Fertility of exceptional bisexual individuals in four dioecious plant species

Exceptional, very rare, bisexual (hermaphroditic or monoecious) individuals have been detected in strictly dioecious populations of Rhamnus alaternus, R. palaestinus, Bryonia syriaca and Populus euphratica (a single individual in each species). While ordinary male plants in each species exhibited fully normal pollen, and female individuals set a considerable amount of fruit, the exceptional bisexual plants showed a reduction in male or female fertility, or both. This inability of the rare bisexual individuals to fully invest in the two genders suggests that trade-offs between male and female functions possibly play a major role in the breeding systems of these dioecious species, and probably implicates resource allocation (among other factors) as a selective force in the evolution of dioecy. Received: 5 April 1999 / Revision accepted: 30 June 1999