Genetic analysis of aspen (Populus tremula L. and Populus tremuloides Michx.) in a diversity experiment

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Poplars are model tree species in ecological and genetic studies since they are
widely distributed, easy to propagate, and since more genetic and genomic resources are
available for poplars than for any other woody plant genus. The poplar diversity
experiment (POPDIV) has been established with European and North American aspen
(Populus tremula L., P. tremuloides Michx.) planted in plots representing either a single
deme only or combinations of two, four and eight demes in order to test the influence of
intraspecific genetic diversity on ecosystem functions and services. In the present study,
the most commonly used molecular markers, SSRs (simple sequence repeats) and AFLPs
(amplified fragment length polymorphisms), were applied to conduct the genetic analysis
of the POPDIV experiment. Both markers identified clonal structures in one Swedish
deme and these clones are non-randomly distributed in the POPDIV experimental field.
Large differences with regard to the genetic diversity within aspen demes were observed.
The genetic diversity estimates based on SSR and AFLP markers showed a high
correlation. The North American P. tremuloides deme had the highest level of genetic
diversity; most private alleles both at SSRs and AFLPs were also observed in this deme
which confirmed that it represents another species different from the European P.
tremula. An analysis of molecular variance (AMOVA) revealed that most of the total
genetic diversity was found within demes by both SSR and AFLP markers, but the
genetic differentiation among demes was also high. Pairwise FST values between demes
showed highly significant differentiation at both SSRs and AFLPs. As expected, the
P. tremuloides deme was strongly differentiated from the other P. tremula demes.
UPGMA dendrograms based on genetic distances obtained from both markers showed
that the North American P. tremuloides deme is obviously outgrouped from the other
European P. tremula demes with high bootstrap support values. The complex patterns of
genetic diversity and differentiation resulted in large differences of the genetic variation
within plots. Genetic diversities within plots with a given number of demes were highly
variable. Across all plots, those with a single German deme and those with two demes
showed the lowest and highest level of genetic diversity, respectively. The plots
containing the North American P. tremuloides deme showed higher levels of genetic
diversity compared to the plots including only P. tremula demes, which was obviously
shown at AFLPs. For the plots with a given number of demes (1, 2, 4, 8), their average
genetic diversity values were increased with increasing number of demes included within
plots. However, the genetic variation within plots strongly depended on the specific
mixture, and the highest diversities were observed for plots with only two demes mixed.
Paternity analysis revealed different numbers of paternal trees contributed to both
German single tree progenies, the deme with a high number of pollen donors showed
high level of genetic diversity. The sibship reconstruction revealed a higher number of
full sibling groups for the North American P. tremuloides deme compared to other
demes. Across demes, the genetic diversity revealed by AFLP markers showed a positive
correlation with the number of full sibling groups, indicating that the number of full
sibling groups reflects the effective number of population size which directly determines
the level of genetic diversity. In conclusion, the high and variable levels of genetic
diversity observed within all demes need to be considered in assessments of the impact of
intraspecific diversity of poplars on the diversity of the associated organisms and
ecosystem functions and services. The genetic diversity in a plot strongly depends on the
specific combination of demes; the number of demes included within a plot is a poor
predictor of its diversity.