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 Madagascan Bulbophyllum. 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 Madagascan Bulbophyllum (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.

中文翻译：

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Madagascan Bulbophyllumspecies (Orchidaceae) 中反复出现的多态交配型变异体现了热带兰花自花授粉的高发生率

从异交到自体受精的转变是被子植物最常见的进化变化之一。兰花科体现了这种进化趋势，但由于普遍缺乏对兰花类群自花授粉的大规模调查，（亚）热带兰花自花授粉的发生率和模式仍然知之甚少。在本研究中，我们评估了马达加斯加 Bulbophyllum 主要单系群的物种内部和物种之间的自花授粉频率和模式。通过在温室和田间进行套袋实验，并辅以绞股蓝的详细花卉微形态研究，研究了自主坐果能力。我们的调查包括 393 个种质，代表至少 78 个物种，因而约占马达加斯加地区该属物种多样性的 37%。我们的研究表明，交配类型与绞股蓝结构直接相关，最常涉及称为“喙”的物理屏障的存在或不存在。作为一个新的和意外的发现，我们确定了马达加斯加 Bulbophyllum 单一谱系的 8 个物种（称为“进化枝 C”），其中自花授粉变种（selfers），要么缺乏喙，要么（很少）拥有柱头喙，co - 与它们依赖传粉者的同种（outcrossers）一起存在。我们假设通过 rostellum 流产的自花授粉具有简单的遗传基础，并且可能通过 rostellum 发育时间（异时性）的细微变化快速和反复地进化。因此，进化枝 C 的物种可能对自花授粉具有内在的遗传和发育不稳定性，允许在有利于繁殖保证的环境，也许是人类干扰的条件下进行快速进化反应。总的来说，这些发现应该会激发对热带兰花交配类型变异的发生、进化和维持的进一步研究，以及它们如何适应不断变化的环境条件。