Variation in mitochondrial DNA reveals multiple distant glacial refugia in black spruce (Picea mariana), a transcontinental North American conifer

Range-wide genetic variation of black spruce (Picea mariana) was studied using polymerase chain reaction-random fragment length polymorphism markers of the mitochondrial genome. Four polymorphic mitochondrial DNA (mtDNA) loci were surveyed and two or three alleles were detected at each locus, resulting in 10 multilocus mtDNA types or mitotypes. A significant subdivision of population genetic diversity was detected (GST = 0.671; NST = 0.726), suggesting low levels of gene flow among populations. The distribution of mitotypes was not random (NST > GST; P < 0.05) and revealed four partially overlapping zones, presumably representative of different glacial populations. Comparison of the genetic structure derived from mtDNA markers and the colonization paths previously deduced from the fossil and pollen records allow us to infer at least three southern and one northeastern glacial populations for black spruce. The patterns revealed in this study suggest that black spruce shares its biogeographical history with other forest-associated North American species.     

Mitochondrial genome recombination in the zone of contact between two hybridizing conifers

Variation in mitochondrial DNA was surveyed at four gene loci in and around the zone of contact between two naturally hybridizing conifers, black spruce (Picea mariana) and red spruce (P. rubens) in northeastern North America. Most of the mtDNA diversity of these species was found in populations next to or into the zone of contact, where some individuals bore rare mitotypes intermediate between the common mitotypes observed in the allopatric areas of each species. Sequence analysis and tests for mtDNA recombination point to this phenomenon, rather than to recurrent mutation, as the most tenable hypothesis for the origin of these rare mitotypes. From the 10 mitotypes observed, at least 4 would be the product of recombination between 4 of the 5 putative ancestral mitotypes. Tests for cytonuclear disequilibrium and geographical structure of the putative recombinant mitotypes suggest that mtDNA recombination is not frequent and relatively recent on the geological time scale. mtDNA recombination would have been promoted by transient heteroplasmy due to leakage of paternal mtDNA since the Holocene secondary contact between the two species.     

Genetic validation and characterization of RAPD markers differentiating black and red spruces: molecular certification of spruce trees and hybrids

 Picea mariana (black spruce) and P. rubens (red spruce) are closely related species which are difficult to differentiate morphologically. RAPD markers differentiating black and red spruces have been previously identified. In the present study, genetic validity of these markers was determined using samples representing range–wide provenances. Their applicability for certifying genetic identity of individual black, red trees and their hybrids from several sympatric and allopatric locations was demonstrated. These diagnostic fragments of both red and black spruce were present at a frequency of over 0.95 in allopatric provenances, but at a lower frequency in some sympatric provenances (0.43–1.00). Natural populations of red spruce exhibiting typical red spruce phenotype contained black spruce diagnostic RAPD fragments and black spruces growing in bogs with typical bog black spruce morphology, contained red spruce-specific RAPD markers. Some major RAPD markers were cloned and sequenced. The results reveal an extremely high degree of identity between the random primer and the primer binding sites on the genome. Amplification of black and red spruce genomic DNA with designed primers flanking the species-diagnostic RAPD markers indicates that most of RAPD markers used to differentiate black spruce from red spruce are not species specific since these sequences were detected in several spruce species using a more sensitive detection method. Received June 17, 2002; accepted August 5, 2002 Published online: February 4, 2003     

Decoupled mitochondrial and chloroplast DNA population structure reveals Holocene collapse and population isolation in a threatened Mexican-endemic conifer

Chihuahua spruce (Picea chihuahuana Martínez) is a montane subtropical conifer endemic to the Sierra Madre Occidental in northwestern México. Range-wide variation was investigated using maternally inherited mitochondrial (mtDNA) and paternally inherited chloroplast (cpDNA) DNA markers. Among the 16 mtDNA regions analysed, only two mitotypes were detected, while the study of six cpDNA microsatellite markers revealed eight different chlorotypes. The average cpDNA diversity (H = 0.415) was low but much higher than that for mtDNA (H = 0). The distribution of mitotypes revealed two clear nonoverlapping areas (G(ST) = N(ST) = 1), one including northern populations and the second one including the southern and central stands, suggesting that these two regions may represent different ancestral populations. The cpDNA markers showed lower population differentiation (G(ST) = 0.362; R(ST) = 0.230), implying that the two ancestral populations continued to exchange pollen after their initial geographic separation. A lack of a phylogeographic structure was revealed by different spatial analyses of cpDNA (G(ST) > R(ST); and samova), and reduced cpDNA gene flow was noted among populations (Nm = 0.873). Some stands deviated significantly from the mutation-drift equilibrium, suggesting recent bottlenecks. Altogether, these various trends are consistent with the hypothesis of a population collapse during the Holocene warming and suggest that most of the modern P. chihuahuana populations are now effectively isolated with their genetic diversity essentially modelled by genetic drift. The conservation efforts should focus on most southern populations and on the northern and central stands exhibiting high levels of genetic diversity. Additional mtDNA sequence analysis confirmed that P. martinezii (Patterson) is not conspecific with P. chihuahuana, and thus deserves separate conservation efforts.     

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.     

Diverging patterns of mitochondrial and nuclear DNA diversity in subarctic black spruce: imprint of a founder effect associated with postglacial colonization

High-latitude ecotonal populations at the species margins may exhibit altered patterns of genetic diversity, resulting from more or less recent founder events and from bottleneck effects in response to climate oscillations. Patterns of genetic diversity were investigated in nine populations of the conifer black spruce (Picea mariana [Mill.] BSP.) in northwestern Québec, Canada, using seed-dispersed mitochondrial (mt) DNA and nuclear (nc) DNA. mtDNA diversity (mitotypes) was assessed at three loci, and ncDNA diversity was estimated for nine expressed sequence tag polymorphism (ESTP) loci. Sampling included populations from the boreal forest and the southern and northern subzones of the subarctic forest-tundra, a fire-born ecotone. For ncDNA, populations from all three vegetation zones were highly diverse with little population differentiation (thetaN = 0.014); even the northernmost populations showed no loss of rare alleles. Patterns of mitotype diversity were strikingly different: within-population diversity and population differentiation were high for boreal forest populations [expected heterozygosity per locus (HE) = 0.58 and thetaM = 0.529], but all subarctic populations were fixed for a single mitotype (HE = 0). This lack of variation suggests a founder event caused by long-distance seed establishment during postglacial colonization, consistent with palaeoecological data. The estimated movement of seeds alone (effective number of migrants per generation, NmM < 2) was much restricted compared to that estimated from nuclear variants, which including pollen movement (NmN > 17). This could account for the conservation of a founder imprint in the mtDNA of subarctic black spruce. After reduction, presumably in the early Holocene, the diversity in ncDNA would have been replenished rapidly by pollen-mediated gene flow, and maintained subsequently through vegetative layering during the current cooler period covering the last 3000 years.     

From glacial refugia to modern populations: new assemblages of organelle genomes generated by differential cytoplasmic gene flow in transcontinental black spruce

Assessing species' range-wide cytoplasmic diversity provides valuable insights regarding their dispersal and adaptive potential in a changing environment. Transcontinental chloroplast (cpDNA) and mitochondrial DNA (mtDNA) population structures were compared to identify putative ancestral and new cytoplasmic genome assemblages in black spruce (Picea mariana), a North American boreal conifer. Mean within-population diversity and allelic richness for cpSSR markers were 0.80 and 4.21, respectively, and diminished westward. Population differentiation based on G(ST) was lower for cpDNA than for mtDNA (G(ST) =0.104 and 0.645, respectively) but appeared comparable when estimated using Jost differentiation index (D=0.459 and 0.537, respectively). Further analyses resulted in the delineation of at least three genetically distinct cpDNA lineages partially congruent with those inferred from mtDNA data, which roughly corresponded to western, central and eastern Canada. Additionally, the patterns of variation in Alaska for both cpDNA and mtDNA markers suggested that black spruce survived the last glacial maximum in this northern region. The range-wide comparison of the geographic extent of cytoplasmic DNA lineages revealed that extensive pollen gene flow between ancestral lineages occurred preferentially from west to east during the postglacial expansion of the species, while seed-mediated gene flow remained geographically restricted. This differential gene flow promoted intraspecific cytoplasmic capture that generated new assemblages of cpDNA and mtDNA genomes during the Holocene. Hence, black spruce postglacial colonization unexpectedly resulted in an increase in genetic diversity with possible adaptive consequences.     

Microsatellite Analysis of Genetic Variation and Population Subdivision for the Black Muntjac, Muntiacus crinifrons

The black muntjac (Muntiacus crinifrons) is a rare deer found only in a restricted region in east China. Recent studies of mitochondrial DNA diversity have shown a markedly low level of nucleotide diversity for the species, and the Suichang population was genetically differentiated from the two other populations, in Huangshan and Tianmushan mountains. In this study, we extended the analysis of genetic diversity and population subdivision for the black muntjac using data from 11 highly polymorphic nuclear DNA microsatellite loci. Contrary to the results based on mtDNA data, the microsatellite loci revealed that the black muntjac retained a rather high nuclear genetic diversity (overall average H (E) = 0.78). Nevertheless, both types of markers supported the idea that the extant black muntjac population is genetically disrupted (overall phi (ST) = 0.16 for mtDNA and overall F (ST) = 0.053 for microsatellite, both P < 0.001). The correlation between genetic differentiation and geographic distance was not significant (Mantel test; P > 0.05), implying that the patterns of genetic differentiation observed in this study might result from recent habitat fragmentation or loss. Based on the results from the mtDNA and nuclear DNA data sets, two management units were defined for the species, Huangshan/Tianmushan and Suichang. We also recommend that a new captive population be established with individuals from the Suichang region as a founder source.     

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.     

The postglacial history of Scots pine (Pinus sylvestris L.) in western Europe: evidence from mitochondrial DNA variation

The geographical structure of mitochondrial (mt)DNA variants (mitotypes) was investigated in 38 western European populations of Scots pine Pinus sylvestris using restriction fragment length polymorphism (RFLP) analysis of total DNA and a homologous cox1 probe. Three major mitotypes (designated a, b and d ) were detected. Within Spain all three major mitotypes were found, gene diversity was high, H_T = 0.586, and this diversity was distributed predominantly among rather than within populations (F_ST(M) = 0.813 for the seven Spanish populations). Mitotype d was present only in the most southerly population from the Sierra Nevada . Elsewhere in Europe, populations showed little or no mtDNA diversity within regions, but there were marked differences between regions. Italian populations were fixed for mitotype b ; populations from northern France, Germany, Poland, Russia and southern Sweden were fixed for mitotype a ; while populations in northern Fennoscandia were fixed for mitotype b . The isolated Scottish populations were predominantly of mitotype a , but mitotype b was present in three of the 20 populations scored. In Scotland, UK gene diversity (H_T = 0.120) and genetic differentiation among populations (F_ST(M) = 0.37) was much lower than in Spain. When interpreted in the light of complementary data from pollen analysis and nuclear genetic markers, the results suggest that present-day populations of P. sylvestris in western Europe have been derived from at least three different sources after glaciation.     

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.     

Cytoplasmic DNA variation and biogeography of Larix Mill. in Northeast Asia

Range‐wide variation in 54 populations of Dahurian larch (Larix gmelinii) and related taxa in Northeast Asia was assessed with four mitochondrial PCR‐RFLP and five chloroplast SSR markers. Eleven mitotypes and 115 chlorotypes were detected. The highest diversity was observed in the southern Russian Far East where hybrids of L. gmelinii, L. olgensis and L. kamtschatica are distributed. In contrast, only two mitotypes occurred in L. cajanderi and L. gmelinii. The Japanese larch (L. kaempferi) was found to be closely related to populations of L. kamtschatica inhabiting the Kuril Islands and South Sakhalin, populations from the northern part of Sakhalin being more closely related to continental species. In general, both mitochondrial (G_ST = 0.786; N_ST = 0.823) and chloroplast (G_ST = 0.144; R_ST = 0.432) markers showed a strong phylogeographical structure and evidence of isolation‐by‐distance. Yet both markers did not allow a clear delineation of species borders. In particular, and contrary to expectations, cpDNA was not significantly better than mtDNA at delineating species borders. This lack of concordance between morphological species and molecular markers could reflect extensive ancestral haplotype sharing and past and ongoing introgression. Finally the distribution of mtDNA and cpDNA variation suggests the presence of several refugia during Pleistocene glacial intervals. In particular, mtDNA and cpDNA reveal weak but visible differentiation between L. gmelinii and L. cajanderi, suggesting independent glacial histories of these species.     

Direction and extent of organelle DNA introgression between two spruce species in the Qinghai-Tibetan Plateau

A recent model has shown that, during range expansion of one species in a territory already occupied by a related species, introgression should take place preferentially from the resident species towards the invading species and genome components experiencing low rates of gene flow should introgress more readily than those experiencing high rates of gene flow. Here, we use molecular markers from two organelle genomes with contrasted rates of gene flow to test these predictions by examining genetic exchanges between two morphologically distinct spruce Picea species growing in the Qinghai-Tibetan Plateau. The haplotypes from both mitochondrial (mt) DNA and chloroplast (cp) DNA cluster into two distinct lineages that differentiate allopatric populations of the two species. By contrast, in sympatry, the species share the same haplotypes, suggesting interspecific genetic exchanges. As predicted by the neutral model, all sympatric populations of the expanding species had received their maternally inherited mtDNA from the resident species, whereas for paternally inherited cpDNA introgression is more limited and not strictly unidirectional. Our results underscore cryptic introgressions of organelle DNAs in plants and the importance of considering rates of gene flow and range shifts to predict direction and extent of interspecific genetic exchanges.     

Population history of European mountain pines Pinus mugo and Pinus uncinata revealed by mitochondrial DNA markers

The dwarf mountain pine (Pinus mugo) and the Pyrenean pine (P. uncinata) constitute a pair of closely related coniferous taxa of poorly resolved evolutionary history and affinity, which inhabit numerous stands scattered over subalpine environments of European mountain ranges. The aim of the study was to investigate their phylogeography and mutual relationships, shedding new light on their taxonomy and the past of the alpine flora. Previous evolutionary reconstructions of the mountain pines relied mainly on bi‐parentally or paternally inherited markers that quickly homogenize between populations, showing rather shallow and recent differentiation of gene pools. Therefore, to contrast these pictures, we analyzed diversity and differentiation within a large set of new mitochondrial loci, inherited in maternal line and distributed by seeds at short geographical distances. Samples of the taxa were taken from 27 natural populations representing their range‐wide distributions—17 populations of P. mugo and 10 of P. uncinata. All markers appeared polymorphic, providing a total of 31 multilocus haplotypes. Two of the loci proved to be species‐diagnostic and nearly fixed between analyzed samples. Distribution of mitotypes indicate that allopatric populations of the taxa constitute separate mitochondrial haplogroups, and the two mountain pines have independent evolutionary history. However, introgression of P. mugo mitotypes by P. uncinata specimens revealed in the species contact zone in Western Alps shows that their speciation is not fully completed.     

Diversifying coevolution between crossbills and black spruce on Newfoundland

Coevolution is increasingly recognized as an important process structuring geographic variation in the form of selection for many populations. Here we consider the importance of a geographic mosaic of coevolution to patterns of crossbill (Loxia) diversity in the northern boreal forests of North America. We examine the relationships between geographic variation in cone morphology, bill morphology, and feeding performance to test the hypothesis that, in the absence of red squirrels (Tamiasciurus hudsonicus), black spruce (Picea mariana) has lost seed defenses directed at Tamiasciurus and that red crossbills (L curvirostra) and black spruce have coevolved in an evolutionary arms race. Comparisons of cone morphology and several indirect lines of evidence suggest that black spruce has evolved defenses in response to Tamiasciurus on mainland North America but has lost these defenses on Newfoundland. Cone traits that deter crossbills, including thicker scales that require larger forces to separate, are elevated in black spruce on Newfoundland, and larger billed crossbills have higher feeding performances than smaller billed crossbills on black spruce cones from Newfoundland. These results imply that the large bill of the Newfoundland crossbill (L. c. percna) evolved as an adaptation to the elevated cone defenses on Newfoundland and that crossbills and black spruce coevolved in an evolutionary arms race on Newfoundland during the last 9000 years since glaciers retreated. On the mainland where black spruce is not as well defended against crossbills, the small-billed white-winged crossbill (L leucoptera leucoptera) is more efficient and specializes on seeds in the partially closed cones. Finally, reciprocal adaptations between crossbills and conifers are replicated in black spruce and Rocky Mountain lodgepole pine (Pinus contorta ssp. latifolia), with coevolution most pronounced in isolated populations where Tamiasciurus are absent as a competitor. This study further supports the role of Tamiasciurus in determining the selection mosaic for crossbills and suggests that a geographic mosaic of coevolution has been a prominent factor underlying the diversification of North American crossbills.     

Development of microsatellite markers for white spruce (Picea glauca) and related species

We report the development of 13 primer pairs that allow the unambiguous amplification of 15 microsatellite (SSR) loci in white spruce (Picea glauca). Fourteen of these loci were polymorphic in trees sampled at three geographically separated regions of western Canada. Segregation analysis carried out on these loci confirmed a Mendelian inheritance pattern for all except two, which showed significant segregation distortion. All of these primer pairs amplified SSR loci in at least one of the other Picea species tested [black spruce (P. mariana), red spruce (P. rubens), Norway spruce (P. abies), Colorado spruce (P. pungens), sitka spruce (P. sitchensis) and Engelmann spruce (P. engelmannii)]. Given the important commercial and ecological roles of these species, this set of markers will be invaluable for their management, the improvement of commercially important traits, and the study of their ecology and genetics. Received: 18 August 2000 / Accepted: 28 September 2000     

Highly Informative Single-Copy Nuclear Microsatellite DNA Markers Developed Using an AFLP-SSR Approach in Black Spruce (Picea mariana) and Red Spruce (P. rubens)

Background

Microsatellites or simple sequence repeats (SSRs) are highly informative molecular markers for various biological studies in plants. In spruce (Picea) and other conifers, the development of single-copy polymorphic genomic microsatellite markers is quite difficult, owing primarily to the large genome size and predominance of repetitive DNA sequences throughout the genome. We have developed highly informative single-locus genomic microsatellite markers in black spruce (Picea mariana) and red spruce (Picea rubens) using a simple but efficient method based on a combination of AFLP and microsatellite technologies.

Principal Findings

A microsatellite-enriched library was constructed from genomic AFLP DNA fragments of black spruce. Sequencing of the 108 putative SSR-containing clones provided 94 unique sequences with microsatellites. Twenty-two of the designed 34 primer pairs yielded scorable amplicons, with single-locus patterns. Fourteen of these microsatellite markers were characterized in 30 black spruce and 30 red spruce individuals drawn from many populations. The number of alleles at a polymorphic locus ranged from 2 to 18, with a mean of 9.3 in black spruce, and from 3 to 15, with a mean of 6.2 alleles in red spruce. The polymorphic information content or expected heterozygosity ranged from 0.340 to 0.909 (mean = 0.67) in black spruce and from 0.161 to 0.851 (mean = 0.62) in red spruce. Ten SSR markers showing inter-parental polymorphism inherited in a single-locus Mendelian mode, with two cases of distorted segregation. Primer pairs for almost all polymorphic SSR loci resolved microsatellites of comparable size in Picea glauca, P. engelmannii, P. sitchensis, and P. abies.

Significance

The AFLP-based microsatellite-enriched library appears to be a rapid, cost-effective approach for isolating and developing single-locus informative genomic microsatellite markers in black spruce. The markers developed should be useful in black spruce, red spruce and other Picea species for various genetics, genomics, breeding, forensics, conservation studies and applications.     

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.     

Large-scale asymmetric introgression of cytoplasmic DNA reveals Holocene range displacement in a North American boreal pine complex

Jack pine (Pinus banksiana) and lodgepole pine (Pinus contorta var. latifolia) are two North American boreal hard pines that hybridize in their zone of contact in western Canada. The main objective of this study was to characterize their patterns of introgression resulting from past and recent gene flow, using cytoplasmic markers having maternal or paternal inheritance. Mitochondrial DNA (mtDNA) and chloroplast DNA (cpDNA) diversity was assessed in allopatric populations of each species and in stands from the current zone of contact containing morphological hybrids. Cluster analyses were used to identify genetic discontinuities among groups of populations. A canonical analysis was also conducted to detect putative associations among cytoplasmic DNA variation, tree morphology, and site ecological features. MtDNA introgression was extensive and asymmetric: it was detected in P. banksiana populations from the hybrid zone and from allopatric areas, but not in P. contorta populations. Very weak cpDNA introgression was observed, and only in P. banksiana populations. The mtDNA introgression pattern indicated that central Canada was first colonized by migrants from a P. contorta glacial population located west of the Rocky Mountains, before being replaced by P. banksiana migrating westward during the Holocene. In contrast, extensive pollen gene flow would have erased the cpDNA traces of this ancient presence of P. contorta. Additional evidence for this process was provided by the results of canonical analysis, which indicated that the current cpDNA background of trees reflected recent pollen gene flow from the surrounding dominant species rather than historical events that took place during the postglacial colonization.     

Genetic structure of an endangered plant, Antirrhinum microphyllum (Scrophulariaceae): allozyme and RAPD analysis

Thirteen allozyme loci and 68 random amplified polymorphic DNA (RAPD) markers were analyzed to assess the genetic diversity and population structure of threatened Antirrhinum microphyllum (Scrophulariaceae), a narrow endemic of central Spain known from only four populations. According to allozyme data, species genetic diversity (p = 46.15%, A = 2.61, and H(e) = 0.218), as well as within-population genetic diversity (p = 44.23%, A = 2.10, and H(e) = 0.204), were high when compared to average estimates for other narrowly distributed plant species. Ninety-four percent of species genetic diversity corresponded to within-population genetic diversity. Nevertheless, significant differences were found among populations in allele frequencies of four of the six polymorphic loci, and three private alleles were detected. Inbreeding coefficients (F(IS)) suggest that populations are structured in genetic neighborhoods. The RAPDs also showed high levels of genetic diversity (p = 89.71% and H(e) = 0.188 at the species level, and p = 67.65% and H(e) = 0.171 at the population level). Nei's genetic distances estimated both from allozymes and RAPDs indicated low differentiation among populations. In spite of this, the low frequencies of certain alleles and the presence of private alleles indicate that efforts should be made to conserve all four remaining populations.