title: Recurrent polymorphic mating type variation in MadagascanBulbophyllumspecies (Orchidaceae) exemplifies a high incidence of auto-pollination in tropical orchids.
abstract: The transition from outcrossing to self-fertilization is one of the most common evolutionary changes in angiosperms. The orchid family exemplifies this evolutionary trend but, because of a general lack of large-scale surveys on auto-pollination in orchid taxa, the incidence and modes of auto-pollination among (sub)tropical orchids remain poorly known. In the present study, we assessed the frequency and mode of auto-pollination within and among species of a largely monophyletic group of MadagascanBulbophyllum. The capacity for autonomous fruit set was investigated by bagging experiments in the greenhouse and the field, complemented with detailed floral micromorphological studies of the gynostemium. Our survey comprises 393 accessions, representing at least 78 species, and thus approximately 37% of the species diversity of the genus in the Madagascan region. Our studies revealed that mating type is directly related to gynostemium structure, most often involving the presence or absence of a physical barrier termed 'rostellum'. As a novel and unexpected finding, we identified eight species of a single lineage of MadagascanBulbophyllum(termed 'clade C'), in which auto-pollinating morphs (selfers), either lacking a rostellum or (rarely) possessing a stigmatic rostellum, co-exist with their pollinator-dependent conspecifics (outcrossers). We hypothesize that auto-pollination via rostellum abortion has a simple genetic basis, and probably evolved rapidly and recurrently by subtle changes in the timing of rostellum development (heterochrony). Thus, species of clade C may have an intrinsic genetic and developmental lability toward auto-pollination, allowing rapid evolutionary response under environmental, perhaps human-disturbed conditions favouring reproductive assurance. Overall, these findings should stimulate further research on the incidence, evolution, and maintenance of mating type variation in tropical orchids, as well as how they adapt(ed) to changing environmental conditions.
authors.affiliation: Department of Organismic Biology, University of Salzburg A-5020, Salzburg, Austria.
authors.forename: Alexander
authors.initials: A
authors.lastname: Gamisch
authors.affiliation: Kadoorie Farm and Botanic Garden Corporation Lam Kam Road, Tai Po, N.T., Hong Kong SAR.
authors.forename: Gunter A
authors.initials: GA
authors.lastname: Fischer
authors.affiliation: Department of Organismic Biology, University of Salzburg A-5020, Salzburg, Austria.
authors.forename: Hans Peter
authors.initials: HP
authors.lastname: Comes
keywords: autogamy; breeding system; evolution; floral morphology; light microscopy; polymorphism; scanning electron microscopy
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