Gender Specialization Across a Climatic Gradient: Experimental Comparison of Monoecious and Dioecious Ecballium

The ecological basis for breeding—system evolution was explored in Ecballium elaterium (L.) A. Rich. (Cucurbitaceae). Two subspecies are parapatric on the Iberian Peninsula: dioicum (dioecious) is found in southern, more arid regions and elaterium (monoecious) is found in northern, wetter regions. Six experimental gardens were set up in three sites that span the climatic gradient of the species' natural range in Spain in order to address two issue: (1) gender specialization in reproductive characters and life history, and (2) ecological specialization of the two subspecies across environments. Overall, the dioecious subspecies outperformed the monoecious subspecies. Monoecious plants, on average, were shorter—lived in all environments. Females of the dioecious subspecies produced similar numbers of female flowers, but more fruits and seeds, than did monoecious plants (cosexuals). The fruits of females contained more, but smaller, seeds than the fruits of cosexuals. Males always produced more male flowers than did cosexuals, disproportionately so with increased plant size (canopy volume); no such disproportionate increase in seed production was found in females. A geographic analysis of gender (proportion of female flowers) in the monoecious plants revealed a striking pattern: dimorphism with increased gender differentiation in drier sites, in contrast to monomorphism with respect to gender in wetter sites. Differences between the subspecies and between habitat types can be related directly to breeding—system differences. The higher overall performance of dioecious plants may be due to enhanced heterozygosity resulting from outcrossing enforced by dioecy. On the other hand, the gender differentiation of the monoecious plants in the drier sites may reflect gender allocation "decisions" enforced by limited resource availability.