Phylogeography and climatic niche evolution in live oaks (Quercus series Virentes) from the tropics to the temperate zone

Aim: We investigated the phylogeography, geographic variation in leaf morphology, freezingtolerance and climatic niches of two widespread evergreen sister oak species (Quercus) in theseries Virentes to gain insight into 1) the historical biogeographic explanations for populationgenetic structure and leaf morphological variation, and 2) the influence of genetically-basedvariation in freezing tolerance within and between species on climatic range limits and geneflow, and 3) to examine the evidence for contrasting impacts of Pleistocene glacial cycles ongenetic structure and diversity in tropical and temperate regions.Location: Southeastern U.S., Mexico and Central AmericaMethods: Nuclear microsatellites, non-recombining nuclear sequences within ITS and NIA-i3and chloroplast DNA sequences were obtained from samples collected throughout the range oftwo sister lineages of live oaks, represented by Quercus virginiana in the temperate zone andQuercus oleoides in the tropics. Divergence times were estimated for the two major geographicand genetic breaks. Differentiation in leaf morphology, analyzed from field specimens, wascompared to the molecular data. Freezing sensitivities of Q. virginiana and Q. oleoidespopulations were assessed in a common garden under controlled tropical and temperate climatesand compared to minimum temperature climatic niches for each species.Results: The geographic break between Q. virginiana and Q. oleoides was associated withstrong genetic differentiation of possible Miocene origin and with differentiation in freezingsensitivity, climatic envelopes and leaf morphology. A second important geographic and geneticbreak within Q. oleoides between Costa Rica and the rest of Central America showed aPleistocene divergence time and no differentiation in leaf morphology. Population geneticdifferentiation was greater but genetic diversity was lower within temperate Q. virginianacompared to tropical Q. oleoides, and genetic breaks largely corresponded to breaks in leafmorphology.Main Conclusions: Two major breaks, one between Mexico and the U.S. at the speciesboundary, and a more recent one within Q. oleoides between Honduras and Costa Rica implicateclimatic changes as potential causes. The latter break may be associated with formation of theCordillera Guanacaste followed by seasonal changes in precipitation. In the former case, an “outof the tropics” scenario is hypothesized, in which acquisition of freezing tolerance in Q.virginiana permitted colonization of and expansion in the temperate zone while differences inclimatic tolerances between the species limited secondary contact. More pronounced Pleistocenechanges in climate and sea level in the southeastern U.S. relative to Central America may explainthe greater population differentiation within temperate Q. virginiana and greater genetic diversityin tropical Q. oleoides. These patterns are predicted to hold for other taxa that span temperateand tropical zones of North and Middle America