Chloroplast DNA polymorphisms in eastern hemlock: range-wide genogeographic analyses and implications for gene conservation

Abstract

The objective of this study was to determine if the genetic diversity of eastern hemlock (Tsuga canadensis (L.) Carrière) is uniformly distributed or structured. Such information is relevant to help guide conservation efforts. Individuals were sampled in 60 range-wide populations of T. canadensis and genotyped at seven polymorphic chloroplast DNA loci. All 16 chlorotypes identified in T. canadensis were highly divergent from the unique chlorotype detected in Carolina hemlock (Tsuga caroliniana Engelm.). Among-population differentiation in T. canadensis was low (GST = 0.020) and the distribution of chlorotypes did not show any strong geographical pattern, which is likely due to the homogenizing effect of pollen gene flow during the Holocene. Nevertheless, a spatial Bayesian approach revealed two distinct groups of populations. Furthermore, an analysis of relative genetic distances indicated that southeastern Appalachian populations harboured greater population differentiation while conserving relatively high allelic richness, which might represent the imprint of an ancient glacial refugium in the region. Thus, these distinctive genetic patterns and the risk of Tsuga decline following climate warming combined with the introduction of the insect Adelges tsugae (Annand) indicate that the southern part of the range should be considered of high priority for ex situ conservation.Résumé : Le but de cette étude était de déterminer si la diversité génétique de la pruche du Canada (Tsuga canadensis (L.) Carrière) était uniforme ou structurée, une information pertinente pour guider les efforts de conservation. Des individus ont été échantillonnés dans 60 populations couvrant l'ensemble de l'aire de distribution de l'espèce et génotypés à sept loci d'ADN chloroplastique polymorphes. Les 16 chlorotypes identifiés chez T. canadensis étaient très divergents de l'unique chlorotype détecté chez la pruche de la Caroline (Tsuga caroliniana Engelm.). La différenciation de populations chez T. canadensis était faible (GST = 0,020) et la distribution des chlorotypes ne montrait pas de patron géographique marqué, probablement en raison de l'homogénéisation par le flux pollinique durant l'Holocène. Néanmoins, une analyse spatiale bayésienne a mis en évidence deux groupes distincts de populations. Aussi, l'analyse des distances génétiques relatives entre populations a révélé que les populations du sud-est des Appalaches étaient fortement différenciées et possédaient une forte richesse allélique, ce qui pourrait représenter l'empreinte d'un ancien refuge glaciaire dans cette région. Ces patrons distinctifs de variation ainsi que les risques élevés d'un déclin de Tsuga résultant du réchauffement climatique et de l'introduction de l'insecte Adelges tsugae (Annand) confèrent donc à la partie sud de l'aire de distribution une forte priorité de conservation ex situ.

Widespread inbreeding and unexpected geographic patterns of genetic variation in eastern hemlock (Tsuga canadensis), an imperiled North American conifer

“…The North genetic cluster descendants of these refugial trees may have migrated northeast along the Appalachians into the northeastern United States and southeastern Canada before colonizing the Great Lakes region. It perhaps made secondary contact with trees from the South Central genetic cluster, with which it appears to have had high levels of historical gene exchange (Table 4), consistent with chloroplast DNA evidence of homogenizing gene flow among eastern hemlock populations during the Holocene (Lemieux et al 2011). Trees from the South Central genetic cluster, meanwhile, also may have made their way up the Appalachians in addition to moving into the Midwest.…”

“…In a study that sampled 20 populations from the Southeastern portion of the range, Potter et al (2008) also found low allozyme diversity in hemlock but reasonably high population differentiation. A range-wide analysis using seven haploid chloroplast DNA loci (Lemieux et al 2011) also detected low among-population differentiation and, consistent with Potter et al (2008), greater differentiation and relatively high allelic richness among southeastern Appalachian populations. These results led both sets of authors to conclude that the majority of eastern hemlock genetic diversity may have originated from a Pleistocene glacial refuge in this region.…”

“…3; Table 4), which according to both clustering approaches account for most of the genetic composition of the sampled populations, may have descended from refuges located in the vicinity of the Southern Appalachian Mountains. Higher levels of allozyme and chloroplast DNA variation in populations in the southeastern Appalachians suggest that at least one glacial refuge area may have been located in or near this region Lemieux et al 2011). Such a pattern, where most of a species' existing distribution descends from a subset of the putative glacial refuges, is common among taxa that increased their ranges and abundances during the glacial-interglacial transition .…”

“…These results led both sets of authors to conclude that the majority of eastern hemlock genetic diversity may have originated from a Pleistocene glacial refuge in this region. Further screening from nuclear or mitochondrial DNA markers is necessary, however, to strengthen inferences regarding the distribution of the species during the last glacial maximum (Lemieux et al 2011). A more robust analysis of genetic variation across the range of the species also is needed to improve the efficiency of existing ex situ and in situ conservation efforts given eastern hemlock's large natural distribution, the existence of disjunct populations that may possess unique adaptive characteristics, and the fact that HWA has already infested and significantly impacted approximately 50% of hemlock ecosystems in the eastern United States (United States Department of Agriculture Forest Service 2011).…”

Less Pollen-Mediated Gene Flow for More Signatures of Glacial Lineages: Congruent Evidence from Balsam Fir cpDNA and mtDNA for Multiple Refugia in Eastern and Central North America

“…The combined use of fossil [ 17 , 18 ] and genetic data [ 6 , 7 , 10 , 19 – 22 ] brought insights regarding the putative location of several of these refugia in North America. In the eastern part of the continent, molecular evidence pointed to the persistence of species in isolated cryptic refugia after their southward retreat.…”