Phylogeographic Analyses and Paleodistribution Modeling Indicate Pleistocene In Situ Survival of Hordeum Species (Poaceae) in Southern Patagonia without Genetic or Spatial Restriction


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Abstract

Although many phylogeographic studies have been conducted to analyze the impact of the ice age on species history of Northern Hemisphere mountain plants, such studies are nearly absent for plants of the Southern Hemisphere, particularly for lowland vegetation units. These species should have been primarily influenced by climate cooling and changes in precipitation regime instead of glaciers covering their distribution areas. It is thought that New World lowland species generally evaded climate changes by equatorial migration during Pleistocene cold cycles and recolonized their habitats at higher latitudes when climate warmed up again. In contrast to Eurasia, latitudinal orientation of the major mountain ranges in the Americas made these migrations easily possible.In the huge steppe of the Patagonian plains and adjacent Andes of southern South America thrives a group of three sympatrically distributed diploid species of the barley genus Hordeum, which originated during the last 1.3 million years (My) from a common progenitor. To get insights into the speciation mode of the taxa and to test the hypothesis of longitudinal migration of steppe vegetation during the Pleistocene, we conducted population genetic and phylogeographic analyses based on sequences of the chloroplast trnL-F region from 922 individuals.We found a high number of chloroplast haplotypes shared among species, which indicate speciation through vicariance events. Analysis of the distribution of genetic diversity within and among species inferred an origin of Hordeum comosum in the Central Argentine Andes, whereas Hordeum patagonicum and Hordeum pubiflorum originated in southern Patagonia. The extant occurrence of H. comosum in southern Patagonia and H. pubiflorum northward along the Argentine Andes was caused by reciprocal migration after the origin of the species.Surprisingly, molecular data provided no evidence for range shifts toward the north during the last glacial maximum and recolonization of southerly habitats afterward, but indicated in situ survival of large populations of Hordeum species within their extant distribution ranges even in southernmost Patagonia and Tierra del Fuego. Ecoclimatic niche modeling used to reconstruct the potential paleodistribution areas of the species during the last glacial maximum shows that climate conditions were sufficient for the species to survive Pleistocene cold cycles in Patagonia without significant geographic restrictions. Molecular data together with ecological niche modeling indicate stable geographic distribution areas in two of the three species for at least the Holocene. As the Hordeum species are characteristic taxa of different steppe habitats, we speculate that the Patagonian steppe might be an old vegetation unit occurring for up to 4.5 My in southern South America.

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