Response of xerophytic plants to glacial cycles in southern South America

Background and aim: Quaternary glaciations strongly affected the distribution of species from arid and semi-arid environments, as temperature drops were accompanied by strong fluctuations in rainfall. In this study, we examined the response of xerophytic species to glacial cycles, determining the genetic patterns and climatic niche of Echinopsis chiloensis var. chiloensis, an endemic columnar cactus of arid and semi-arid regions of Chile.

Methods: We analysed 11 polymorphic microsatellites for 130 individuals from 13 populations distributed across the entire distribution of the species. We examined genetic diversity and structure, identified possible patterns of isolation by distance (IBD) and tested two competing population history scenarios using Approximate Bayesian Computation. The first scenario assumes a constant population size while the second includes a bottleneck in the southern population. The latter scenario assumed that the southernmost populations experienced a strong contraction during glaciation, followed by a postglacial expansion; by contrast, the area of the northernmost populations remained as a stable refugium. We also used ecological niche modelling (ENM) to evaluate the location and extension of suitable areas during the Last Glacial Maximum (LGM) and the mid-Holocene.

Key results: We found a decline in genetic diversity towards high latitudes and a significant IBD pattern that together with ENM predictions suggest that E. chiloensis var. chiloensis experienced range contraction northwards during wet-cold conditions of the LGM, followed by expansion during aridification of the mid-Holocene. In addition to IBD, we detected the presence of a strong barrier to gene flow at 32°30'S, which according to coalescence analysis occurred 44 kyr BP. The resulting genetic clusters differed in realized climatic niche, particularly in the variables related to precipitation.

Conclusions: Our results suggest that the cactus E. chiloensis var. chiloensis experienced range contraction and fragmentation during the wet-cold conditions of the LGM, which may have facilitated ecological differentiation between northern and southern populations, promoting incipient speciation.