The genusSorbus in Europe contains five diploid species,Sorbus aria,Sorbus aucuparia,Sorbus torminalis,Sorbus chamaemespilus andSorbus domestica, classified into five different subgenera. The subgenusAria (or theS. 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 theS. aria aggregate and a species from another subgenus (eitherS. aucuparia,S. torminalis orS. chamaemespilus). We have used restriction fragment length polymorphism (RFLP) analysis on a total of 178Sorbus 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 theS. 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' speciesS. intermedia is shown to have genomes fromS aria,S aucuparia andS 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 theS aria aggregate.

Authors and Affiliations

1. Department of Plant Sciences, University of Cambridge, Downing Street, Cambridge, CB2 3EA, England, , , , ,

   E. Nelson-Jones,  D. Briggs &  A. Smith

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