Two sides of the same coin? Transient hybridization in refugia and rapid postglacial ecological divergence ensures the evolutionary persistence of sister Nothofagus.

Glacial periods have been considered inhospitable environments that consisted of treeless vegetation at higher latitudes. The fossil record suggests many species survived the Last Glacial Maximum (LGM) within refugia, usually at lower latitudes. However, phylogeographic studies have given support to the existence of previously unknown high-latitude refugia not detected in the fossil record. Here we test the hypothesis that cold-tolerant trees of Patagonia survived cold periods in microclimatically favourable locales where hybridisation occurred between sister taxa. To study local presence through glacial periods in multiple refugia we used pollen records and genetic information (isozymes, microsatellites, and combined nuclear and chloroplast DNA sequences) of population pairs of Nothofagus antarctica and N. pumilio, that belong to the ancient subgenus Nothofagus which can potentially hybridize in nature, along their entire latitudinal range in Patagonia. Studied species share the N. dombeyi type pollen which was abundant >20% at the northern-most latitudinal bands (35-43°S), even during the LGM. Mid- and southern latitudinal records (44-55°S) yielded lower abundances of ~10% that increased after c. 15.0 cal. ka BP. Therefore, fossil pollen evidence suggests a long-lasting local presence of Nothofagus throughout glacial-interglacial cycles but mostly as small populations between 44-51°S. We found species-specific and shared genetic variants, the latter of which attained relatively high frequencies thus providing evidence of ancestral polymorphisms. Populations of each species were similarly diverse suggesting survival throughout the latitudinal range. Estimates of coalescent divergence times were broadly synchronous across latitudes suggesting that regional climates similarly affected populations and species that hybridized through climate cycles fostering local persistence.