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.

Genetic Patterns of Myrceugenia correifolia, a Rare Species of Fog-Dependent Forests of Mediterranean Chile: Is It a Climatic Relict?

Rare species frequently occur in areas with microclimatic conditions that are atypical for their regions, but that were more common in the past, and that probably have operated as climatic refugia for a long time. Myrceugenia correifolia is a rare arboreal species that grows in deep canyons and hilltops of the Coast Range of north-central Chile between 30° and 35°S. In the northern edge of its distribution M. correifolia grows in small patches of fog-dependent forest surrounding by xeric vegetation. These forest formations are thought to be remnants of an ancient and continuous rainforest that according to some authors became fragmented during aridization of the Neogene (Neogene relict) and to others during warm-dry cycles of the Pleistocene (glacial relicts). Here we asked whether the northernmost populations of M. correifolia are Neogene relicts, glacial relicts, or the result of a recent northward colonization. To answer this question we examined genetic diversity and population divergence of M. correifolia using microsatellite markers, tested various competing population history scenarios with an approximate Bayesian computation (ABC) method, and complemented these data with ecological niche modeling (ENM). We detected three genetic clusters with a distinctive latitudinal pattern (north, center, and south) and high levels of differentiation (FST = 0.36). Demographic inference supported an admixture event 31 kya between two populations that diverged from an ancient population 139 kya. The admixture time coincides with the beginning of a period of wet conditions in north-central Chile that extended from 33 to 19 kya and was preceded by dry and cold conditions. These results suggest that increased precipitation during glacial periods triggered northward expansion of the range of M. correifolia, with subsequent admixture between populations that remained separated during interglacial periods. Accordingly, ENM models showed that suitable habitats for M. correifolia in north-central Chile were larger and less fragmented during the Last Glacial Maximum than at present, suggesting that northernmost populations of this species are glacial relicts.