Diversification of Eulychnia in the Atacama Desert

The cactus family (Cactaceae) is a major lineage of succulent plants that is thought to have diversified during the Miocene expansion of New World arid environments. In southern South America, the Bolivian and northern Argentinian South Central Andes as well as the Atacama and Peruvian deserts are considered as centers of diversity for Cactaceae. The Atacama and Peruvian deserts are among the driest places on earth, yet a surprisingly high number of plant species and a high percentage of endemic lineages have been recorded from these regions. The genus Eulychnia has eight currently recognized species and ca. nine synonyms and is endemic to the Atacama and Peruvian deserts. In the present study we investigate the phylogeny of this group based on material from the type localities of all published names in order to shed new light on species delimitation and biogeographic history. We employ a multi-step approach including a family-wide Bayesian molecular clock dating based on plastid sequence data in order to estimate the age of the genus and date the split from its sister genus Austrocactus. We present a comprehensive genotyping-by-sequencing (GBS) dataset analyzed with maximum likelihood and coalescent based phylogenetic reconstructions, using the family-wide age estimate as a secondary calibration to date the GBS phylogeny. We also infer ancestral ranges, employing the dispersal extinction cladogenesis approach. Our GBS phylogeny of Eulychnia is fully resolved with high support values nearly throughout the phylogeny. Our data indicate that the split from Austrocactus took place in the late Miocene, with a subsequent diversification of Eulychnia during the early Quaternary, coinciding with the onset of hyper aridity in the Atacama and Peruvian deserts. Three lineages are retrieved: Eulychnia ritteri from Peru is sister to all Chilean species, which in turn fall into two sister clades of three and four species respectively. Diversification in the Chilean clades started in the middle Pleistocene.