Apomictic Mountain Whitebeam (Sorbus austriaca, Rosaceae) Comprises Several Genetically and Morphologically Divergent Lineages

Abstract

The interplay of polyploidisation, hybridization, and apomixis contributed to the exceptional diversity of Sorbus (Rosaceae), giving rise to a mosaic of genetic and morphological entities. The Sorbus austriaca species complex from the mountains of Central and South-eastern Europe represents an allopolyploid apomictic system of populations that originated following hybridisation between S. aria and S. aucuparia. However, the mode and frequency of such allopolyploidisations and the relationships among different, morphologically more or less similar populations that have often been described as different taxa remain largely unexplored. We used amplified fragment length polymorphism (AFLP) fingerprinting, plastid DNA sequencing, and analyses of nuclear microsatellites, along with multivariate morphometrics and ploidy data, to disentangle the relationships among populations within this intricate complex. Our results revealed a mosaic of genetic lineages-many of which have not been taxonomically recognised-that originated via multiple allopolyploidisations. The clonal structure within and among populations was then maintained via apomixis. Our results thus support previous findings that hybridisation, polyploidization, and apomixis are the main drivers of Sorbus diversification in Europe.

Keywords: Sorbus austriaca; apomixis; hybridisation; multiple origins; polyploidy.

Figure 1

Geographic origin (A) and amplified fragment length polymorphism (AFLP) variation (B) of analysed Sorbus accessions. Sampled localities are numbered; details are in Table S1. AFLP clusters within Sorbus subgen. Soraria are colour-coded and labelled with the letters a–l; in (A) their presence in each sampled locality is indicated (the parental taxa S. aria and S. aucuparia are in white). The NeighborNet diagram is supplemented with bootstrap values > 85% derived from a Neighbour-joining analysis (Figure S1); multilocus genotypes (MG’s) derived from nuclear microsatellites; and ploidy data obtained by flow cytometry; dashed lines denote the plastid haplotype affiliation according to the plastid trnT–trnF phylogenetic analysis. Note that only a subset of individuals was sequenced for plastid DNA variation.

Figure 2

Principal coordinate analysis of Jaccard distances among the 25 multilocus genotypes found in 110 accessions of Sorbus subgen. Soraria accessions based on nuclear microsatellite data. Locality numbers correspond to Figure S1A and Table 1 and colours and letters to the AFLP clusters in Figure 1B. Multilocus genotypes belonging to the same AFLP cluster are connected with dashed lines. The numbers in parentheses denote the number of clones per locality.

Figure 3

Bayesian consensus phylogram inferred from phylogenetic analyses of plastid trnT–trnF sequences. Numbers above branches are posterior probabilities > 0.90 and those below branches maximum parsimony bootstrap values > 50%. Locality numbers correspond to Table S1.

Figure 4

Scatterplot of absolute genome size values (2C pg) for analysed Sorbus accessions. Colours correspond to the AFLP clusters (Figure 1B) and locality numbers followed by individual numbers to Figure 1A and Table S1.

Figure 5

Canonical discriminant analysis ((A) DF1 vs. DF2; (B) DF1 vs. DF3) of nine predefined Sorbus austriaca groups (corresponding to the AFLP clusters shown in Figure 1) based on 18 morphological leaf characters.