Origin and evolution of invasive naturalized Rhododendron ponticum in the British Isles
Information concerning the area of origin, genetic diversity and possible acquisition of germplasm through hybridization is fundamental to understanding the evolution, ecology and possible control measures for an introduced invasive plant species. Rhododendron ponticum (EPPO list of invasive alien plants) is extensively naturalized in the British Isles, where it has very few natural enemies. It is recognized as a threat to native communities and is a pest of forestry. The species is native of the area near the south of Black Sea (i.e. Caucasus, northern Turkey and the southeast corner of Bulgaria) and, disjunctly, Lebanon and three small areas in the Iberian Peninsula, i.e. in Southwest Spain, and Southern and Central Portugal. It is not known whether native material in Turkey, Spain or Portugal gave rise to the naturalized material, or to what extent introgression has affected this material. Chloroplast (cp) and nuclear ribosomal DNA (rDNA) restriction fragment length polymorphisms (RFLPs) were sought which could distinguish between native material of R. ponticum, and between 15 other Rhododendron species including the closest relatives of R. ponticum. Thereafter, a total of 260 naturalized accessions of R. ponticum from throughout the British Isles was examined with respect to informative polymorphisms. It was found that 89% of these accessions possessed a cpDNA haplotype that occurred in native material of R. ponticum derived almost entirely from Spain, while 10% of accessions had a haplotype unique to Portuguese material. These results therefore indicated an Iberian origin for British material. rDNA or cpDNA evidence of introgression from R. catawbiense was found in 27 British accessions of R. ponticum, and such accessions were significantly more abundant in the coldest region of Britain, eastern Scotland, than elsewhere. This could indicate that introgression from R. catawbiense confers improved cold tolerance. Introgression from R. maximum and an unidentified species was also detected.
Milne RI, Abbott RJ (2000) Origin and evolution of invasive naturalized material of Rhododendron ponticum L. in the British Isles. Molecular Ecology. 9, 541-556