Phylogenetic analyses of Begonia sect. Coelocentrum and allied limestone species of China shed light on the evolution of Sino-Vietnamese karst flora

Based on our phylogenetic study, Begonia sect. Coelocentrum is recircumscribed and expanded to include other members of the Clade SVLB (sect. Diploclinium: B. cavaleriei, B. pulvinifera, and B. wangii; sect. Leprosae: B. cylindrica and B. leprosa; sect. Petermannia: B. sinofloribunda). Because species of Clade SVLB have strong niche conservatism to retain in their ancestral habitats in cave-like microhabitats and Begonia are generally poor dispersers prone to diversify allopatrically, we propose that extensive and continuous karstification of the Sino-Vietnamese limestone region facilitated by the onset of East Asian monsoon since the late Miocene has been the major driving force for species accumulation via geographic isolation in Clade SVLB. Morphologically species of Clade SVLB differ mainly in vegetative traits without apparent adaptive value, suggesting that limestone Begonia radiation is better characterized as non-adaptive, an underappreciated speciation mode crucial for rapid species accumulations in organisms of low vagility and strong niche conservatism.