Department of Biosciences, University of Salzburg, Hellbrunnerstrasse 34, Salzburg, 5020, Austria.
Smithsonian Tropical Research Institute, Apartado, Balboa, Ancón, 0843-03092, Republic of Panama.
New Phytol. 2021 Aug;231(3):1236-1248. doi: 10.1111/nph.17437. Epub 2021 Jun 4.
Despite growing evidence that niche shifts are more common in flowering plants than previously thought, little is known of whether such shifts are promoted by changes in photosynthetic pathways. Here we combine the most complete phylogeny for epiphytic Malagasy Bulbophyllum orchids (c. 210 spp.) with climatic niche and carbon isotope ratios to infer the group's spatial-temporal history, and the role of strongly expressed crassulacean acid metabolism (CAM) in facilitating niche shifts and diversification. We find that most extant species still retain niche (Central Highland) and photosynthesis (C ) states as present in the single mid-Miocene (c. 12.70 million yr ago (Ma)) ancestor colonizing Madagascar. However, we also infer a major transition to CAM, linked to a late Miocene (c. 7.36 Ma) invasion of species from the sub-humid highland first into the island's humid eastern coastal, and then into the seasonally dry 'Northwest Sambirano' rainforests, yet without significant effect on diversification rates. These findings indicate that CAM in tropical epiphytes may be selectively advantageous even in high rainfall habitats, rather than presenting a mere adaptation to dry environments or epiphytism per se. Overall, our study qualifies CAM as an evolutionary 'gateway' trait that considerably widened the spatial-ecological amplitude of Madagascar's most species-rich orchid genus.
尽管越来越多的证据表明，生态位转移在开花植物中比以前认为的更为常见，但对于这种转移是否是由光合作用途径的变化所促进的，人们知之甚少。在这里，我们结合了最完整的马达加斯加附生 Bulbophyllum 兰花（约 210 种）的系统发育关系、气候生态位和碳同位素比值，以推断该类群的时空历史，以及强烈表达的景天酸代谢（CAM）在促进生态位转移和多样化方面的作用。我们发现，大多数现存物种仍然保留着在单一的中新世中期（约 1270 万年前）祖先中存在的生态位（中央高地）和光合作用（C ）状态，当时它们殖民了马达加斯加。然而，我们还推断出一种主要的向 CAM 的转变，这与晚中新世（约 736 万年前）来自次湿润高地的物种的入侵有关，这些物种首先进入岛屿潮湿的东部沿海地区，然后进入季节性干旱的“西北桑比瑞纳”雨林，但对多样化率没有显著影响。这些发现表明，CAM 在热带附生植物中可能具有选择性优势，即使在高降雨量的生境中也是如此，而不仅仅是对干燥环境或附生本身的适应。总的来说，我们的研究将 CAM 确定为一种进化的“门户”特征，它极大地拓宽了马达加斯加最具物种多样性的兰花属的空间生态幅度。
