Castela (Simaroubaceae), an impressive New World radiation of thorny shrubs destined for edaphically dry habitats

Castela Turpin is composed of 16 known species and is found throughout edaphically dry zones across the American continents from the Sonoran Desert to the southern South American Chaco biogeographic region and Caribbean seasonally dry tropical forests (SDTF). Castela is most diverse in the SDTF of the Greater Antilles, with seven of the 16 known species occurring there. We reconstructed the phylogeny of Castela sampling 15 of the 16 known species using a novel plastome dataset. Our phylogeny was used to test the biogeographic history and evolution of morphological characters across the clade. Castela was composed of four major subclades: the Holacantha, Depressa, Caribbean, and Chaco clades. Castela most likely originated in the Mojave/Sonoran Desert/Baja California region and then moved into the Chihuahuan Desert, Chaco, Greater and Lesser Antilles and northern South America, suggesting multiple long-distance dispersal events. Taxa occurring in arid areas, such as the Sonoran and Chihuahuan deserts and parts of the Caribbean SDTF, have a tendency to quickly lose or drastically reduce the size of their leaves, and most photosynthesis appears to be taken over by photosynthetic stems. The Holacantha clade exhibits the plesiomorphic yellow-green flowers so common in much of the rest of the family, while Castela s.s. mostly has derived red/rose-colored flowers. The putative Miocene origin of Castela and subsequent diversification is likely tied to the expansion of arid lands in the Americas; however, speciation within those clades may have been driven by peripheral isolate speciation and vicariance through habitat fragmentation resulting from more recent climatic change.