High molecular diversity in the true service tree (Sorbus domestica) despite rareness: data from Europe with special reference to the Austrian occurrence

Abstract

Background and aims: Sorbus domestica (Rosaceae) is one of the rarest deciduous tree species in Europe and is characterized by a scattered distribution. To date, no large-scale geographic studies on population genetics have been carried out. Therefore, the aims of this study were to infer levels of molecular diversity across the major part of the European distribution of S. domestica and to determine its population differentiation and structure. In addition, spatial genetic structure was examined together with the patterns of historic and recent gene flow between two adjacent populations.

Methods: Leaf or cambium samples were collected from 17 populations covering major parts of the European native range from north-west France to south-east Bulgaria. Seven nuclear microsatellites and one chloroplast minisatellite were examined and analysed using a variety of methods.

Key results: Allelic richness was unexpectedly high for both markers within populations (mean per locus: 3·868 for nSSR and 1·647 for chloroplast minisatellite). Moreover, there was no evidence of inbreeding (mean Fis = -0·047). The Italian Peninsula was characterized as a geographic region with comparatively high genetic diversity for both genomes. Overall population differentiation was moderate (FST = 0·138) and it was clear that populations formed three groups in Europe, namely France, Mediterranean/Balkan and Austria. Historic gene flow between two local Austrian populations was high and asymmetric, while recent gene flow seemed to be disrupted.

Conclusions: It is concluded that molecular mechanisms such as self-incompatibility and high gene flow distances are responsible for the observed level of allelic richness as well as for population differentiation. However, human influence could have contributed to the present genetic pattern, especially in the Mediterranean region. Comparison of historic and recent gene flow may mirror the progress of habitat fragmentation in eastern Austria.

Keywords: Chloroplast DNA; Rosaceae; Sorbus domestica; gene flow; genetic variability; habitat fragmentation; nuclear microsatellites; self-incompatibility; true service tree.

Fig. 1.

cpDNA diversity of Sorbus domestica L. in Europe. Circle size is proportional to sample size. The barplot on the left shows the overall frequency of each allele. The green layer indicates the actual distribution of S. domestica according to the EUFORGEN-network.

Fig. 2.

UPGMA consensus tree created out of 1000 bootstrapped matrices using Cavalli-Sforza and Edwards chord distance. Numbers beside the branches indicate bootstrap values.

Fig. 3.

Results of the principal co-ordinate analysis (PCoA) using Nei’s standard genetic distance. The first two axes are shown and explain 74·1 % of the observed variation.

Fig. 4.

Allelic distance according to Gregorius (1974). The distance of each group to the residual gene pool is represented by the radius and the indicated numbers in parentheses. The angle of each segment shows the relative sample size. The thin line shows the average allelic distance.

Fig. 5.

Genetic clustering barplot using the software Structure. The results for K-values from 2 to 4 are shown.