A molecular biogeography of the New World cypresses (Callitropsis, Hesperocyparis; Cupressaceae)

Abstract

Previous studies of phylogenetic relationships among cypresses have recovered separate Old and New World lineages, resolved a Southeast Asian species sister to the New World clade, and split the New World group into two principal lineages (i.e., the Macrocarpa and Arizonica clades) having fundamentally different geographic distributions. Collectively, these observations suggested a number of intriguing hypotheses regarding the origin and biogeographic history of the New World cypresses (NWC). In this study, we use DNA sequence data to examine the historical biogeography of NWC. Divergence times are estimated in BEAST using fossil-calibrated minimum age constraints, and a spatial history of the group reconstructed using Statistical Dispersal-Vicariance Analysis. Results presented here suggest ancestral NWC colonized the New World from Asia, perhaps by a more widespread trans-Beringian ancestor, in the late Cretaceous or early Cenozoic. Strong positive correlations between species age and geographic location (i.e., latitude and longitude) suggest current distributions were influenced by directional migration (northwest to southeast) as climates cooled and became increasingly arid in the latter half of the Cenozoic. Although our data support a middle Eocene (45 Mya) origin for NWC, nearly all species are apparently no more than late Miocene (6 Mya) in age, and net diversification rates in the group are among the highest reported to date for gymnosperms. Key coastal to interior migrations underlie the fundamentally different biogeographies of the Macrocarpa and Arizonica clades, with the Transverse Ranges of southern California being a barrier to north–south migration of certain species from these two lineages.