateral fixed cost. The importance of attractive structures in A. italicum was further supported by a disproportionate increased allocation, in larger inflorescences, to the appendix (an attractive structure) compared to fertile flowers. In addition, an increase in inflorescence mass involved a disproportionate increase in mass allocation to male, rather than female, flowers. This pattern also contrasts with a size-related gender change from male to female in Arisaema species. These findings were consistent with sex allocation model predictions and shed light on the evolution of sex lability in Arisaema species.