Molecular systematics of 'Rhododendron ponticum' L. and its close allies

Abstract

Rhododendron ponticum, introduced to the British Isles in 1763, has become a noxious invasive pest species, particularly in the west. Material of R. ponticum and its close allies in subsection Pontica were subjected to chloroplast DNA RFLP analysis. A preliminary phylogeny based on all cpDNA variation detected is presented. Suggestions regarding the interrelationships of these species are made, and a cpDNA-type of unknown taxonomic identity is tentatively identified. Chloroplast DNA differences were detected between native material of R. ponticum from Turkey, Spain and Portugal. Based on these differences, it was determined that approximately 90% of material naturalised in the British Isles originates from Spain and 10% from Portugal. These two types of material occur in roughly the same proportion throughout the British Isles. About 10% of naturalised accessions were found to contain an rDNA marker indicating nuclear introgression from R. catawbiense. Introgressed individuals are shown to be most common in the coldest region of the British Isles surveyed, i.e. E. Scotland, and it is suggested that introgression from R. catawbiense may confer increased frost tolerance to R. ponticum. The occurrence of the rDNA marker was not correlated with that of morphological markers indicating introgression. Correlations were found between morphological characteristics, which may result from introgression from cultivated species, such as R. maximum. Molecular evidence for the involvement of R. maximum in the ancestry of British R. ponticum was found in two accessions which resembled R. maximum. in certain morphological characteristics. It was established that another unidentified species was involved in the parentage of naturalised material. Five natural hybrid combinations amongst Turkish Rhododendron species were detected using morphological, cpDNA and nuclear rDNA markers. These were R. ponticum x R. ungernii, with introgression in both directions; R. ponticum x R. smirnovii; R. smirnovii x R. ungernii with introgression at least towards R. ungernii; R. smirnovii x R. caucasicum; and R. ponticum x R. caucasicum. The unusual characteristics of the last combination are discussed in some detail.