The origin of intermediate species of the genus Sorbus
The genus Sorbus in Europe contains five diploid species, Sorbus aria, Sorbus aucuparia, Sorbus torminalis, Sorbus chamaemespilus and Sorbus domestica, classified into five different subgenera. The subgenus Aria (or the S. aria aggregate) contains apomictic triploid and tetraploid species. Within the genus there are, in addition, a number of species, morphologically intermediate between plants of the four main subgenera, which are considered to reproduce partly, or exclusively, by apomixis. These are believed to have originated by hybridisation between species in the S. aria aggregate and a species from another subgenus (either S. aucuparia, S. torminalis or S. chamaemespilus). We have used restriction fragment length polymorphism (RFLP) analysis on a total of 178 Sorbus accessions to test this model. The genome relationships of the different groups have been assessed, and the hybrid nature of the major intermediate groups is unequivocally demonstrated. Polyploid species in the S. aria aggregate show genetic variation, indicating the possibility of multiple origins and/or facultative apomictic breeding behaviour. A major finding, confirmed by microsatellite analysis, is that the 'intermediate' species S. intermedia is shown to have genomes from S aria, S aucuparia and S torminalis. Polymorphic mitochondrial DNA markers were used to determine the direction of the crosses that gave rise to new 'hybrid species'; in the majority of cases the pollen was provided by the parent from the S aria aggregate.