Genetic diversity and structure were investigated in Fraxinus excelsior, a common
broadleaved tree in Britain. Samples, with geographical coordinates, were collected
from six semi-natural populations, three each from south and north Wales. Six
microsatellite loci were amplified and genetic parameters were calculated. High levels
of polymorphism were found, and expected heterozygosity averaged 0.862 within
populations. An excess of homozygotes was found in all populations, as F,s was 0.184
overall. Low levels of differentiation among populations were recorded, with overall
FsTestimated at 0.017.
In the most intensively studied stand spatial autocorrelation identified weak small-scale
genetic structure. Individuals up to around 700 m showed significant genetic
autocorrelation. In another stand, samples were collected from mature trees, seeds,
naturally regenerated seedlings, and seedlings grown from seed in a nursery.
Comparisons of the genetic diversity among these cohorts showed no differences in
allelic richness, heterozygosity, or homozygote excess. Differentiation between these
different cohorts was also negligible with FsT values of less than 0.02. Analysis of 200
seeds showed outcrossing rates close to one, with most pollen donors unsampled, and
low levels of correlated paternity.
It is concluded that little differentiation exists among populations within the scale
studied here, and so forest regenerative material for planting woodlands may be sourced
widely. It is also concluded that genetic diversity is maintained equally by natural or
artificial regeneration in the present situation where a large number of seed trees are
available. The excess of homozygotes found in all populations is similar to that found
by other workers and may be partially explained by null alleles. It is also possible that
anomalous single-locus patterns may indicate that one or more microsatellite loci are
not neutral but are linked to a gene or genes under selection.