Origin and evolution of endemic Galápagos Varronia species (Cordiaceae)

Four endemic Varronia species (Cordiaceae) occupy the Galápagos archipelago. Three comprise the V. leucophlyctis complex (V. anderssonii, V. leucophlyctis, V. scouleri), whose species' limits are not well defined but that is morphologically distinct from the fourth endemic species, V. revoluta. Sequence data from the nuclear rDNA ITS region and the cpDNA ndhF gene were gathered from 49 accessions of Varronia from five Galápagos islands in order to test the evolutionary relationships of endemic Varronia species, determine the number of immigration events to the islands and estimate their age of origin. All endemic species nest within the clade of species belonging to Varronia, which is an entirely American genus. We find little evidence of phylogenetic structuring of the V. leucophlyctis complex but divergent phylogenetic signals from nuclear and chloroplast genomes regarding its relationship to V. revoluta. Results are consistent with a hybridization event involving ancestral Galapagean lineages, with chloroplast and nuclear data suggesting one or two dispersal events from the Americas to the Galápagos, respectively. Fossil-based divergence time estimates indicate endemic species diverged from American continental species as early as 4.5 Myr ago and radiated 1.12 Myr, which coincides with ages of exposed and subsided Galápagos islands.