Comparison of fine-scale genetic structure using nuclear microsatellites within two British oakwoods differing in population history

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Abstract

Two oak woods in northern Britain which contained both Quercus robur and Quercus petraea were selected for study. The woods differed in the degree of human interference they had experienced; Roudsea, in the Lake District, is a native wood with no record of planting but has experienced coppicing; Dalkeith, in southern Scotland, has experienced extensive planting and management. All the trees in a selected area of each wood (nearly 700 trees) were mapped and genotyped using six nuclear microsatellite loci. A range of statistical methods was used to assess the diversity and degree of spatial genetic structure present in each wood, and results are compared with published results for a natural oakwood in central France (Petite Charnie). For the unplanted wood at Roudsea, significantly higher genetic diversity and greater spatial genetic structure, at relatively short distances, was detected in the Q. petraea component of the wood relative to that of Q. robur. These observations agree with those from Petit Charnie and indicate that native oak woods across Europe maintain similar levels and structure of genetic variation, and is probably due to differences in seed dispersal mechanism and regeneration dynamics of the species. At Dalkeith, however, significant spatial structure of genetic variation occurred in both species examined, and the additional influence of planting is offered to account for increased genetic structure within this wood. At Roudsea, the influence of coppicing is also discussed to explain the lack of population genetic equilibrium and increased genetic diversity at this site.

Introduction

Many studies of genetic variation within plants have concentrated at the macrogeographic level. Studies of trees, have shown that genetic variation is relatively high and that the majority is partitioned within rather than between populations. However, much less is known about the distribution of variation at the microgeographic, i.e. within population level, although it is recognised that such information would help in assessing the importance of microgeographic evolutionary processes such as the establishment of sibling neighbourhoods and microsite selection effects. There is currently little information regarding the forces which bring about positive structuration over local spatial scales although factors such as assortative mating, restricted seed and pollen dispersal, competition, differential mortality, gap recruitment and human interference can be implicated. As Epperson, 1990, Epperson, 1992 points out, both forest management practices and conservation strategies require selection of genotypes in space and information on intrapopulation genetic diversity is essential for well informed management of the forest ecosystem. Considerable effort has therefore been expended to devise methods of assessing the structure of genetic variation within tree populations using field-collected molecular data and computer simulations (Linhart et al., 1981, Epperson and Allard, 1989, Perry and Knowles, 1991, Schnabel et al., 1991, Wagner et al., 1991, Bacilieri et al., 1994, Berg and Hamrick, 1995, Streiff et al., 1998). The development of cheap, effective, PCR-based molecular marker methods has made it possible to study DNA-encoded variation in a large number of individuals. Microsatellites provide one such applicable marker system, and are loci containing tandemly repeated units of 1–5 bp. Nuclear-encoded microsatellites have now been developed for many tree species including Gliricidia (Dawson et al., 1997), Fraxinus (Lefort et al., 1999), Larix (Volkaert, 1995), Picea (Morgante et al., 1996, Pfeiffer et al., 1997, van de Ven and McNicol, 1996) and Pinus (Smith and Devey, 1994, Kostia et al., 1995, Echt et al., 1996, Soranzo et al., 1998). Due to their high levels of variability, codominantly expressed microsatellites provide ideal markers for the study of intraspecific diversity and population genetic structure within woods. For example, microsatellites were used in a study of Picea abies and demonstrated that the genetic variation found in three Austrian populations was randomly distributed within each wood (Geburek et al., 1998). The European white oaks (Quercus spp.) are an economically important timber group and were one of the first tree genera for which microsatellites were developed (Quercus macrocarpa; Dow et al., 1995). Since then nearly 50 loci have been isolated from white oaks (Quercus petraea (Matt.) Liebl., Steinkellner et al., 1997; Quercus robur L., Streiff et al., 1998). Recent work by Streiff et al. (1998) used isozyme and six microsatellite loci to study the diversity and fine-scale genetic structure within a native French oak wood containing both Q. robur and Q. petraea. The wood (“Petite Charnie”) maintains a high level of genetic diversity. Novel methods of statistical analysis developed for their study demonstrated the wood had a significant, but low spatial genetic structure which was greater for Q. petraea than Q. robur. It remains unknown whether this intrapopulation genetic structure is typical of other oak woods occurring elsewhere in Europe.

Many native British woods are owned by large estates and have reliable records allowing the management and planting history to be assessed for selected populations. For the current study, two woods were selected that contain mixtures of the two native oak species, Q. robur and Q. petraea. The first wood, Roudsea, is located in the Lake District in northern England, and historic estate records confirm its native and autochthonous state. The second wood, Dalkeith, is located in southern Scotland, and although it was originally a native wood, it has a history of management and planting over a long period. The objective of the study was to use microsatellites to measure the level of diversity and spatial genetic structure within the two woods, and to compare the results with those of Streiff et al. (1998) for the oak wood in France (Petite Charnie). In addition, a comparison of the native wood at Roudsea with the partly planted wood at Dalkeith might provide some insight into the impact of management on population diversity and spatial structure.

Section snippets

The location and history of the sampled woods

Two woods containing both Q. robur and Q. petraea were selected for study. Roudsea wood is located near Haverthwaite, Cumbria, England (54.23°N, 3.03°W), detailed estate records show that the wood was managed under a regular coppicing system, but they provide no account of any planting of oak having occurred. The earliest record of the wood in 1650 indicates a similar size of wood to that of today (Petly-Jones, personal communication). Nearly, all the coppiced trees were thinned to a single

Genetic diversity

Every tree sampled at each site displayed a unique genotype when analysed at six microsatellite loci. The six microsatellite loci exhibited high levels of polymorphism in both species with up to 34 alleles detected in Roudsea at the most variable loci (MSQ4 and AG36) (Table 1). The distribution of alleles varied from a normal distribution for locus AG104 to a bimodal pattern for MSQ13. The most frequent allele at each locus was either the same in the two sampled woods or differed by no more

Discussion

The data presented here come from two intensively sampled plots totalling nearly 700 trees, and the study represents one of the largest surveys of fine-scale spatial structure undertaken to date. The results are compared to another dataset of over 300 trees from a French wood, Petite Charnie (Streiff et al., 1998). All studies were undertaken using identical methods which have been checked and validated (Mariette et al., in press). This allows meaningful comparisons of variation in spatial

Acknowledgements

This study was carried out with financial support from the Forestry Commission, the Natural Environment Research Council and the Commission of the European Communities, Agriculture and Fisheries (FAIR) specific RTD programme (CT-FAIR1 PL95-0297). We thank the Duke of Buccleuch and English Nature for granting access to the two sites. We are grateful to Cathleen Baldwin for assistance in labelling and mapping the position of trees in Dalkeith Old Wood. The invaluable help of the Technical Support

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