![]() Assessing large-scale relationships of populations should elucidate the importance of range expansions and contractions in producing the current distribution patterns of species, as well as generate hypotheses regarding the genetic structure and diversity of temperate tree lineages affected by glaciation in the latter part of the Cenozoic.Īcer saccharum (sugar maple) is a widespread temperate tree species. Limited geographical sampling reduces the capacity to reconstruct historical refugia and identify the spatial location of genetic breaks in temperate tree species at a continental level. Periods of connectivity and disjunction among North American and Central American refugial areas and their consequences on the genetic structure of the temperate tree populations are not known for most species. These fluctuations may have involved expansions into low elevation areas at low latitudes during glacial episodes, and contractions to high elevation refuges during warmer interglacial periods. These isolated populations may have originated by ancient vicariant events or through range fluctuations. Additionally, the distributions of many North American temperate species include disjunct populations in cloud forests in Mexico and Central America. mexicana) also show genetic differentiation related to the history of migration and isolation during glacial and interglacial periods. Populations of temperate tree species in the subtropics (e.g., Pinus chiapensis, Picea chihuahuana, Pseudotsuga menziesii, Fagus grandifolia var. saccharinum, Quercus rubra) that survived in multiple refugia closer to the ice margin. Furthermore, there is evidence of the impact of the Quaternary glaciations on the population genetic structure of species (e.g., Acer rubrum, A. For instance, patterns of genetic diversity in temperate North American trees (e.g., Carya illinoinensis, Liquidambar styraciflua, Fagus grandifolia) include higher diversity in southern, never-glaciated areas, and lower diversity in northern areas where glaciated landmasses existed. Studies indicate different genetic responses to range contraction and expansion. Repeated glacial and interglacial periods promoted range shifts of a number of tree species during late Pliocene and the Quaternary. Geological events have affected the distributions and levels of gene flow among populations of North American tree species. ![]() The time to the most common ancestor values revealed that populations from the Midwest and Northeast USA represented different haplotype lineages, indicating major divergence of haplotypes lineages before the Last Glacial Maximum and suggesting the existence of multiple glacial refugia. Past connectivity among populations in the southeastern USA and eastern Mexico and Guatemala possible occurred through gene flow during the Pleistocene. The divergence of western Mexican populations from the rest of the sugar maples likely resulted from orographic and volcanic barriers to gene flow. Current southern Mexican and Guatemalan populations have reduced population sizes, genetic bottlenecks and tend toward homozygosity, as indicated using nuclear and chloroplast markers. Our data also suggested that multiple Pleistocene refugia (tropics-southeastern USA, midwestern, and northeastern USA), but not western Mexico (Jalisco), contributed to post-glacial northward expansion of ranges. The most frequent and widespread haplotype occurred in half of the sites (Guatemala, eastern Mexico, southeastern USA, and Ohio). Splits during the Pleistocene separated the rest of the phylogroups. The time of the most recent common ancestor of the western Mexico haplotype lineage was dated to the Pliocene (5.9 Ma, 95 % HPD: 4.3–7.3 Ma). ![]() Nuclear and chloroplast data indicated that populations in midwestern USA and western Mexico were highly differentiated from populations in the rest of the sites. We examined genetic subdivisions, explored the locations of ancestral haplotypes, analyzed genetic data to explore the presence of a single or multiple glacial refugia, and tested whether genetic lineages are temporally consistent with a Pleistocene or older divergence. The studied populations span a geographic range from Maine, USA (46°N), to El Progreso, Guatemala (15°N). skutchii) using nuclear and chloroplast data. saccharum) and its only subspecies in tropical America ( Acer saccharum subsp. We determined the regional structuring of genetic variation of sugar maple ( Acer saccharum subsp. The biogeographical history of temperate vegetation that spans large ranges of latitude is complex, involving multiple latitudinal shifts that might have occurred via different migration routes. Geological events in the latter Cenozoic have influenced the distribution, abundance and genetic structure of tree populations in temperate and tropical North America. ![]()
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