Phylogeographic structure of jack pine (Pinus banksiana; Pinaceae) supports the existence of a coastal glacial refugium in northeastern North America

? Premise of the study: The genetic structure of jack pine (Pinus banksiana Lamb.), a North American boreal conifer with large longitudinal distribution, was investigated to test for the possible existence of a genetically distinct lineage in the Maritimes region in northeastern North America, which could be indicative of a mid-latitude coastal refuge during the last glaciation. ? Methods: One maternally inherited mitochondrial DNA (mtDNA) minisatellite marker and four paternally inherited chloroplast DNA (cpDNA) microsatellite markers were used to assess the range-wide geographical structure of jack pine populations with particular focus on northeastern North America. ? Key results: The populations from the Maritimes region presented a unique mtDNA background characterized by very low diversity and the preponderance of a distinctive mitotype. The distribution of cpDNA diversity was not spatially structured, though three chlorotypes were restricted to the east. ? Conclusions: MtDNA data suggest that populations from the Maritimes region derive from a genetically depauperated north-coastal refugium. Contrastingly, the much higher geographical uniformity observed for cpDNA variation indicates that gene flow by pollen had been much more effective than seed gene flow at homogenizing population structure.     

Extensive Mitochondrial Introgression from Pinus pumila to P. parviflora var. pentaphylla (Pinaceae)

Pinus species exhibit a paternal chloroplast inheritance and a maternal mitochondrial inheritance. The levels and patterns of cpDNA and mtDNA introgression between the two pine species, P. pumila and P. parviflora var. pentaphylla, were examined at three mountain sites in Japan. The pine species were examined by using PCR-based diagnostic genetic markers of cpDNA and mtDNA. The survey which was carried out in multiple hybrid zones demonstrated a generality in the uni-directional pattern of cytoplasmic gene flow between the two pine species, i.e. paternal cpDNA flowed from P. parviflora var. pentaphylla to P. pumila, and in contrast, maternal mtDNA flowed from P. pumila to P. parviflora var. pentaphylla. Whenever plants which had a non-native combination of cpDNA and mtDNA were observed, they always had the cpDNA haplotype of P. parviflora var. pentaphylla and the mtDNA haplotype of P. pumila. The existence of only this type of cytoplasmic chimera may suggest that F₁ hybrids are successfully produced only in the crossing of P. pumila as the maternal parent and P. parviflora var. pentaphylla as the paternal parent. The present study also detected extensive mtDNA capture in populations of P. parviflora var. pentaphylla located in the southern and middle parts of the Ohu Mountains, Tohoku, Japan. In that area, nearly all of the plants examined had the mtDNA haplotype of P. pumila. The extensive mtDNA introgression suggests that seed flow could be an effective medium for interspecific gene exchange. Received 17 August 1998/ Accepted in revised form 7 January 1999     

Paternally Inherited Chloroplast Polymorphism in Pinus: Estimation of Diversity and Population Subdivision, and Tests of Disequilibrium with a Maternally Inherited Mitochondrial Polymorphism

We have surveyed a chloroplast DNA restriction fragment length polymorphism in 745 individuals, distributed rangewide in eight allopatric natural populations of jack pine (Pinus banksiana Lamb.) and eight allopatric natural populations of lodgepole pine (Pinus contorta Dougl.). The polymorphic region of the chloroplast genome is located near duplicated psbA genes. Fourteen length variants were found in the survey, and these variants distinguished the two species qualitatively. Variant diversities were high in both species (h(es) = 0.43 in jack pine; h(es) = 0.44 in lodgepole pine). Population subdivision was weak within and among lodgepole pine subspecies and in jack pine (i.e., θvalues were less than 0.05). This weak subdivision is compatible with theoretical predictions for paternally inherited markers in wind-pollinated outcrossers, as well as for polymorphisms with high length mutation rates. If these populations are at a drift-migration equilibrium, the chloroplast DNA restriction fragment data and previous mitochondrial frequency data from the same individuals are consistent with gene flow that is differential through seeds and pollen. The new data have permitted the first empirical tests of disequilibrium between maternally and paternally inherited factors. As expected, these tests failed to detect convincing evidence of non-random association between chloroplast and mitochondrial variants.     

HISTORICAL SEPARATION AND PRESENT GENE FLOW THROUGH A ZONE OF SECONDARY CONTACT IN PONDEROSA PINE

I examined the effects of historical division and secondary contact between eastern and western varieties of ponderosa pine (Pinus ponderosa Laws Pinaceae) on extant patterns of genetic variation. Fossil and biogeographic evidence both indicate that the current point of contact between these two varieties represents secondary contact following historical separation during the Wisconsin glaciation. Current gene flow was assessed by observing the degree of introgression of paternally inherited cpDNA and maternally inherited mtDNA polymorphisms. Both seeds and pollen are wind dispersed in ponderosa pine. Introgression was primarily from west to east, the direction of the prevailing wind, for both organelles, but introgression of cpDNA far exceeded that of mtDNA. Thus pollen is the main agent of contemporary gene flow between the two varieties. Neither seeds nor pollen showed enough introgression since secondary contact to have homogenized the two gene pools. However, allozyme differentiation was minimal. This calls into question assumptions of selective neutrality for at least some of the markers. Theory predicts that nuclear markers will show a high locus-to-locus variance of F_ST following historical separation. This prediction is confirmed by the allozyme data for ponderosa pine, and may provide a useful means of identifying historical separations from allele frequency data.     

GENE FLOW AND SPECIES DELIMITATION: A CASE STUDY OF TWO PINE SPECIES WITH OVERLAPPING DISTRIBUTIONS IN SOUTHEAST CHINA

Species delimitation detected by molecular markers is complicated by introgression and incomplete lineage sorting between species. Recent modeling suggests that fixed genetic differences between species are highly related to rates of intraspecific gene flow. However, it remains unclear whether such differences are due to high levels of intraspecific gene flow overriding the spread of introgressed alleles or favoring rapid lineage sorting between species. In pines, chloroplast (cp) and mitochondrial (mt) DNAs are normally paternally and maternally inherited, respectively, and thus their relative rates of intraspecific gene flow are expected to be high and low, respectively. In this study, we used two pine species with overlapping geographical distributions in southeast China, P. massoniana and P. hwangshanensis, as a model system to examine the association between organelle gene flow and variation within and between species. We found that cpDNA variation across these two pine species is more species specific than mtDNA variation and almost delimits taxonomic boundaries. The shared mt/cp DNA genetic variation between species shows no bias in regard to parapatric versus allopatric species’ distributions. Our results therefore support the hypothesis that high intraspecific gene flow has accelerated cpDNA lineage sorting between these two pine species.     

Interspecific phylogenetic analysis enhances intraspecific phylogeographical inference: a case study in Pinus lambertiana

Pinus lambertiana (sugar pine) is an economically and ecologically important conifer with a 1600-km latitudinal range extending from Oregon, USA, to northern Baja California, Mexico. Like all North American white pines (subsect. Strobus), sugar pine is highly susceptible to white pine blister rust, a disease caused by the fungus Cronartium ribicola. We conducted a chloroplast DNA (cpDNA) survey of Pinus subsect. Strobus with comprehensive geographical sampling of P. lambertiana. Sequence analysis of 12 sugar pine individuals revealed strong geographical differentiation for two chloroplast haplotypes. A diagnostic restriction site survey of an additional 72 individuals demarcated a narrow 150-km contact zone in northeastern California. In the contact zone, maternal (megagametophtye) and paternal (embryo) haplotypes were identified in 31 single seeds, demonstrating bidirectional pollen flow extending beyond the range of maternal haplotypes. The frequencies of the Cr1 allele for white pine blister rust major gene resistance, previously determined for 41 seed zones, differ significantly among seed zones that are fixed for the alternate haplotypes, or contain a mixture of both haplotypes. Interspecific phylogenetic analysis reveals that the northern sugar pine haplotype belongs to a clade that includes Pinus albicaulis (whitebark pine) and all of the East Asian white pines. Furthermore, there is little cpDNA divergence between northern sugar pine and whitebark pine (dS = 0.00058). These results are consistent with a Pleistocene migration of whitebark pine into North America and subsequent chloroplast introgression from whitebark pine to sugar pine. This study demonstrates the importance of placing phylogeographical results in a broader phylogenetic context.     

Population structure of a lodgepole pine (Pinus contorta) and jack pine (P. banksiana) complex as revealed by random amplified polymorphic DNA.

We studied the population structure of a lodgepole (Pinus contorta Dougl.) and jack pine (Pinus banksiana Lamb.) complex in west central Alberta and neighboring areas by assessing random amplified polymorphic DNA (RAPD) variability in 23 lodgepole pine, 9 jack pine, and 8 putative hybrid populations. Of 200 random primers screened, 10 that amplified 39 sharp and reproducible RAPDs were chosen for the study. None of the 39 RAPDs were unique to the parental species. RAPD diversity ranged from 0.085 to 0.190 among populations and averaged 0.143 for lodgepole pine, 0.156 for jack pine, 0.152 for hybrids, and 0.148 for all 40 populations. The estimated population differentiation based on G(ST) was 0.168 for hybrids, 0.162 for lodgepole pine, 0.155 for jack pine, and 0.247 across all 40 populations. Cluster analysis of genetic distances generally separated jack pine from lodgepole pine and hybrids, but no division could be identified that further separated lodgepole pine from hybrids. The observed weak to mild trend of "introgression by distance" in the complex and neighbouring areas was consistent with the view that introgressive hybridization between lodgepole and jack pines within and outside the hybrid zone may have been through secondary contact and primary intergradation, respectively.     

DNA from bird-dispersed seed and wind-disseminated pollen provides insights into postglacial colonization and population genetic structure of whitebark pine (Pinus albicaulis)

Uniparentally inherited mitochondrial (mt)DNA and chloroplast (cp)DNA microsatellites (cpSSRs) were used to examine population genetic structure and biogeographic patterns of bird-dispersed seed and wind-disseminated pollen of whitebark pine (Pinus albicaulis Engelm.). Sampling was conducted from 41 populations throughout the range of the species. Analyses provide evidence for an ancestral haplotype and two derived mtDNA haplotypes with distinct regional distributions. An abrupt contact zone between mtDNA haplotypes in the Cascade Range suggests postglacial biogeographic movements. Among three cpSSR loci, 42 haplotypes were detected within 28 cpSSR sample populations that were aggregated into six regions. Analysis of molecular variance (amova) was used to determine the hierarchical genetic structure of cpSSRs. amova and population pairwise comparisons (FST ) of cpSSR, and geographical distribution of mtDNA haplotypes provide insights into historical changes in biogeography. The genetic data suggest that whitebark pine has been intimately tied to climatic change and associated glaciation, which has led to range movements facilitated by seed dispersal by Clark's nutcracker (Nucifraga columbiana Wilson). The two hypotheses proposed to explain the genetic structure are: (i) a northward expansion into Canada and the northern Cascades in the early Holocene; and (ii) historical gene flow between Idaho and the Oregon Cascades when more continuous habitat existed in Central Oregon during the late Pleistocene. Genetic structure and insights gained from historical seed movements provide a basis on which to develop recovery plans for a species that is at risk from multiple threats.     

Spatial distribution of cpDNA and mtDNA haplotypes in a hybrid zone betweenPinus pumila andP. parviflora var.pentaphylla (Pinaceae)

Pinus species exhibit paternal chloroplast inheritance and maternal mitochondrial inheritance. This independent inheritance of two cytoplasmic genomes provides an exceptional environment for discriminating female (seeds) and male (pollen) components of gene flow across hybridizing species. We obtained mitochondrial genetic markers diagnostic toP. parviflora var.pentaphylla andP. pumila by PCR amplification of the intron ofnad1 on mtDNA, and examined the spatial-distribution pattern of the mtDNA haplotypes in a hybrid zone betweenP. parviflora var.pentaphylla andP. pumila in the Tanigawa Mountains of Japan. These data, in conjunction with previous information on cpDNA haplotypes and needle morphology, revealed contrastive patterns of introgression of two cytoplasmic genomes. CpDNA introgression has occurred uni-directionally fromP. parviflora var.pentaphylla toP. pumila. Conversely, mtDNA introgression has occurred in the opposite direction, fromP. pumila toP. parviflora var.pentaphylla. Levels of introgression are roughly equivalent for cpDNA and mtDNA. The contrastive spatial distribution pattern of cpDNA and mtDNA haplotypes could be caused by differential movement of seeds and pollen for interspecific genetic exchange.     

Cytoplasmic composition in Pinus densata and population establishment of the diploid hybrid pine

Sequence and restriction site analyses of the paternally inherited chloroplast rbcL gene and maternally inherited mitochondrial nad1 fragments from the same set of populations and individuals were used to investigate cytoplasmic composition and population establishment of Pinus densata, a diploid pine that originated through hybridization between P. tabuliformis and P. yunnanensis. Two variable sites and three chlorotypes (TT, TC and GC) were detected on the rbcL gene of the three pines. P. densata harboured the three chlorotypes, two of which (TT, GC) were characteristic of the parental species, respectively. The third chlorotype (TC) was distributed extensively in seven of the 10 P. densata populations analysed, and might represent a mutation type or have been derived from an extinct parent. The distribution of chlorotypes, together with that of mitotypes, indicated that significant founder effect and backcross happened during the population establishment of the hybrid pine. P. tabuliformis and P. yunnanensis had acted as both mother and father donors, i.e. bi-directional gene flow existed between the two parental species in the past. Population differentiation of P. densata is high, as detected from the cytoplasmic genomes: GST = 0.533 for cpDNA and GST = 0.905 for mtDNA. The differences in cytoplasmic composition among the hybrid populations suggest that the local populations have undergone different evolutionary histories.     

Assessing genetic diversity and structure of fragmented populations of eastern white pine (Pinus strobus) and western white pine (P. monticola) for conservation management

Aims Many pine populations in Canada have fragmented distributions resulting from the effects of glaciations, overharvesting and white pine blister rust infections. Forest fragmentation can modify gene flow and reduce genetic diversity. Selective logging can reduce the density of trees, thereby altering mating patterns and increasing inbreeding. The hypothesis of the present study is that forest fragmentation will not increase inbreeding and will have no effect on genetic diversity parameters in the Canadian Pinus moniticola and P. strobus populations targeted because of (i) the long life span of the pine species, (ii) outbreeding and self-incompatibility of P. monticola and P. strobus and (iii) wind pollination resulting in high gene flow among populations. We studied the genetic diversity of P. strobus across its range in Canada, and we completed a detailed analysis of the genetic structure of P. monticola populations from western Canada using microsatellites genetic markers.Methods Seed samples from 10 P. monticola populations and 10 P. strobus populations were collected from western and eastern Canada, respectively. The mother trees included in seed lots were representative of each stand. Genomic DNA extracted from each sample was amplified with microsatellite primers. The intra- and interpopulation genetic diversity parameters were assessed using Popgene and Genepop softwares and the genetic distances among populations within each species using the PowerMarker software.Important findings Pinus monticola and P. strobus exhibited moderate to high genetic diversity. Also, both species showed low levels of inbreeding despite the geographic isolation and small stand size. Gene flow estimates were high and population differentiation values were relatively low for these fragmented forest sites.     

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 phylogeographic structure and postglacial history of balsam fir (Abies balsamea), a transcontinental North American boreal conifer, was inferred using mitochondrial DNA (mtDNA) and chloroplast DNA (cpDNA) markers. Genetic structure among 107 populations (mtDNA data) and 75 populations (cpDNA data) was analyzed using Bayesian and genetic distance approaches. Population differentiation was high for mtDNA (dispersed by seeds only), but also for cpDNA (dispersed by seeds and pollen), indicating that pollen gene flow is more restricted in balsam fir than in other boreal conifers. Low cpDNA gene flow in balsam fir may relate to low pollen production due to the inherent biology of the species and populations being decimated by recurrent spruce budworm epidemics, and/or to low dispersal of pollen grains due to their peculiar structural properties. Accordingly, a phylogeographic structure was detected using both mtDNA and cpDNA markers and population structure analyses supported the existence of at least five genetically distinct glacial lineages in central and eastern North America. Four of these would originate from glacial refugia located south of the Laurentide ice sheet, while the last one would have persisted in the northern Labrador region. As expected due to reduced pollen-mediated gene flow, congruence between the geographic distribution of mtDNA and cpDNA lineages was higher than in other North American conifers. However, concordance was not complete, reflecting that restricted but nonetheless detectable cpDNA gene flow among glacial lineages occurred during the Holocene. As a result, new cpDNA and mtDNA genome combinations indicative of cytoplasmic genome capture were observed.     

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.     

DIRECT AND INDIRECT ESTIMATES OF SEED VERSUS POLLEN MOVEMENT WITHIN A POPULATION OF PONDEROSA PINE

We examined the spatial distribution of maternally inherited mitochondrial DNA and paternally inherited chloroplast DNA polymorphisms in a permanently marked stand of ponderosa pine (Pinus ponderosa Laws). Movement of maternally inherited mtDNA occurs only via seed dispersal, and mtDNA haplotypes showed significant patch structure. Moreover, individuals within patches identified by mtDNA haplotypes were related approximately as half-sibs based upon analysis of allozyme genotypes. Thus, seed dispersal is limited within the population, and creates matrilineal clusters in space. By contrast, paternally inherited cpDNA is dispersed by movement of both seed and pollen. Chloroplast DNA polymorphisms showed no evidence of patch structure, but rather a weak (and nonsignificant) trend toward hyperdispersion, suggesting nearly unlimited movement of pollen among trees within this stand. Two of the trees had unique allozyme alleles, which were used to directly measure pollen movement away from those trees. Marked pollen was as likely to disperse across the population as it was to fertilize near neighbors.     

Comparative population structure and genetic diversity of Arceuthobium americanum (Viscaceae) and its Pinus host species: insight into host-parasite evolution in parasitic angiosperms

In a recent study we revealed that the parasitic angiosperm Arceuthobium americanum is comprised of three distinct genetic races, each associated with a different host in regions of allopatry. In order to assess the role of host identity and geographical isolation on race formation in A. americanum, we compared the genetic population structure of this parasite with that of its three principal hosts, Pinus banksiana, Pinus contorta var. latifolia and Pinus contorta var. murrayana. Despite the fact that A. americanum was divided into three genetic races, hosts were divided into only two genetic groups: (i) Pinus banksiana and hybrids, and (ii) P. contorta var. latifolia and var. murrayana. These findings suggest that factors such as geographical isolation and adaptation to different environmental conditions are important for race formation in the absence of host-driven selection pressures. To assess factors impacting population structure at the fine-scale, genetic and geographical distance matrices of host and parasite were compared within A. americanum races. The lack of a relationship between genetic and geographical distance matrices suggests that isolation-by-distance plays a negligible role at this level. The effect of geographical isolation may have been diminished because of the influence of factors such as random seed dispersal by animal vectors or adaptation to nongeographically patterned environmental conditions. Host-parasite interactions might also have impacted the fine-scale structure of A. americanum because the parasite and host were found to have similar patterns of gene flow.     

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.     

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.     

More introgression with less gene flow: chloroplast vs. mitochondrial DNA in the Picea asperata complex in China, and comparison with other Conifers

Recent work has suggested that rates of introgression should be inversely related to levels of gene flow because introgressed populations cannot be 'rescued' by intraspecific gene flow if it is too low. Mitochondrial and chloroplast DNA (mtDNA and cpDNA) experience very different levels of gene flow in conifers due to their contrasted maternal and paternal modes of transmission, hence the prediction that mtDNA should introgress more readily than cpDNA in this group. Here, we use sequence data from both mtDNA and cpDNA to test this hypothesis in a group of closely related spruces species, the Picea asperata complex from China. Nine mitochondrial and nine chloroplast haplotypes were recovered from 459 individuals in 46 natural populations belonging to five species of the Picea asperata complex. Low variation was found in the two mtDNA introns along with a high level of differentiation among populations ( G _ST = 0.90). In contrast, we detected higher variation and lower differentiation among populations at cpDNA markers ( G _ST = 0.56), a trend shared by most conifer species studied so far. We found that cpDNA variation, although far from being fully diagnostic, is more species-specific than mtDNA variation: four groups of populations were identified using cpDNA markers, all of them related to species or groups of species, whereas for mtDNA, geographical variation prevails over species differentiation. The literature suggests that mtDNA haplotypes are often shared among related conifer species, whereas cpDNA haplotypes are more species-specific. Hence, increased intraspecific gene flow appears to decrease differentiation within species but not among species.     

HYBRIDIZATION AND CHLOROPLAST DNA VARIATION IN A PINUS SPECIES COMPLEX FROM ASIA

Heterologous hybridization of chloroplast DNA (cpDNA) involving 30 endonucleaseprobe combinations was used to analyze cpDNA variation in multiple individuals and populations of Pinus tabulaeformis (Carr.), Pinus yunnanensis (Franchèt) and Pinus massoniana (Lamb.). Restriction fragment patterns detected by several combinations distinguished among the three species. The obtained cpDNA markers were subsequently used to examine cpDNA variation of Pinus densata (Masters), a putative tertiary hybrid between P. tabulaeformis and P. yunnanensis. The analysis demonstrated that P. densata populations harbor three different haplotypes. Two of these haplotypes are characteristic of P. tabulaeformis and P. yunnanensis. However, the third haplotype found in P. densata appears to be absent in other extant Asian Pinus species. It is suggested that the observed cpDNA composition of P. densata populations is a result of past hybridization involving P. tabulaeformis, P. yunnanensis, and a third unknown or extinct taxon. Chloroplast DNA polymorphism in P. densata was much greater than that for nuclear allozyme markers in this and the other Pinus species. Population differentiation was also substantial in P. densata and exceeded that for allozyme markers. In contrast, no cpDNA polymorphism was detected in populations of P. tabulaeformis, P. yunnanensis, and P. massoniana. The study suggests that interspecific gene exchange may lead to the creation of stable cpDNA polymorphism in conifer hybrids.     

A mitochondrial DNA minisatellite reveals the postglacial history of jack pine (Pinus banksiana), a broad-range North American conifer

Jack pine (Pinus banksiana Lamb.) is a broadly distributed North American conifer and its current range was covered by the Laurentian ice sheet during the last glacial maximum. To infer about the history and postglacial colonization of this boreal species, range-wide genetic variation was assessed using a new and highly variable minisatellite-like marker of the mitochondrial genome. Among the 543 trees analysed, 14 distinct haplotypes were detected, which corresponded to different repeat numbers of the 32-nucleotide minisatellite-like motif. Several haplotypes were rare with limited distribution, suggesting recent mutation events during the Holocene. At the population level, an average of 2.6 haplotypes and a mean haplotype diversity (H) of 0.328 were estimated. Population subdivision of genetic diversity was quite high with G(ST) and R(ST) values of 0.569 and 0.472, respectively. Spatial analyses identified three relatively homogeneous groups of populations presumably representative of genetically distinct glacial populations, one west and one east of the Appalachian Mountains in the United States and a third one presumably on the unglaciated northeastern coastal area in Canada. These results indicate the significant role of the northern part of the US Appalachian Mountains as a factor of vicariance during the ice age. A fourth distinct group of populations was observed in central Québec where the continental glacier retreated last. It included populations harbouring haplotypes present into the three previous groups, and it had higher level of haplotype diversity per population (H = 0.548) and lower population differentiation (G(ST) = 0.265), which indicates a zone of suture or secondary contact between the migration fronts of the three glacial populations. Introgression from Pinus contorta Dougl. var. latifolia Engelm. was apparent in one western population from Alberta. Altogether, these results indicate that the mitochondrial DNA variation of jack pine is geographically highly structured and it correlates well with large-scale patterns emerging from recent phylogeographical studies of other tree boreal species in North America.