RAPD variations among pure and hybrid populations ofPicea mariana, P. rubens, andP. glauca (Pinaceae), and cytogenetic stability ofPicea hybrids: Identification of species-specific RAPD markers

Random amplified polymorphic DNA (RAPD) analysis was used to determine genetic relationships amongP. mariana (black spruce),P. rubens (red spruce), andP. glauca (white spruce) and to assess the degree of polymorphism within populations from different provenances and among spruce hybrids. Eleven arbitrary decamer primers were used to amplify genomic DNAs extracted from embryogenic cultures and seedlings. Species-specific RAPD markers were identified.Picea mariana andP. rubens showed similar RAPD profiles confirming their close genetic relationship. Species-specific RAPD markers were identified and were useful in distinguishing white spruce from black and red spruces. RAPD differentiation between populations within each species was small. The level of polymorphism was much higher in spruce hybrid populations than in the pure species. Cytological analysis ofP. mariana ×P. rubens hybrids showed normal mitotic behaviour at prophase, metaphase, anaphase, and telophase. All the hybrids analyzed from different cross combinations were euploids.     

Comparison of naturally and synthetically baited spruce beetle trapping systems in the central Rocky Mountains

We compared naturally baited trapping systems to synthetically baited funnel traps and fallen trap trees for suppressing preoutbreak spruce beetle, Dendroctonus rufipennis Kirby, populations. Lures for the traps were fresh spruce (Picea spp.) bolts or bark sections, augmented by adding female spruce beetles to create secondary attraction. In 2003, we compared a naturally baited system ("bolt trap") with fallen trap trees and with synthetically baited funnel traps. Trap performance was evaluated by comparing total beetle captures and spillover of attacks into nearby host trees. Overall, the trap systems did not significantly differ in spruce beetle captures, although bolt traps caught 6 to 7 times more beetles than funnel traps during the first 4 wk of testing. Funnel traps with synthetic lures had significantly more spillover than either trap trees or bolt traps. The study was repeated in 2004 with modifications including an enhanced blend synthetic lure. Again, trap captures were generally similar among naturally and synthetically baited traps, but naturally baited traps had significantly less spillover. Although relatively labor-intensive, the bolt trap could be used to suppress preoutbreak beetle populations, especially when spillover is undesirable. Our work provides additional avenues for management of spruce beetles and suggests that currently used synthetic lures can be improved.     

Strong spatial genetic structure in peripheral but not core populations of Sitka spruce [Picea sitchensis (Bong.) Carr.

We examined spatial genetic structure within eight populations of Sitka spruce classified as core or peripheral based on ecological niche, and continuous or disjunct based on species distribution. In each population, 200 trees were spatially mapped and genotyped for eight cDNA-based sequence tagged site (STS) codominant markers. Spatial autocorrelation was assessed by estimating p(ij), the average co-ancestry coefficient, between individuals within distance intervals. The distribution of alleles and genotypes within core populations was almost random, with nonsignificant co-ancestry values among trees as close as 50 m in core populations. In contrast, the distribution of alleles and genotypes within peripheral populations revealed an aggregation of similar multilocus genotypes, with co-ancestry values greater than 0.20 among trees up to 50 m apart and significant, positive values between trees up to 500 m. The relatively high density of reproductive adults in core populations may lead to highly overlapping seed shadows that limit development of spatial genetic structure. However, in peripheral populations with a lower density of adults, the distribution of alleles and genotypes was highly structured, likely due to offspring establishment near maternal trees and subsequent biparental inbreeding, as well as more recent population establishment at the leading edge of post-Pleistocene range expansion. Conserving genetic diversity in peripheral populations may require larger reserves for in situ conservation than required in core populations. These data on spatial genetic structure can be used to provide guidance for sampling strategies for both ex situ conservation and research collections.     

GENETIC STRUCTURE OF NORWAY SPRUCE (PICEA ABIES): CONCORDANCE OF MORPHOLOGICAL AND ALLOZYMIC VARIATION

This study describes the population structure of Norway spruce (Picea abies) as revealed by protein polymorphisms and morphological variation. Electrophoretically detectable genetic variability was examined at 22 protein loci in 70 populations from the natural range of the species in Europe. Like other conifers, Norway spruce exhibits a relatively large amount of genetic variability and little differentiation among populations. Sixteen polymorphic loci (73%) segregate for a total of 51 alleles, and average heterozygosity per population is 0.115. Approximately 5% of the total genetic diversity is explained by differences between populations (G_ST = 0.052), and Nei's standard genetic distance is less than 0.04 in all cases. We suggest that the population structure largely reflects relatively recent historical events related to the last glaciation and that Norway spruce is still in a process of adaptation and differentiation. There is a clear geographic pattern in the variation of allele frequencies. A major part of the allelefrequency variation can be accounted for by a few synthetic variables (principal components), and 80% of the variation of the first principal component is “explained” by latitude and longitude. The central European populations are consistently depauperate of genetic variability, most likely as an effect of severe restrictions of population size during the last glaciation. The pattern of differentiation at protein loci is very similar to that observed for seven morphological traits examined. This similarity suggests that the same evolutionary forces have acted upon both sets of characters.     

Genetic adaptation of plants in the subarctic environment

The general feature when the northern limit of distribution of spruce Picea abies L. and meadow fescue Festuca pratensis Huds. is approached, is that individuals tend to become more homozygous at their gene loci, but different alleles are still retained in the populations. This is due to the fact that individuals within populations differ strongly in their genotypes – i.e. not one but many different genotypes are retained also at the species margin. This polymorphism in the marginal populations is probably due to some form of frequency dependent selection. Clines in gene frequency are much more pronounced in spruce than in meadow fescue probably due to a much higher environmental autocorrelation in the spruce case with its very long generation interval.     

Interactions between green spruce aphid (Elatobium abietinum (Walker)) and Norway and Sitka spruce under high and low nutrient conditions

1 Green spruce aphid (Elatobium abietinum) is a serious pest of spruce (Picea spp.) in north‐west Europe, causing defoliation of one‐year‐old and older needles. 2 Relationships between population development of E. abietinum, needle loss and tree growth were compared for five pure genotypes of Sitka spruce and mixed‐genotype material of Sitka and Norway spruce, grown under high and low nutrient conditions. 3 Despite wide differences in flushing date between spruce genotypes, E. abietinum populations peaked on the same date on each genotype and on the mixed‐genotype material, irrespective of nutrient supply. 4 Larger aphid populations developed on trees grown under high nutrient conditions than under low nutrients. However, more needles were lost per aphid in the low nutrient treatment and overall defoliation rates in the two nutrient treatments were similar. 5 Total aphid numbers differed significantly between Sitka spruce genotypes within nutrient treatments, but not in relation to bud‐burst or needle terpene content. Reductions in height growth caused by infestation were greater (15–44%), and related to mean aphid densities and defoliation, in the low nutrient treatment, but were smaller (11–27%) and not related to aphid density and defoliation in the high nutrient treatment. 6 Development of E. abietinum populations was similar on Norway and Sitka spruce, but Norway spruce lost fewer needles. However, the effects of infestation on tree growth were more closely related to aphid density and were similar for Norway and Sitka spruce. 7 Infestation caused a decrease in total root dry weight of Norway and Sitka spruce in proportion to the reductions observed in above‐ground growth.     

Negative effects on survival and performance of Norway spruce seedlings colonized by dark septate root endophytes are primarily isolate-dependent

Root endophytes are common and genetically highly diverse suggesting important ecological roles. Yet, relative to above-ground endophytes, little is known about them. Dark septate endophytic fungi of the Phialocephala fortinii s.l.-Acephala applanata species complex (PAC) are ubiquitous root colonizers of conifers and Ericaceae, but their ecological function is largely unknown. Responses of Norway spruce seedlings of two seed provenances to inoculations with isolates of four PAC species were studied in vitro. In addition, isolates of Phialocephala subalpina from two populations within and one outside the natural range of Norway spruce were also included to study the effect of the geographic origin of P. subalpina on host response. The interaction of PAC with Norway spruce ranged from neutral to highly virulent and was primarily isolate-dependent. Variation in virulence was much higher within than among species, nonetheless only isolates of P. subalpina were highly virulent. Disease caused by P. subalpina genotypes from the native range of Norway spruce was more severe than that induced by genotypes from outside the natural distribution of Norway spruce. Virulence was not correlated with the phylogenetic relatedness of the isolates but was positively correlated with the extent of fungal colonization as measured by quantitative real-time PCR.     

Genetic differentiation of the green spruce aphid (Elatobium abietinum Walker), a recent invader to Iceland

1 The RAPD method (Random Amplified Polymorphic DNA) was used to investigate genetic diversity of the green spruce aphid, Elatobium abietinum Walker, a pest introduced recently to Iceland. 2 This aphid in Iceland comprised two polymorphic populations, one in the east and the other in the west of the country. The genetic variation between sites within a population was continuous and appeared to be in good agreement with geographical distances. 3 In the eastern population the variation was greater between sites than within sites, whereas in the western population the pattern of variation appeared to be the opposite. This overall greater genetic variation in the eastern population could be due to its having been colonized earlier than the western one. 4 The study also demonstrated a close relationship between the green spruce aphid in Iceland and aphids from Denmark, which agrees with their assumed origin. The differences in introduction time, adaptation and competitiveness between the two Icelandic populations are discussed.     

New evidence from mitochondrial DNA of a progenitor-derivative species relationship between black spruce and red spruce (Pinaceae)

Mitochondrial DNA (mtDNA) markers were used to assess the genetic diversity in allopatric populations of black spruce (Picea mariana [Mill.] BSP) and red spruce (P. rubens Sarg.). Patterns of mitochondrial haplotypes (mitotypes) were strikingly different between the two species. All mtDNA markers surveyed were polymorphic in black spruce, revealing four different mitotypes and high levels of mtDNA diversity (P(p) = 100%, A = 2.0, H = 0.496). In contrast, populations of red spruce had only two mitotypes and harbored low levels of ggenetic diversity (P(p) = 13.2%, A = 1.1, H = 0.120). When the southernmost allopatric populations of red spruce were considered, only one mitotype was detected. As previously reported for nuclear gene loci, the diversity observed for mtDNA in red spruce was a subset of that found in black spruce. Comparison of present and previously published data supports the hypothesis of a recent progenitor-derivative relationship between these species, red spruce presumably being derived by allopatric speciation of an isolated population of black spruce during the Pleistocene.     

Genetic diversity and differentiation of Siberian spruce populations at nuclear microsatellite loci

The results of the study of 21 populations of Siberian spruce (Picea obovata Ledeb.) from different parts of the species natural range by microsatellite (SSR) analysis of nuclear DNA are presented. Using nine loci developed for Picea abies (L.) Karst. and Picea glauca (Moench) Voss and detecting variation in Picea obovata, the parameters of intra- and interpopulation genetic diversity, as well as the degree of population differentiation, were determined. It was demonstrated that the population of Siberian spruce in the study was characterized by a relatively high average level of intrapopulation variability (H_o = 0.408; H_e = 0.423) and low interpopulation differentiation (F_st = 0.048, P = 0.001) at this class of DNA markers. The genetic distance between populations ranged from 0.009 to 0.167, averaging 0.039. The isolated Magadan population, located in the extreme Northeast of Russia at a considerable distance from the main species range and characterized by the lowest genetic diversity among the studied populations, was maximally differentiated from the rest of the spruce populations. In addition, the steppe Ubukun population from Buryatia and the population from the Bogd Khan Uul Biosphere Reserve, Mongolia, were considerably different in the genetic structure from most populations of Siberian spruce, although to a lesser extent than the Magadan population. These findings are consistent with the results of previous studies of this species carried out using allozyme and microsatellite loci of chloroplast DNA and point to the prospects of using nuclear microsatellites as DNA markers to analyze the population genetic structure of Siberian spruce.     

Genetic diversity impacts of forest fires,forest harvesting,and alternative reforestation practices in black spruce (Picea mariana)

Benchmarks were established for genetic diversity inherent in natural mature populations, and genetic diversity impacts of forest fires, clearcut harvesting and alternative natural and artificial silvicultural regeneration practices were determined in black spruce (Picea mariana). Allozymes of 32 loci were used to determine and compare genetic diversity and genetic relationships of adjacent or nearby four stand types: post-fire natural mature (FNM), post-fire natural young (FNR), post-harvest natural young (HNR) and post-harvest plantation (PLT), of black spruce at each of the four study sites located in two ecoregions in Manitoba: Ecoregion 90-Lac Seul Upland (Eastern) and Ecoregion 158 - Mid-Boreal Lowland (Northern). Both allelic- and genotypic-based genetic diversity parameters, as well as latent genetic potential, were determined. Black spruce populations showed typical moderate to high levels of allozyme genetic diversity. The mean genetic diversity parameters over the 16 black spruce populations sampled were as follows: percent loci polymorphic - 67%, mean number of alleles per locus - 2.52, effective number of alleles per locus - 1.70, observed heterozygosity - 0.222, expected heterozygosity - 0.308, mean number of observed genotypes per locus - 3.65, mean number of expected genotypes per locus - 5.03, genotype additivity (observed) - 116.8, genotype additivity (expected) - 161, genotype multiplicity (observed) - 6.16 x 10(15), genotype multiplicity (expected) - 2.06 x 10(19) and latent genetic potential - 26.12. The four stand types (FNM, FNR, HNR and PLT) had comparable and statistically similar genetic diversity levels at each of the four study sites as well as overall. No significant differences in black spruce genetic diversity levels were observed between the two ecoregions in Manitoba, as well as between the post-fire and post-harvest regenerated stands. No particular order of genetic relatedness among the four stand types was observed. Black spruce populations showed some sort of site-related differentiation in their genetic constitution. Allelic heterogeneity and genetic distances among populations within stand types and among four stand types suggest that the genetic diversity was maintained at the landscape level in black spruce. The results of our study demonstrate that forest fires and currently used clearcut harvesting, and alternative natural and artificial silvicultural regeneration practices, do not adversely affect genetic diversity in black spruce, and that the genetic diversity effects of clearcut harvesting are not significantly different from those due to forest fires in black spruce.     

Factors influencing size inequality in peatland black spruce and tamarack: evidence from post-drainage release growth

1 We used tree ring analysis to determine stem radius and thus examine size variation over time in two even-aged (approximately 40-year-old) mixed populations of black spruce and tamarack established on peatlands in a boreal forest. We also followed the response of one of these populations to improved edaphic conditions over 8 years following drainage.
2 Populations of trees in undrained areas showed a decline in size variability over time until age 20–25 years, after which size heterogeneity was relatively stable.
3 For trees in undrained areas there was a relationship between age and size for the first 20–25 years, but this relationship then broke down due to a period where relative growth rate and size were inversely related.
4 For the population of trees in the drained area, smaller trees (i.e. those that had been growing more slowly prior to drainage) showed significantly greater drainage-induced release growth, while larger trees (those growing faster prior to drainage) showed an initial reduction in growth following drainage and, overall, less release growth.
5 The response of tamarack to drainage was more dramatic than for black spruce.
6 Despite extensive variation in tree size, drainage dramatically reduced variability in growth rate among trees of each species, such that size variability in the populations declined.
7 We postulate that the heterogeneity of microsites with respect to edaphic conditions, perhaps associated with the hummock to hollow microtopographic gradient, has a major influence on growth variation, and hence size inequality, in peatland populations of black spruce and tamarack.     

Survival and growth patterns of white spruce (Picea glauca [Moench] Voss) rangewide provenances and their implications for climate change adaptation

Intraspecific assisted migration (ISAM) through seed transfer during artificial forest regeneration has been suggested as an adaptation strategy to enhance forest resilience and productivity under future climate. In this study, we assessed the risks and benefits of ISAM in white spruce based on long‐term and multilocation, rangewide provenance test data. Our results indicate that the adaptive capacity and growth potential of white spruce varied considerably among 245 range‐wide provenances sampled across North America; however, the results revealed that local populations could be outperformed by nonlocal ones. Provenances originating from south‐central Ontario and southwestern Québec, Canada, close to the southern edge of the species' natural distribution, demonstrated superior growth in more northerly environments compared with local populations and performed much better than populations from western Canada and Alaska, United States. During the 19–28 years between planting and measurement, the southern provenances have not been more susceptible to freezing damage compared with local populations, indicating they have the potential to be used now for the reforestation of more northerly planting sites; based on changing temperature, these seed sources potentially could maintain or increase white spruce productivity at or above historical levels at northern sites. A universal response function (URF), which uses climatic variables to predict provenance performance across field trials, indicated a relatively weak relationship between provenance performance and the climate at provenance origin. Consequently, the URF from this study did not provide information useful to ISAM. The ecological and economic importance of conserving white spruce genetic resources in south‐central Ontario and southwestern Québec for use in ISAM is discussed.     

Natural hybridization between black spruce and red spruce

Using species-specific random amplified polymorphic DNA (RAPD) markers and morphological characters, natural hybridization between the closely related black spruce Picea mariana (Mill.) B.S.P. and red spruce P. rubens Sarg. was evaluated in natural populations from north-eastern North America. Sampling included populations from both areas of allopatry and also 14 populations from part of the area of sympatry located in the province of Québec and covering several thousands of square kilometres. Classification results from RAPD species-specific markers and from a discriminant function based on morphology were compared. Molecular analysis of the allopatric populations indicated a small amount of interspecific gene leakage with no asymmetric directionality to introgression. A high occurrence of hybrid/introgressant individuals was observed within sympatric populations, suggesting weak reproductive isolation. As expected, the detection of such individuals was more efficient using molecular markers than with morphological traits. The hybrid zone appeared extensive with variable species structure and, in some stands, the main component composed of hybrid/introgressant trees. Implications for the genecology and genetic management of these species are discussed.     

Genetic structure of a rare European conifer, Serbian spruce (Picea omorika (Panč.) Purk.)

Genetic variation in 13 populations of a Balkan endemic, Serbian spruce (Picea omorika (Panč.) Purk.), was investigated using 16 isozyme loci. Serbian spruce is characterized by low levels of genetic variation (average proportion of polymorphic loci was 20.9% and average expected heterozygosity was 0.067). In most populations, a significant surplus of heterozygotes was observed, indicating a strong selection against inbreds. In the largest populations, fixation indices were positive, probably due to within-population differentiation and the associated Wahlund effect. Despite a strong overall differentiation (F_ST=0.261), no geographical trends in the genetic variation could be identified. Genetic drift caused by small effective population sizes and a strong fragmentation is the most plausible explanation for such variation patterns. A Bayesian analysis of population structure revealed the existence of two clusters, which are supposed to be possible remnants of ancient differentiation within a large range of the predecessor of Serbian spruce, Picea omoricoides Weber. Ex situ conservation measures, namely establishing seed orchards from trees of mixed origin, are proposed as a necessary complement to in situ protection of natural stands.     

Genetic polymorphism of Siberian spruce (Picea obovata Ledeb) in middle Siberia

Based on the analysis of 22 loci controlling allozyme variation of MDH, SKDH, 6-PGD, IDH, PEPCA, GOT, FDH, LAP, PGI, PGM, SOD, and GDH, the data on within- and among-population variation for nine cenopopulations of Siberian spruce (Picea obovata Ledeb.) located along the Yenisei meridian, from 65 degrees 50' NL to 52degrees 14' NL, were obtained. It was demonstrated that 86.36% of the loci, tested in the species analyzed, were polymorphic. The mean number of alleles per locus was 2.91, and the observed and expected heterozygosity constituted 0.161 and 0.168, respectively. More than 97% of total variation occurred within the populations, while the proportion of the among-population variation constituted only 2.3% (Fst =0.0230). Genetic distance (DN) between the populations examined varied from 0.0019 to 0.0115, averaging at 0.0051. It was shown that in the part of the Siberian spruce range examined there was no close association between the level of the genetic differentiation of the populations and the geographic distance between them. It seems likely, that this finding can be associated with the fact that Siberian spruce, growing on this territory, is the intrazonal species, i.e., it is not attached to a certain forest zone, and its distribution is mostly determined by local ecological conditions.     

Risk of genetic maladaptation due to climate change in three major European tree species

Tree populations usually show adaptations to their local environments as a result of natural selection. As climates change, populations can become locally maladapted and decline in fitness. Evaluating the expected degree of genetic maladaptation due to climate change will allow forest managers to assess forest vulnerability, and develop strategies to preserve forest health and productivity. We studied potential genetic maladaptation to future climates in three major European tree species, Norway spruce (Picea abies), silver fir (Abies alba), and European beech (Fagus sylvatica). A common garden experiment was conducted to evaluate the quantitative genetic variation in growth and phenology of seedlings from 77 to 92 native populations of each species from across Switzerland. We used multivariate genecological models to associate population variation with past seed source climates, and to estimate relative risk of maladaptation to current and future climates based on key phenotypic traits and three regional climate projections within the A1B scenario. Current risks from climate change were similar to average risks from current seed transfer practices. For all three climate models, future risks increased in spruce and beech until the end of the century, but remained low in fir. Largest average risks associated with climate projections for the period 2061–2090 were found for spruce seedling height (0.64), and for beech bud break and leaf senescence (0.52 and 0.46). Future risks for spruce were high across Switzerland. However, areas of high risk were also found in drought‐prone regions for beech and in the southern Alps for fir. Genetic maladaptation to future climates is likely to become a problem for spruce and beech by the end of this century, but probably not for fir. Consequently, forest management strategies should be adjusted in the study area for spruce and beech to maintain productive and healthy forests in the future.     

Continent‐wide population genomic structure and phylogeography of North America’s most destructive conifer defoliator,the spruce budworm (Choristoneura fumiferana)

The spruce budworm, Choristoneura fumiferana, is presumed to be panmictic across vast regions of North America. We examined the extent of panmixia by genotyping 3,650 single nucleotide polymorphism (SNP) loci in 1975 individuals from 128 collections across the continent. We found three spatially structured subpopulations: Western (Alaska, Yukon), Central (southeastern Yukon to the Manitoba–Ontario border), and Eastern (Manitoba–Ontario border to the Atlantic). Additionally, the most diagnostic genetic differentiation between the Central and Eastern subpopulations was chromosomally restricted to a single block of SNPs that may constitute an island of differentiation within the species. Geographic differentiation in the spruce budworm parallels that of its principal larval host, white spruce (Picea glauca), providing evidence that spruce budworm and spruce trees survived in the Beringian refugium through the Last Glacial Maximum and that at least two isolated spruce budworm populations diverged with spruce/fir south of the ice sheets. Gene flow in the spruce budworm may also be affected by mountains in western North America, habitat isolation in West Virginia, regional adaptations, factors related to dispersal, and proximity of other species in the spruce budworm species complex. The central and eastern geographic regions contain individuals that assign to Eastern and Central subpopulations, respectively, indicating that these barriers are not complete. Our discovery of previously undetected geographic and genomic structure in the spruce budworm suggests that further population modelling of this ecologically important insect should consider regional differentiation, potentially co‐adapted blocks of genes, and gene flow between subpopulations.     

A cyclophilin gene marker confirming geographical differentiation of Norway spruce populations and indicating viability response on excess soil-born salinity

A newly identified cyclophilin-encoding cDNA clone was used to design a codominant inherited EST-PCR marker in Norway spruce. The study of the current minor polymorphism revealed a geographically structured differentiation pattern across 17 test populations, showing a slight clinal variation south-north through Europe. Based on the frequency of alleles, isolation-by-distance analysis and the Ewens-Watterson test, we conclude that a selectively neutral random-drift mutation recently occurred within the Alpine population group, thus being responsible for the genetic variation detected. Analysis of tolerant and susceptible subsets of two adjacent Bavarian spruce populations affected by soil-born NaCl pollution, revealed that the cyclophilin marker locus also confirms biased genotype frequencies. Considering an unlinked PCR marker of a ribosomal protein-encoding EST clone, deviations between pooled tolerant subsets and pooled sensitive subsets were proven to be more significant for two-locus homozygous genotypes than for each locus alone. We suggest that both loci are linked to adaptive genomic regions. Their potential to test the feasibility of marker-assisted selection of both NaCl-tolerant and drought resistant tree populations is discussed.     

Tracking the progression of speciation: variable patterns of introgression across the genome provide insights on the species delimitation between progenitor–derivative spruces (Picea mariana × P. rubens)

The genic species concept implies that while most of the genome can be exchanged somewhat freely between species through introgression, some genomic regions remain impermeable to interspecific gene flow. Hence, interspecific differences can be maintained despite ongoing gene exchange within contact zones. This study assessed the heterogeneous patterns of introgression at gene loci across the hybrid zone of an incipient progenitor–derivative species pair, Picea mariana (black spruce) and Picea rubens (red spruce). The spruce taxa likely diverged in geographic isolation during the Pleistocene and came into secondary contact during late Holocene. A total of 300 SNPs distributed across the 12 linkage groups (LG) of black spruce were genotyped for 385 individual trees from 33 populations distributed across the allopatric zone of each species and within the zone of sympatry. An integrative framework combining three population genomic approaches was used to scan the genomes, revealing heterogeneous patterns of introgression. A total of 23 SNPs scattered over 10 LG were considered impermeable to introgression and putatively under diverging selection. These loci revealed the existence of impermeable genomic regions forming the species boundary and are thus indicative of ongoing speciation between these two genetic lineages. Another 238 SNPs reflected selectively neutral diffusion across the porous species barrier. Finally, 39 highly permeable SNPs suggested ancestral polymorphism along with balancing selection. The heterogeneous patterns of introgression across the genome indicated that the speciation process between black spruce and red spruce is young and incomplete, albeit some interspecific differences are maintained, allowing ongoing species divergence even in sympatry. The approach developed in this study can be used to track the progression of ongoing speciation processes.