Reproductive systems and crossing potential in three species of Bulbophyllum (Orchidaceae) occurring in Brazilian 'campo rupestre' vegetation

Experimental self-, cross- and interspecific pollinations were carried out in Bulbophyllum weddellii, B. involutum and B. ipanemense (Orchidaceae). The last two species are closely related, but B. weddellii is somewhat more distant. The three species are self-compatible, with pollination by a vector necessary for fruit formation. In all crossing types, high rates of fruit abortion and formation of seeds without embryos were observed, particularly in interspecific crosses that involved B. weddellii. Crosses between the more closely related B. involutum and B. ipanemense produced fruit formation rates and seed viability similar to those obtained in intraspecific crosses. These results agree with current hypotheses that suggest that interspecific crossing rates reflect phylogenetic proximity. Examination of pollen tube growth and aborted fruits suggests that a series of factors may be involved in the high abortion rate for fruits and large proportion of seeds without embryos. The low interspecific fertility noted for B. weddellii and B. involutum is important in the maintenance of isolation between these species that are sympatric, flower in the same period and share the same pollinators.