Hybridization drives evolution of apomicts in Rubus subgenus Rubus: evidence from microsatellite markers

Abstract

Background and aims: Rubus subgenus Rubus is a group of mostly apomictic and polyploid species with a complicated taxonomy and history of ongoing hybridization. The only polyploid series with prevailing sexuality is the series Glandulosi , although the apomictic series Discolores and Radula also retain a high degree of sexuality, which is influenced by environmental conditions and/or pollen donors. The aim of this study is to detect sources of genetic variability, determine the origin of apomictic taxa and validate microsatellite markers by cloning and sequencing.

Methods: A total of 206 individuals from two central European regions were genotyped for 11 nuclear microsatellite loci and the chloroplast trn L- trn F region. Microsatellite alleles were further sequenced in order to determine the exact repeat number and to detect size homoplasy due to insertions/deletions in flanking regions.

Key results: The results confirm that apomictic microspecies of ser. Radula are derived from crosses between sexual series Glandulosi and apomictic series Discolores , whereby the apomict acts as pollen donor. Each apomictic microspecies is derived from a single distinct genotype differing from the parental taxa, suggesting stabilized clonal reproduction. Intraspecific variation within apomicts is considerably low compared with sexual series Glandulosi , and reflects somatic mutation accumulation. While facultative apomicts produce clonal offspring, sexual species are the conduits of origin for new genetically different apomictic lineages.

Conclusions: One of the main driving forces of evolution and speciation in the highly apomictic subgenus Rubus in central Europe is sexuality in the series Glandulosi . Palaeovegetation data suggest that initial hybridizations took place over different time periods in the two studied regions, and that the successful origin and spread of apomictic microspecies of the series Radula took place over several millennia. Additionally, the cloning and sequencing show that standard evaluations of microsatellite repeat numbers underestimate genetic variability considering homoplasy in allele size.

Keywords: Apomixis; Rubus subgenus Rubus; hybridization; microevolution; microsatellites; polyploidy.

Fig. 1.

Haplotype network derived from sequences of the chloroplast trnL–trnF region of different taxa of the subgen. Rubus, using R. caesius as an outgroup. Black dots indicate the number of mutational changes, species belonging to ser. Radula are underlined, and species belonging to ser. Discolores are in bold.

Fig. 2.

Principal co-ordinate analysis based on Tomiuk–Loeschcke (1991) identities of 11 loci of all studied Rubus taxa. The first two axes explained 72·6 % variability. Ellipses delimit taxa involved in hybridization processes. bm, Bohemian Massif; ca, the Carpathians.