LAUSR

BACKGROUND AND AIMS The genus Buxus has high levels of endemism in the Caribbean flora with c. 50 taxa. In Cuba, 82% grow on ultramafic substrates and 59% are nickel (Ni)-accumulators or Ni-hyperaccumulators. Hence it is an ideal model group to study if this diversification could be related to the adaptation to ultramafic substrates and to the Ni-hyperaccumulation. METHODS We generated a well-resolved molecular phylogeny, including nearly all of the Neotropical and Caribbean Buxus taxa. To obtain robust divergence times we tested for the effect of different calibration scenarios, and we reconstructed ancestral areas and ancestral character states. Phylogenetic trees were examined for trait-independent shifts in diversification rates and we used multi-state models to test for state-dependent speciation and extinction rates. KEY RESULTS We found a Caribbean Buxus clade with Mexican ancestors, encompassing three major subclades, which started to radiate during the middle Miocene (13.25 Mya). Other Caribbean islands and northern South America were reached from c. 3 Mya onwards. CONCLUSIONS An evolutionary scenario is evident in which Buxus plants able to grow on ultramafic substrates by exaptation became ultramafic substrate endemics and stepwise evolved from Ni tolerance over Ni-accumulation to Ni-hyperaccumulation, which has triggered species diversification of Buxus in Cuba. Storms could have contributed to Cuba acting as a species pump and to reach other Caribbean islands and northern South America.