¹Institute of Ecology and Resource Management, University of Edinburgh, King's Buildings, Mayfield Rd, Edinburgh EH9 3JU, Scotland

Correspondence: RA Ennos, School of Biological Sciences, University of Edinburgh, King's Buildings, Mayfield Rd, Edinburgh EH9 3JU, Scotland. E-mail: rennos@ed.ac.uk

²Current address: Department of Plant Sciences, University of Oxford, South Parks Rd, Oxford OX1 3RB, UK.

³Current address: School of Conservation Sciences, Bournemouth University, Talbot Campus, Poole, Dorset BH12 5BB, UK.

Received 23 July 2003; Accepted 24 January 2004; Published online 14 July 2004.

Abstract

The Arran whitebeams Sorbus arranensis and S. pseudofennica are two endemic woody plant taxa that have evolved on Arran through hybridisation. S. arranensis is a triploid hybrid between the widespread diploid S. aucuparia and the rare tetraploid S. rupicola. S. pseudofennica is a tetraploid formed by crossing between S. arranensis and S. aucuparia. In order to determine the mating systems of the two endemic species six maternal trees of each taxon together with 10–12 of their seed offspring were scored for their phenotype at three microsatellite loci and one nuclear intron locus. All seeds of S. arranensis were identical in phenotype to their maternal parents. In S. pseudofennica, 17.5% of all seeds differed in marker phenotype from their maternal parent. The proportion of seed with nonmaternal phenotypes varied significantly among maternal trees of S. pseudofennica. The results suggest that the triploid S. arranensis is an obligate apomict, whereas the tetraploid S. pseudofennica is a facultative apomict. Molecular marker analysis of three trees from Arran with an unusual leaf morphology indicates that they are the product of sexual reproduction by S. pseudofennica, and may originate from hybridisation with S. aucuparia. This research demonstrates that the Sorbus taxa on Arran are participants in an active evolutionary process generating novel biodiversity. Conservation programmes for these taxa should aim to preserve this evolutionary process rather than the individual taxonomic entities that it produces.