Historical and ecological divergence among populations of Monttea chilensis (Plantaginaceae), an endemic endangered shrub bordering the Atacama Desert, Chile

Abstract

The coastal deserts of northern Chile show an important latitudinal gradient of aridity with more arid areas to the north of the Atacama Desert than to the south. Several plant species have disjunct distributions that correspond with the extremes of this latitudinal gradient. In this study, using genetic (chloroplast and nuclear DNA), morphological (vegetative and floral traits of various kinds) and climatic and topographic information, we explored ecological and historical events that have putatively shaped patterns of variation among Monttea chilensis populations ? a species that shows this disjunct distribution. Through phylogeographic and phylogenetic analyses, two divergent lineages were identified located at the latitudinal extremes. The lineage located north (NG) of the Atacama Desert showed more genetic diversity and better-resolved phylogeographic structure than the southern (SG) lineage. Considerable morphological variation across the geographical range corresponds with these genetic groups. We observed contrasting relationships between floral and vegetative traits: populations from the most arid region (NG) possessed larger flowers, but smaller vegetative values, and vice versa. Niche modelling and multivariate analyses, including environmental data, revealed different environmental requirements for each lineage. NG plants occur in regions with warmer and drier climatic conditions and at higher altitudes, while SG populations inhabit colder and more humid environments and lower altitudes. The evolutionary history of M. chilensis exhibits a phylogeographical footprint consistent with past fragmentation and allopatric differentiation, where the hyper-arid zone formed by the Atacama Desert clearly acted as an important gene flow barrier. This barrier has led to considerable differentiation in morphology and ecology, resulting in two ecotypes or geographical races, suggesting incipient speciation promoted by local adaptation and geographical isolation.