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Aim: Long-distance dispersal (LDD) plays an important role in shaping the distribution of global biodiversity. Polyploidy could favour invasion and thereby facilitate LDD. However, how and to what extent polyploidy interacts with LDD remain unclear. Here, we test the putative role of polyploidy in the global dispersal of a cosmopolitan genus Rorippa. Location: Global. Time Period: Late Miocene to present. Major Taxa Studied: Rorippa Scop., Brassicaceae. Methods: We traced the biogeographical and speciation history for 17 diploids and 41 polyploids of Rorippa using variation from plastid genomes and multiple nuclear loci. The ploidy role in dispersal rate difference was demonstrated using trait-dependent biogeographical modelling. Results: LDD shaped the amphitropical disjunction of Rorippa, during which polyploids showed higher dispersal rates than those of diploids, with 5.6× increase under the best-fitted model. Five diploids and 21 polyploids were identified as products of transoceanic speciation events. Polyploidy-involved LDD was more common in terms of polyploidization following LDD than those preceding LDD. Main Conclusions: We demonstrate that polyploidy would be not only a driver but also a responder of LDD in Rorippa, highlighting a synergistic relationship between them. Our results provide a framework to uncover the biogeographical consequences of polyploidization and the joint roles of polyploidy and LDD in shaping the distribution of biodiversity.

1. Epiphytic communities offer an original framework to disentangle the contributions of environmental filters, biotic interactions and dispersal limitations to community structure at fine spatial scales. We determine here whether variations in light, microclimatic conditions and host tree size affect the variation in species composition and phylogenetic structure of epiphytic bryophyte communities, and hence, assess the contribution of environmental filtering, phylogenetic constraints and competition to community assembly. 2. A canopy crane giving access to 1.1 ha of tropical rainforest in Yunnan (China) was employed to record hourly light and microclimatic conditions from 54 dataloggers and epiphytic bryophyte communities from 408 plots. Generalized Dissimilarity Modelling was implemented to analyse the relationship between taxonomic and phylogenetic turnover among epiphytic communities, host-tree characteristics and microclimatic variation. 3. Within-tree vertical turnover of bryophyte communities was significantly about 30% higher than horizontal turnover among-trees. Thus, the sharp vertical variations in microclimatic conditions from tree base to canopy are more important than differences in age, reflecting the likelihood of colonization, area, and habitat conditions between young and old trees, in shaping the composition of epiphytic bryophyte communities. 4. Our models, to which microclimatic factors contributed most (83–98%), accounted for 33% and 18% of the variation in vertical turnover in mosses and liverworts, respectively. Phylogenetic turnover shifted from significantly negative or non-significant within communities to significantly positive among communities, and was slightly, but significantly, correlated with microclimatic variation. These patterns highlight the crucial role of microclimates in determining the composition and phylogenetic structure of epiphytic communities. 5. Synthesis. The mostly non-significant phylogenetic turnover observed wit