Application of RAPDs to the critical taxonomy of the English endemic elm Ulmus plotii Druce

Abstract

The taxonomy of the British elms is notoriously complicated and a satisfactory consensus classification remains elusive. This taxonomic complexity appears to be attributable to the reproductive biology of the species. Ulmus glabra Huds. reproduces sexually and its taxonomic status is widely (albeit not universally) accepted. In contrast, the suckering elms of the U. minor complex (U. minor Mill. emend. Richens sensu latissimo) rarely reproduce by seed in Britain. Instead they perpetuate predominantly by vegetative reproduction; arguments regarding their taxonomy are legion. We have used molecular markers (RAPDs) to investigate the amounts and partitioning of clonal diversity and taxon inter-relationships in the British elms, focusing on a particularly enigmatic suckering elm, U. plotii Druce. Our molecular data suggest that all samples of U. plotii that precisely match the type description are ramets of a single genet, the distribution of which is attributable to human planting. Morphologically similar samples, which have many but not all of the U. plotii diagnostic characters, do not cluster with U. plotii when the RAPD data are analysed using principal coordinates analysis (PCO). Instead, they are scattered on the PCO plots throughout the broader range of variability of the U. minor complex. The implications of these results for the taxonomy of the British elms are discussed, and the need to combine knowledge of population structure with taxonomic pragmatism is emphasized.