Reproductive biology and functional aspects of the floral morphology of Rauvolfia sellowii Mull. Arg. (Apocynaceae; Rauvolfioideae) - a report of dioecy in Apocynaceae
Koch, Ingrid; Bittrich, Volker; Kinoshita, Luiza; Sumiko,
published: Aug 5, 2002
During studies on the reproductive biology of Rauvolfia sellowii Müll. Arg. the occurrence of functional dioecy was verified. This is the first case of dioecy in Apocynaceae s. str. demonstrated unequivocally. The occurrence of gynodioecy in R. vomitoria Afzel. is also demonstrated and its occurrence in Carissa grandiflora (E.Meyer) A.DC. confirmed. Our data reveal another case of a taxon described as having hermaphroditic flowers that has in fact functionally dioecious flowers. Behaviour of the pollinators and small populations might have favoured the development of dioecy in R. sellowii to avoid endogamic depression. The nonfunctional flower organs are particularly interesting: male flowers show all female structures, including well-developed ovules. Only callose deposition in the ovules and the absence of fruit development reveal their nature. Female flowers have empty anthers. Such organs still play an important role for the reproductive mechanism and we believe that this is responsible for their persistence. Although the possibility that the evolutionary time elapsed since the development of dioecy was insufficient for their suppression cannot be completely ruled out. Although it has been suggested that stigmatic receptivity is restricted to a region below the circular scraper structure at the base of the style head, pollen tube growth demonstrated that the median secretory part is also receptive in these cases. Secondary sexual characters and mechanisms of the pollination process are also discussed. Earlier studies dealing with the functional importance of floral structures during pollination are briefly reviewed.