CROSSING STUDIES IN CYPHOMANDRA (SOLANACEAE) AND THEIR SYSTEMATIC AND EVOLUTIONARY SIGNIFICANCE

A crossing program involving eight species of Cyphomandra was instituted to elucidate systematic relationships between the taxa and evolutionary mechanisms operating in the genus. The results show that gametophytic self-incompatibility is widespread in Cyphomandra. Pollen tubes were able to reach the ovules in nearly all interspecific crosses. Most of these crosses failed, however, indicating that postzygotic barriers are important in reproductive isolation. Only five of 44 interspecific combinations attempted have produced viable seeds. F₁ plants have been evaluated from three of the five hybrid combinations: C. betacea × acuminata, C. uniloba × betacea, and C. corymbiflora × diploconos. All three hybrid combinations show reduced fertility as compared to the parents, apparently caused in part by differences in chromosome structure between parent species. Crossing success was congruent with morphological similarity in C. betacea, C. acuminata, and C. uniloba, but not in C. corymbiflora and C. diploconos. External isolating mechanisms such as geography, ecology, phenology, and pollinator specificity may also operate to maintain species integrity in nature.