Joint inferences from cytoplasmic DNA and fossil data provide evidence for glacial vicariance and contrasted post‐glacial dynamics in tamarack, a transcontinental conifer

Abstract

Aim Tamarack (Larix laricina) is an early-successional transcontinental boreal conifer occurring within the spruce-fir dominated forest. The aim was to infer the species biogeographical history and to assess the putative genetic imprint left by interspecific competition during post-glacial migration, using cytoplasmic DNA and fossil data.Location Forty-five locations were sampled across the transcontinental range spanning the North American boreal forest.Methods A total of 621 trees were scanned for mitochondrial and chloroplast DNA polymorphisms to reveal geographical patterns of genetic diversity, differentiation, and population structure throughout the species range. Published pollen records were analysed to assess the chronology of post-glacial colonization of Larix sp. relative to more competitive tree taxa, Picea sp. and Abies sp..Results Genotyping resulted in two mitotypes (one locus) and 24 chlorotypes (three cpSSR loci). Bayesian assignment test based on cpDNA data detected three groups: eastern North America, western North America and Alaska. CpDNA population differentiation was higher in the western part of the range relative to the eastern part. Post-glacial colonization chronology derived from fossil data indicated that Larix colonized western North America at least 4000 years after Picea and Abies, but shortly preceded them in eastern North America.Main conclusions Cytoplasmic and fossil data provided support for four distinct glacial lineages. Two lineages would have persisted south of the Laurentide ice sheet, while the two remaining ones likely originate from northern refugia located in Beringia and Labrador. Larix establishment was possibly hindered by earlier establishment of more competitive taxa in western North America, which resulted in high genetic differentiation among western populations. These results provide support for a putative role of interspecific competition in structuring the standing genetic variation at the time of post-glacial colonization.

Ice‐age persistence and genetic isolation of the disjunct distribution of larch in Alaska

“…The clear division of the L. laricina populations between eastern Canada, western Canada, and Alaska, as indicated by the cpDNA Bayesian analysis suggests the existence of three distinct ice‐age refugial lineages (Figure ). These three glacial lineages are further supported by the presence of chlorotypes endemic to each BAPS group (Figure ; Warren et al, ). The two lineages in eastern and western Canada were previously reported based on allozyme and cpDNA polymorphisms (Cheliak, Wang, & Pitel, ; Warren et al, ).…”

“…The two lineages in eastern and western Canada were previously reported based on allozyme and cpDNA polymorphisms (Cheliak, Wang, & Pitel, ; Warren et al, ). Our results provide the first robust evidence for a third genetic lineage in Alaska, supporting prior speculation of a glacial refugium based upon the cpDNA analysis of a single Alaskan population (Warren et al, ).…”

“…Tamarack (eastern larch, Larix laricina ) is a transcontinental North American conifer, with a prominent disjunction in the Yukon that isolates the Alaskan distribution from the larger range in Canada (Ritchie, ; Warren et al, ). The isolated distribution in Alaska may have persisted through the LGM, or resulted from postglacial colonization and long‐distance migration from south of the ice sheets.…”

Invoking adaptation to decipher the genetic legacy of past climate change

“…Paleoecological records provide examples of tree populations that failed to keep up with postglacial climate warming (e.g., Pennington , Payette , Warren et al. ). A major inference from phylogeographic surveys over the past two decades is that small populations survived closer to the ice sheets during the last glaciation than previously suggested from fossil pollen data.…”

“…), to explore the role of interspecific competition in structuring the genetic variation during postglacial colonization (Warren et al. ), or to examine the critical role of small local refugial populations in postglacial colonization (Hao et al. ).…”