Speciation by triparental hybridization in genus Sorbus (Rosaceae)

Hybridization associated with polyploidization and apomixis is a frequent mechanism of speciation. Sorbus is a genus with ongoing hybridization resulting in a polyploid complex with different parental lineage. Triparens is the smallest hybridogenous subgenus of Sorbus so far known to comprise only two taxa, S. intermedia and S. × liljeforsii that combine the genomes of three taxa (S. aria agg., S. aucuparia and S. torminalis). To elucidate the origins of S. dacica, S. paxiana and S. tauricola, three new trigenomic candidates formerly believed to be of biparental origin with either S. aria agg. × S. aucuparia or S. aria agg. × S. torminalis lineage we combined data from HPLC and chloroplast DNA analysing additional 33 related taxa as well. We concluded that the 'torminalis-type' flavonoid profile and the 'aucuparia-type' plastid indicate the participation of both S. torminalis and S. aucuparia resulting in the formation of S. dacica, S. paxiana and S. tauricola. Sorbus aria agg. as the third ancestor and as a necessary link to meet genes of S. torminalis and S. aucuparia in one genome is obvious from morphological features (densely tomentose undersides of leaves). The tetraploid cytotypes and obligate pseudogamy of S. dacica and S. paxiana were determined by flow cytometry and are published here for the first time. The most probable evolutionary scenario for Triparens species is: 1. a diploid sexual S. aucuparia as pollen acceptor hybridized with a tetraploid apomictic taxon from the S. aria agg. producing a triploid apomictic taxon with 'aucuparia-type' plastid inherited maternally; 2. during a second crossing event this subgenus Soraria hybrid as maternal progenitor hybridized with the sexual diploid S. torminalis (providing gene(s) of apigenin O-glucuronide synthesis) forming a tetraploid Triparens hybrid with 'aucuparia-type' plastid and 'torminalis-type' flavonoids.