Molecular phylogenetics and floral evolution of the Cirrhopetalum alliance (Bulbophyllum, Orchidaceae): Evolutionary transitions and phylogenetic signal variation

Character mapping reveals the emended Cirrhopetalum alliance clade (CAC) is well supported by a set of synapomorphies, viz.: a subumbellate inflorescence, lateral sepals basally twisted with connate upper margins (inset photos: a & c), whereas significant floral character transitions are revealed in Subclade D4, viz.: a compressed (or elongated) racemose inflorescence with flowers in which the lateral sepals are free and are similar in shape and size to the dorsal sepal (inset photos: b & d). [Display omitted] •Robust molecular phylogeny of Cirrhopetalum alliance (Bulbophyllum, Orchidaceae).•The emended Cirrhopetalum alliance is a well-supported monophyletic group.•Extensive non-monophyly of sections revealed within the Cirrhopetalum alliance.•Inclusion of sect. Desmosanthes highlights significant evolutionary transitions.•Contrasting evolution of lateral sepals and labellum linked to functional roles in pollination. The Cirrhopetalum alliance is a loosely circumscribed species-rich group within the mega-diverse genus Bulbophyllum (Orchidaceae). The monophyletic status of the alliance has been challenged by previous studies, although established sectional classifications have yet to be tested in a phylogenetic context. We used maximum likelihood and Bayesian analyses of DNA sequence data (cpDNA: matK and psbA-trnH; nrDNA: ITS and Xdh; 3509 aligned characters; 117 taxa), including all sections putatively associated with the Cirrhopetalum alliance, to reconstruct the phylogeny. We mapped 11 selected categorical floral characters onto the phylogeny to identify synapomorphies and assess potential evolutionary transitions across major clades. Our results unequivocally support the recognition of an amended Cirrhopetalum alliance as a well-supported monophyletic group characterized by clear synapomorphies, following the inclusion of sect. Desmosanthes and the exclusion of five putative Cirrhopetalum-allied sections. Most sections within the Cirrhopetalum alliance are demonstrated to be polyphyletic or paraphyletic, necessitating a new sectional classification. The inclusion of sect. Desmosanthes revolutionizes our understanding of the alliance, with significant evolutionary transitions in floral characters detected. We further investigated six continuously variable characters of the sepals and labellum, and detect phylogenetic conservatism in labellum width and the evolutionary lability of lateral sepal length, which can partly be explained by the different funct