MORPHOLOGY, ISOZYME VARIATION, CYTOLOGY, AND REPRODUCTION OF HYBRIDS BETWEEN SORBUS-ARIA (L) CRANTZ AND S-TORMINALIS (L) CRANTZ

In the Reppischtal valley (near Zurich, Switzerland) Sorbus aria and S. torminalis grow as well as numerous presumably hybridogeneous forms (S. latifolia). This study was based on the hypothesis that these are hybrids and part of an apomictic taxon. In order to examine this hypothesis, the morphological as well as the isozyme and cytological variation of these trees were analysed and investigations on their reproduction were carried out. Furthermore, isozyme analyses were effected in S. latifolia growing on another site (Birnberg, to the south of Baden, Germany). S. latifolia varies morphologically. Apart from the intermediate individuals, there are individuals that are more or less similar to S. aria. S. latifolia and S. torminalis differ dearly from each other. The isozyme analyses revealed that the seven tested (morphologically intermediate) individuals from Reppischtal are in fact six different genotypes. Also the tested individuals from Birnberg showed genotypic variation. S. latifolia produces viable seeds sexually. The seedlings are mainly diploid (2n = 34), one plant proved to be triploid (3n = 51). The results obtained suggest the conclusion that S. latifolia in the Reppischtal valley is not a genetically uniform, apomictic taxon. It is much more likely that the individuals are a complex of hybrids, at least partially capable of sexual reproduction, and various forms of back-crosses more or less similar to S. aria. However, the possibility that apomictic forms occur within this complex cannot be ruled out on the basis of this study: Further investigations will be necessary. The taxonomic consequences of this variation are being discussed.