Variation in self-incompatibility and interspecific compatibility in a lineage of the mostly self-compatible genus Bulbophyllum (B. sect. Micranthae–Orchidaceae)

A vast majority of Orchidaceae species are self-compatible, and many present mechanical or temporal barriers that prevent self-pollination. Self-incompatibility is present in some orchid groups with pollinated by insects that pay long visits to a flower, in addition to visiting many flowers of the same plant, as observed in Diptera. The genus Bulbophyllum, despite being pollinated by flies, predominantly comprises self-compatible species, and some of them present pre-pollination selfing barriers. However, there are some reports of Bulbophyllum sect. Micranthae presenting self-incompatibility. This section comprises 12 species, some of which show complex taxonomy possibly due by introgression. Here, we analyzed the morphology and development of pollen tube following intra- and interspecific experimental pollinations in eight species of Bulbophyllum sect. Micranthae to understand their reproductive system and isolation. All species in the section are self-incompatible, except for the species sister to the remaining section, Bulbophyllum mentosum, and need vectors for pollination. Epifluorescence microscopy revealed that the major site of incompatibility is present at the beginning of the column while in some others the pollen grains did not germinate. However, in some species with partial self-incompatibility, pollen tubes turned tortuous and anomalous upon reaching the ovary. We believe that this self-incompatibility reaction in the ovary is an extended gametophytic self-incompatibility reaction, as reported in other myophilous orchids, but diallelic crossing experiments are necessary to confirm our hypotheses. In the studied species of B. sect. Micranthae, reproductive isolation is mainly due to pre-pollination barriers, and fruit set is possible in most interspecific crossings, except for those involving the basal most species B. mentosum. [ABSTRACT FROM AUTHOR]