Admixture facilitates adaptation from standing variation in the European aspen (Populus tremulaL.), a widespread forest tree

Abstract

Adaptation to new environments can start from new mutations or from standing variation already present in natural populations. Whether admixture constrains or facilitates adaptation from standing variation is largely unknown, especially in ecological keystone or foundation species. We examined patterns of neutral and adaptive population divergence in Populus tremula L., a widespread forest tree, using mapped molecular genetic markers. We detected the genetic signature of postglacial admixture between a Western and an Eastern lineage of P. tremula in Scandinavia, an area suspected to represent a zone of postglacial contact for many species of animals and plants. Stringent divergence‐based neutrality tests provided clear indications for locally varying selection at the European scale. Six of 12 polymorphisms under selection were located less than 1 kb away from the nearest gene predicted by the Populus trichocarpa genome sequence. Few of these loci exhibited a signature of ‘selective sweeps’ in diversity‐based tests, which is to be expected if adaptation occurs primarily from standing variation. In Scandinavia, admixture explained genomic patterns of ancestry and the nature of clinal variation and strength of selection for bud set, a phenological trait of great adaptive significance in temperate trees, measured in a common garden trial. Our data provide a hitherto missing direct link between past range shifts because of climatic oscillations, and levels of standing variation currently available for selection and adaptation in a terrestrial foundation species.

Genetic Differentiation, Clinal Variation and Phenotypic Associations With Growth Cessation Across the Populus tremula Photoperiodic Pathway

“…Second, Swedish populations of P. tremula show evidence for recent admixture between lineages originating from different glacial refugia (De Carvalho et al 2010). The clines we observe could therefore be a product of gene flow between two partly divergent populations (De Carvalho et al 2010), a view that is further supported by the presence of weak, but significant, isolation by distance we found in our study populations based on microsatellite data (Hall et al 2007). We do not, however, observe any isolation by distance in the SNP data presented here (Mantel's test, P ¼ 0.413).…”

Genomic Admixture Analysis in European Populus spp. Reveals Unexpected Patterns of Reproductive Isolation and Mating

“…Inbreeding coefficients (F IS ) were significantly positive, consistent with previously observed Wahlund effects arising from wide local sampling of the parental reference populations (De Carvalho et al 2010) and widespread departures from random mating due to assortative mating in hybrid zones (van Loo et al 2008).…”

“…These included 83 microsatellites, eight sequence-based markers, and two insertiondeletion polymorphisms (supporting information, Table S1). Seventy-six of the markers were developed from genomic libraries or directly from the P. trichocarpa genome assembly v.1 (http:/ /www.ornl.gov/sci/ipgc; van der Schoot et al 2000; Smulders et al 2001;Tuskan et al 2004), and 17 were developed from expressed sequence tags (ESTs) De Carvalho et al 2010). The markers were distributed across all 19 chromosomes of the Populus genome with an average of 5 6 1 (SE) markers per linkage group (Table S1).…”

Interspecific gene flow and ecological selection in a pine (Pinus sp.) contact zone

“…Natural selection can cause fixation of advantageous alleles that have a positive fitness effect and potential to speed up adaptation in new genetic background of hybrids (De Carvalho et al 2010;Martin et al 2006). Two loci in our hybrid dataset showed increased frequency towards alleles specific to P. sylvestris at calcium-dependent protein kinase (Pr4-5) and alleles specific to P. mugo at mys transcription factor (Pr4-10).…”