Origin and evolution of invasive naturalized material of Rhododendron ponticum L. in the British Isles

Information concerning the area of origin, genetic diversity and possible a cquisition of germplasm through hybridization is fundamental to understandi ng the evolution, ecology and possible control measures for an introduced i nvasive plant species. Rhododendron ponticum is extensively naturalized in the British Isles, but it is not known whether native material in Turkey, S pain or Portugal gave rise to the naturalized material, or to what extent i ntrogression has affected this material. Chloroplast (cp) and nuclear ribos omal DNA (rDNA) restriction fragment length polymorphisms (RFLPs) were soug ht which could distinguish between native material of R. ponticum, and betw een 15 other Rhododendron species including R. ponticum's closest relatives . Thereafter, a total of 260 naturalized accessions of R. ponticum from thr oughout the British Isles was examined with respect to informative polymorp hisms. It was found that 89% of these accessions possessed a cpDNA haplotyp e that occurred in native material of R. ponticum derived almost entirely f rom Spain, while 10% of accessions had a haplotype unique to Portuguese mat erial. These results therefore indicated an Iberian origin for British mate rial. rDNA or cpDNA evidence of introgression from R. catawbiense was found in 27 British accessions of R. ponticum, and such accessions were signific antly more abundant in Britain's coldest region, eastern Scotland, than els ewhere. This could indicate that introgression from R. catawbiense confers improved cold tolerance. Introgression from R. maximum and an unidentified species was also detected.