Species-specific RAPD fingerprints for the closely related Picea mariana and P. rubens

Postglacial phylogeography, admixture, and evolution of red spruce (Picea rubens Sarg.) in Eastern North America

Climate change is a major evolutionary force that can affect the structure of forest ecosystems worldwide. Red spruce (Picea rubens Sarg.) has recently faced a considerable decline in the Southern Appalachians due to rapid environmental change, which includes historical land use, and atmospheric pollution. In the northern part of its range, red spruce is sympatric with closely related black spruce (Picea mariana (Mill.) B.S.P.), where introgressive hybridization commonly occurs. We investigated range-wide population genetic diversity and structure and inferred postglacial migration patterns and evolution of red spruce using nuclear microsatellites. Moderate genetic diversity and differentiation were observed in red spruce. Genetic distance, maximum likelihood and Bayesian analyses identified two distinct population clusters: southern glacial populations, and the evolutionarily younger northern populations. Approximate Bayesian computation suggests that patterns of admixture are the result of divergence of red spruce and black spruce from a common ancestor and then introgressive hybridization during post-glacial migration. Genetic diversity, effective population size (Ne) and genetic differentiation were higher in the northern than in the southern populations. Our results along with previously available fossil data suggest that Picea rubens and Picea mariana occupied separate southern refugia during the last glaciation. After initial expansion in the early Holocene, these two species faced a period of recession and formed a secondary coastal refugium, where introgressive hybridization occurred, and then both species migrated northward. As a result, various levels of black spruce alleles are present in the sympatric red spruce populations. Allopatric populations of P. rubens and P. mariana have many species-specific alleles and much fewer alleles from common ancestry. The pure southern red spruce populations may become critically endangered under projected climate change conditions as their ecological niche may disappear.

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.

The genomic architecture and association genetics of adaptive characters using a candidate SNP approach in boreal black spruce

BACKGROUND

The genomic architecture of adaptive traits remains poorly understood in non-model plants. Various approaches can be used to bridge this gap, including the mapping of quantitative trait loci (QTL) in pedigrees, and genetic association studies in non-structured populations. Here we present results on the genomic architecture of adaptive traits in black spruce, which is a widely distributed conifer of the North American boreal forest. As an alternative to the usual candidate gene approach, a candidate SNP approach was developed for association testing.

RESULTS

A genetic map containing 231 gene loci was used to identify QTL that were related to budset timing and to tree height assessed over multiple years and sites. Twenty-two unique genomic regions were identified, including 20 that were related to budset timing and 6 that were related to tree height. From results of outlier detection and bulk segregant analysis for adaptive traits using DNA pool sequencing of 434 genes, 52 candidate SNPs were identified and subsequently tested in genetic association studies for budset timing and tree height assessed over multiple years and sites. A total of 34 (65%) SNPs were significantly associated with budset timing, or tree height, or both. Although the percentages of explained variance (PVE) by individual SNPs were small, several significant SNPs were shared between sites and among years.

CONCLUSIONS

The sharing of genomic regions and significant SNPs between budset timing and tree height indicates pleiotropic effects. Significant QTLs and SNPs differed quite greatly among years, suggesting that different sets of genes for the same characters are involved at different stages in the tree's life history. The functional diversity of genes carrying significant SNPs and low observed PVE further indicated that a large number of polymorphisms are involved in adaptive genetic variation. Accordingly, for undomesticated species such as black spruce with natural populations of large effective size and low linkage disequilibrium, efficient marker systems that are predictive of adaptation should require the survey of large numbers of SNPs. Candidate SNP approaches like the one developed in the present study could contribute to reducing these numbers.

Species-diagnostic and species-specific DNA sequences evenly distributed throughout pine and spruce chromosomes

Genome organization in the family Pinaceae is complex and largely unknown. The main purpose of the present study was to develop and physically map species-diagnostic and species-specific molecular markers in pine and spruce. Five RAPD (random amplified polymorphic DNA) and one ISSR (inter-simple sequence repeat) species-diagnostic or species-specific markers for Picea mariana, Picea rubens, Pinus strobus, or Pinus monticola were identified, cloned, and sequenced. In situ hybridization of these sequences to spruce and pine chromosomes showed the sequences to be present in high copy number and evenly distributed throughout the genome. The analysis of centromeric and telomeric regions revealed the absence of significant clustering of species-diagnostic and species-specific sequences in all the chromosomes of the four species studied. Both RAPD and ISSR markers showed similar patterns.

Genetic diversity of Picea asperata populations based on RAPDs

The genetic diversity of ten natural populations of Picea asperata Mast. were studied using RAPD markers. A total of 160 reproducible fragments were produced from the ten primers used. The mean number of fragments detected per individual was 114.7. Altogether 120 fragments were polymorphic among the ten populations, none of them were found to be population-specific. Nei's expected heterozygosity (H (e)) ranged from 0.233 to 0.269, and the average was 0.247. The analysis of molecular variance revealed that the coefficient of gene differentiation among populations, based on F (ST) and the unbiased estimate PhiST, equaled 0.224 and 0.290, respectively. Such high values indicate that there is significant differentiation among populations, which could result from several factors, including restricted gene flow between populations (Nm = 0.866). Founder events may be another factor attributing to the high level of genetic differentiation. In addition, it was discovered that the geographic distribution is not correlated with the genetic distances among the populations of P. asperata.

A composite linkage map from two crosses for the species complex Picea mariana x Picea rubens and analysis of synteny with other Pinaceae

Four individual linkage maps were constructed from two crosses for the species complex Picea mariana (Mill.) B.S.P. x Picea rubens Sarg in order to integrate their information into a composite map and to compare with other Pinaceae. For all individual linkage maps, 12 major linkage groups were recovered with 306 markers per map on average. Before building the composite linkage map, the common male parent between the two crosses made it possible to construct a reference linkage map to validate the relative position of homologous markers. The final composite map had a length of 2,319 cM (Haldane) and contained a total of 1,124 positioned markers, including 1,014 AFLPs, 3 RAPDs, 53 SSRs, and 54 ESTPs, assembled into 12 major linkage groups. Marker density of the composite map was statistically homogenous and was much higher (one marker every 2.1 cM) than that of the individual linkage maps (one marker every 5.7 to 7.1 cM). Synteny was well conserved between individual, reference, and composite linkage maps and 94% of homologous markers were colinear between the reference and composite maps. The combined information from the two crosses increased by about 24% the number of anchor markers compared to the information from any single cross. With a total number of 107 anchor markers (SSRs and ESTPs), the composite linkage map is a useful starting point for large-scale genome comparisons at the intergeneric level in the Pinaceae. Comparisons of this map with those in Pinus and Pseudotsuga allowed the identification of one breakdown in synteny where one linkage group homologous to both Picea and Pinus corresponded to two linkage groups in Pseudotsuga. Implications for the evolution of the Pinaceae genome are discussed.

Application of ISSR, RAPD, and cytological markers to the certification of Picea mariana, P. glauca, and P. engelmannii trees, and their putative hybrids

Picea glauca (white spruce) and P. engelmannii (Engelmann spruce) are so similar and integrated that it is impossible to distinguish between them and their hybrids using morphological characteristics. Although natural hybrids between P. glauca and P. mariana (black spruce) do not generally occur, even though the 2 species are sympatric in North America, a first-generation hybrid, called the Rosendahl spruce, has been reported in the literature. In this study, several inter-simple sequence repeat (ISSR) markers were developed, as were randomly amplified polymorphic DNA (RAPD) markers, to certify spruce trees and their hybrids. ISSR fingerprinting was more efficient than RAPD assay; it detected 70% polymorphic DNA markers among the spruce species analyzed, whereas RAPD fingerprinting detected only 53%. Species-diagnostic ISSR and RAPD markers differentiating P. glauca from P. engelmannii and P. mariana were cloned and sequenced. Molecular certification of the spruce samples analyzed confirmed that all the seeds from interior spruce populations were true hybrids of P. glauca and P. engelmannii. But the analysis of seeds derived from the putative Rosendahl spruce indicated that this tree is likely a pure P. glauca genotype, rather than a hybrid of P. glauca and P. mariana. These data were confirmed by cytological analyses. Further analysis, using a more sensitive DNA amplification method with designed primers flanking the species-diagnostic ISSR and RAPD markers, revealed that such sequences are not generally species-specific because they are present in other spruce 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.

Reproductive barriers and hybridity in two spruces, Picea rubens and Picea mariana, sympatric in eastern North America

Hybridization between red spruce (Picea rubens Sarg.) and black spruce (Picea mariana (Mill.) BSP), late- and early-successional species, respectively, has resulted in identification and management problems. We investigated the nature and magnitude of reproductive and life-cycle success barriers in controlled intra- and inter-specific crosses of red and black spruce. We quantified a number of reproductive, germination, phenological, and performance traits, and examined traits by parental pedigree and hybrid index. Species' pollen had no effect on number of aborted or nonpollinated ovules. Controlled intraspecific crosses had, on average, 6.6 times more filled seeds than interspecific crosses. Cone and seed morphometric traits were species specific, with seed traits showing negative hybridization effects on both species. Germination, cotyledon number, and seedling height had significant species-specific traits, with hybrids showing an additive or slightly negative heterosis. Severe, negative heterosis appears to be of limited importance as an isolating barrier between red and black spruce. Reproductive phenology was remarkably similar among species and hybrid progenies when grown in common garden experiments. Crossability barriers are clearly paramount in maintaining the separation of the species. Ecological separation based on ecophysiological differences (e.g., shade tolerance) also represents an important prezygotic barrier for minimizing the negative effects of hybridization (e.g., postzygotic inviability) on reproductive fitness.Key words: cone, seed and seedling traits, genetic variation, phenology, red and black spruce.

Molecular genetic diversity of the French-American grapevine hybrids cultivated in North America

French-American hybrid grapevines are most popular in eastern and mid-western North America: they are hardy cultivars derived from crosses between the European Vitis vinifera and American wild vines. The aim of this study was to characterize their genetic background using 6 microsatellite (SSR) markers and a set of 33 diagnostic RAPD markers. The latter were reproducible with different PCR thermal cyclers. Two SSR loci were found to be synonymous, VrZAG47 and VVMD27. The DNA profile frequencies estimated for each cultivar were much lower with multi-locus SSR data than that obtained from multi-fragment RAPD data. There was no significant correlation between the multi-locus DNA profile frequencies derived from SSRs and those from RAPDs. Estimates of genetic diversity derived from SSRs were generally higher and the average similarity between cultivars was generally lower than values reported for subgroups of V. vinifera, in accordance with expectations for hybrid cultivars. The phenetic relationships depicted by UPGMA (unweighted pair-group method with arithmetic averaging) and neighbor-joining analyses of microsatellite data were congruent and, to a large extent, in agreement with the known pedigree or history of each cultivar. A major dichotomy was observed between one group where the known genetic background was dominated by the North American Vitis riparia and Vitis labrusca, and another one where the genetic background was dominated by the European V. vinifera. Two Kulhmann varieties thought to be synonymous were found to be different, though closely related.

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.

Old-growth red spruce forests as reservoirs of genetic diversity and reproductive fitness

Old-growth forests are assumed to be potential reservoirs of genetic diversity for the dominant tree species, yet there is little empirical evidence for this assumption. Our aim was to characterize the relationship of stand traits, such as age, height and stem diameter, with the genetic and reproductive status of old-growth and older second-growth stands of red spruce (Picea rubens Sarg.) in eastern Canada. We found strong relationships between height growth (a fitness trait) and measures of genetic diversity based on allozyme analyses in red spruce. The negative relationship between height and the proportion of rare alleles suggests that high proportions of these rare alleles may be deleterious to growth performance. Latent genetic potential, however, showed a significant and positive relationship with height. Stand age was not correlated to height, but was correlated to seedling progeny height. In late-successional species such as red spruce, age and size (e.g., height and stem diameter) relationships may be strongly influenced by local stand disturbance dynamics that determine availability of light, growing space, moisture and nutrients. In larger and older stands, age appeared to provide a good surrogate measure or indicator for genetic diversity and progeny height growth. However, in smaller and more isolated populations, these age and fitness relationships may be strongly influenced by the effects of inbreeding and genetic drift. Therefore, older populations or old-growth forests may represent superior seed sources, but only if they are also of sufficient size and structure (e.g., stem density and spatial family structure) to avoid the effects of inbreeding and genetic drift. Thus, larger and older forests appear to have an important evolutionary role as reservoirs of both genetic diversity and reproductive fitness. Given the rapid environmental changes anticipated (as a result of climate change, increasing population isolation through fragmentation, or following the introduction of exotic pests and diseases) these older populations of trees may have a valuable function in maintaining the adaptive potential of tree species.

Architectural and genotypic variation in the clonal shrub Taxus canadensis as determined from random amplified polymorphic DNA and amplified fragment length polymorphism

Canada yew (Taxus canadensis Marsh.) is a clonal species with two types of ramets. The most common type has bilateral symmetry and alternate branching. The radial symmetry and opposite branching of the rarer type are similar to the architecture expressed in a young plant grown from seed. Architectural analysis was conducted on four colonies of increasing age and size. A genetic analysis using random amplified polymorphic DNA (RAPD) and amplified fragment length polymorphism (AFLP) was conducted on 122 ramets, 93 sampled from nine colonies from a single population, 24 from eight other populations, and five from other Taxus species. Analysis of the architectural development sequence shows that yew follows the same architectural model as, for example, species of Abies or Picea. This model constitutes a level of organization intermediate between that of the ramet and of the genet. The molecular analyses confirm the greater variability of AFLPs over RAPDs and indicate that architecture is not directly related to genotypic variation. Ramets with radial symmetry have slightly higher genetic variation, on average, than ramets with bilateral symmetry with 95% degree of confidence. The molecular analyses show that the level of resolution of the AFLPs is adequate to characterize 98% of the ramets of a colony. Between pairs of physically connected ramets, some random genotypic differences are observed, supporting the hypothesis of genetic mosaicism in plants, due to somaclonal variations.Key words: architecture, clonal plant, intraclonal variation, molecular markers, somaclonal mutations.

Molecular evidence of natural hybridization between abies veitchii and A. homolepis (Pinaceae) revealed by chloroplast, mitochondrial and nuclear DNA markers

Sub-alpine Abies veitchii and A. homolepis are distributed in the central part of Honshu Island, Japan, and their habitats are segregated vertically. These species sometimes form a mixed forest in the overlapping area of the two species, that is, in the upper limit of the A. homolepis habitat and the lower limit of A. veitchii. These species have been considered to be distantly related because they were classified into different sections by most conventional classifications. No natural hybridization has been reported between the two species. The aim of this study was to demonstrate, through the use of molecular markers, whether natural hybridization takes place between these two species at two experimental sites on Mt. Fuji, where the species occur naturally. DNA markers from paternally inherited chloroplast DNA (cpDNA), maternally inherited mitochondrial DNA (mtDNA) and biparentally inherited nuclear DNA (nDNA), were used for this study. As organelle DNA markers, polymerase chain reaction-single strand conformation polymorphism (PCR-SSCP) markers were developed to determine the maternal and paternal species for each individual. Two of 334 individuals possessed a cpDNA haplotype derived from A. homolepis and a mtDNA haplotype from A. veitchii. Furthermore, the nDNA of these two individuals was analysed using the random amplified polymorphic DNA (RAPD) assay to investigate their genomic composition. RAPD analysis indicated that the nuclear genomes of the two individuals were derived from both species. We conclude that A. veitchii and A. homolepis produce natural hybrids, and that their systematic relationship should be re-evaluated.

Evidence from sequence-tagged-site markers of a recent progenitor-derivative species pair in conifers

Black spruce (Picea mariana [B.S.P.] Mill.) and red spruce (Picea rubens Sarg.) are two conifer species known to hybridize naturally in northeastern North America. We hypothesized that there is a progenitor-derivative relationship between these two taxa and conducted a genetic investigation by using sequence-tagged-site markers of expressed genes. Based on the 26 sequence-tagged-site loci assayed in this study, the unbiased genetic identity between the two taxa was quite high with a value of 0.920. The mean number of polymorphic loci, the mean number of alleles per polymorphic locus, and the average observed heterozygosity were lower in red spruce (P = 35%, A(P) = 2.1, H(o) = 0.069) than in black spruce (P = 54%, A(P) = 2.9, H(o) = 0.103). No unique alleles were found in red spruce, and the observed patterns of allele distribution indicated that the genetic diversity of red spruce was essentially a subset of that found in black spruce. When considered in combination with ecological evidence and simulation results, these observations clearly support the existence of a progenitor-derivative relationship and suggest that the reduced level of genetic diversity in red spruce may result from allopatric speciation through glaciation-induced isolation of a preexisting black spruce population during the Pleistocene era. Our observations signal a need for a thorough reexamination of several conifer species complexes in which natural hybridization is known to occur.

Indicators of population viability in red spruce, Picea rubens. II. Genetic diversity, population structure, and mating behavior

Red spruce (Picea rubens Sarg.) has become increasingly rare across large portions of its range in eastern North America as a result of a general and widespread decline over the past century. Genetic diversity, population genetic structure, outcrossing rates in the filled seeds, and actual inbreeding levels were characterized in five small, isolated, remnant red spruce populations from the disjunct northwestern limits of its range in Ontario and five populations from the larger, more extensive Maritime populations of Nova Scotia and New Brunswick to determine genetic and reproductive status, to provide some benchmarks for monitoring genetic changes resulting from isolation and restricted population sizes, and to assist the development of restoration and conservation strategies. Thirty-seven allozyme loci coding for 15 enzymes were used for genetic diversity assessments, and six of the most polymorphic loci were used for mating system determination. On average, 29.1% (95% criterion) of the loci were polymorphic, the number of alleles per locus was 1.60, and the observed and expected heterozygosities were 0.097 and 0.100, respectively. The Ontario populations were comparable to or slightly less genetically variable than those from the Maritimes. Only 4.7% of the detected genetic variation was among stands; the remainder was among individuals within stands. The Maritime populations were genetically less differentiated from each other than those in Ontario. With the exception of three Maritime populations clustering tightly in one group, there was no clear separation of Ontario red spruce populations from Maritime red spruce populations based on genetic distance as well as canonical discriminant analyses. The average multilocus (tm) and single-locus (ts) population outcrossing rates were 0.595 and 0.558, respectively, indicating a comparatively high tolerance for inbreeding up to the filled seed stage of development in red spruce. The Ontario populations, on average, showed higher outcrossing rates (tm = 0.654, ts = 0.641) than the Maritime populations (tm = 0.535, ts = 0.475). Individual family outcrossing rates were similar to their respective population outcrossing rates and no significant differences were observed among families within populations for the multilocus estimates. When such high levels of inbreeding in filled seeds were combined with the proportions of empty (post-pollination-aborted) seeds, it appears that actual inbreeding levels may vary from 48 to 86%. The highest inbreeding levels occurred in the smallest, most isolated Ontario populations and in those populations most likely to have been affected by poorer pollination conditions. Allozyme variation indicates that in the short term, extant remnants of Ontario red spruce have maintained their genetic diversity and integrity. For artificial restoration of red spruce in Ontario, local seed sources could be used without undue concern over losses of genetic diversity. However, over the longer term, genetic drift and inbreeding may be expected to result in further losses of genetic diversity and (or) reproductive fitness if population sizes, numbers, and distribution continue to decline.Key words: Picea rubens, allozymes, gene conservation, restoration, genetic diversity, population structure, outcrossing rates, inbreeding.

Indicators of population viability in red spruce, Picea rubens. I. Reproductive traits and fecundity

Red spruce (Picea rubens Sarg.) has experienced a substantial decline across most of its range in eastern North America over the past century and probably also in the disjunct Ontario populations where it now occurs only in small isolated stands. Measurements of cone and seed traits from natural populations were used as indicators of the reproductive and genetic status of red spruce across the northern margins of its range in Canada. Cone and seed traits were quantified to provide reproductive benchmarks for assessing and monitoring population viability. Reduced fecundity and seedling height growth were observed in some of the smallest Ontario populations, suggesting some inbreeding depression in both vegetative and reproductive components of fitness. Nevertheless, the reproductive status of these small isolated Ontario populations compared favorably with the much larger, more extensive Maritime populations in Nova Scotia and New Brunswick. Significantly higher proportions of aborted (nonpollinated) seeds and lower proportions of filled seeds suggested poorer pollination conditions in the Maritimes in 1996. The proportion of empty seed, which was used to estimate inbreeding levels, was significantly and negatively related to seedling height growth. In the short-term, the Ontario populations, which probably represent relatively recent remnants of a broader past distribution, generally appeared to be quite resilient to the effects of small population size on fecundity and progeny fitness. In the longer term, continuing decline in population sizes and numbers may be expected to erode reproductive success and genetic diversity through the effects of inbreeding, genetic drift, and changes in mating behavior. The reproductive indicators described here have general validity for assessing and monitoring reproductive and genetic aspects of population viability in conifers.Key words: Picea rubens, reproductive success, reproductive fitness indicators, inbreeding, population viability, conservation.

RAPD of controlled crosses and clones from the field suggests that hybrids are rare in the Salix alba-Salix fragilis complex

The polyploid Salix alba-Salix fragilis hybrid complex is rather difficult to study when using only morphological characters. Most of the features have a low diagnostic value for unambiguously identifying the hybrids, introgression patterns and population structures, though morphological traits have proved to be useful in making a hybrid index. Morphology and molecular variation from RAPDs were investigated in several case studies on willows from Belgium. A thorough screening of full-sib progenies of interspecific controlled crosses was made to select homologous amplification products. The selected amplified products proved to be useful in a principal coordinate analysis for the estimation of variability of hybrid progenies. On the basis of genetic similarities and ordination analysis, a method for the identification of clones in the field was established using presumed pure species and presumed introgressants. The chosen reference clones were checked against additional European samples of putative pure species to ensure the reliability of the method beyond a regional scale. The RAPDs suggested that both species have kept their gene pools well separated and that hybridization actually does not seem to be a dominating process. The observation that molecular markers do not always follow the morphological traits or allozyme data is discussed.

Negative heterosis not apparent in 22-year-old hybrids of Picea mariana and Picea rubens

Work from the 1970s indicated that, relative to either parent species, crosses between red spruce (Picea rubens Sarg.) and black spruce (Picea mariana (Mill.) B.S.P.) were inferior with respect to both growth and photosynthesis. We re-examined the hypothesis that there is negative heterosis in hybrids of red and black spruce using 22-year-old trees in a common garden study planted on two sites. The trees were the product of controlled crossings and represent a continuum from hybrid class 0 (pure black spruce) to hybrid class 1 (pure red spruce). Progeny of all controlled crosses were measured for height and diameter. A subset of families were measured for gas exchange and were assessed using a hybrid index based on needle color, needle configuration, twig ridges, twig bark color, vegetative bud color, and cone scale morphology. Tree growth rate linearly declined with the increasing proportion of red spruce germplasm (increasing hybrid index). In 1994, intermediate hybrid index classes did not differ in gas exchange from either pure black spruce (hybrid index class 0) or pure red spruce (hybrid index class 1), and in 1996, hybrids displayed slightly higher rates of gas exchange. Thus, negative heterosis was not apparent in 22-year-old trees. Individual tree hybrid index generally agreed with expectation based on midparent means, although the relationship was stronger on the higher productivity site (r2 = 0.91) than the poorer productivity site (r2 = 0.54).Key words: black spruce, heterosis, hybrid, photosynthesis, red spruce.

Comparison of RAPD technique and somatic incompatibility tests for the identification of Phlebiopsis gigantea strains

Somatic incompatibility assays and random amplified polymorphic DNA (RAPD) analysis were assessed and compared for use in monitoring the survival of introduced strains of Phlebiopsis gigantea, a fungal biocontrol agent used against Heterobasidion annosum root rot. Overall there was concordance between the two methods. All incompatible strains exhibited different RAPD profiles whilst compatible ones, with two exceptions, had the same RAPD profile. Somatic incompatibility tests and RAPD profiles generated by three primers, from which 11 markers were retained, could distinguish field-tested strains from among 60 strains from a population collected in Quebec and Ontario. Furthermore, both methods allowed us to clearly demonstrate the presence of inoculated strains 1 year after treatment. The combined use of these techniques provides a valuable tool for discriminating between different P. gigantea strains and conducting epidemiological studies. Key words: Heterobasidion annosum, vegetative incompatibility, random amplified polymorphic DNA, biological control, Pinus resinosa.