English elm is a 2,000-year-old Roman clone

This tree's genetic uniformity may have helped to fell entire European populations.

Abstract

The outbreak of Dutch elm disease in the 1970s ravaged European elm populations, killing more than 25 million trees in Britain alone; the greatest impact was on Ulmus procera, otherwise known as the English elm¹. Here we use molecular and historical information to show that this elm derives from a single clone that the Romans transported from Italy to the Iberian peninsula, and from there to Britain, for the purpose of supporting and training vines. Its highly efficient vegetative reproduction and its inability to set seeds have preserved this clone unaltered for 2,000 years as the core of the English elm population — and the preponderance of this susceptible variety may have favoured a rapid spread of the disease.

Main

Following the results from the European Union project RESGEN CT96-78 on elm genetic resources, we studied the variability of chloroplast DNA in the two elm species native to Spain (the field elm, U. minor Miller sensu latissimo², and the wych elm, U. glabra Huds.) and in elm samples from France, Greece, Italy and Britain (for methods, see supplementary information).

We analysed restriction fragment-length polymorphisms in chloroplast DNA after amplification by polymerase chain reaction and detected four major chloroplast lineages, which are maternally inherited in Ulmus (see supplementary information). Lineage A appears in Greece; lineage B in northern Italy; lineage C in Italy, the Iberian peninsula and Britain (but not in France); and lineage D throughout all of Spain, northern Italy, France and Britain. Lineage-D haplotypes appear in both U. minor and U. glabra samples. By contrast, in Spain and Britain, lineage-C haplotypes are observed only in U. minor, although they appear in both species in Italy (mainly in elms from the central region).

Except for the presence of lineage C in the Iberian peninsula and Britain, the distribution of lineages is very similar to those described for other trees³. Haplotype sharing among tree species is reckoned to indicate their presence in common glacial refugia, where chloroplasts would have been exchanged through hybridization and introgression⁴. Absence of lineage-C haplotypes in Spanish U. glabra highlights the lack of refugia for this lineage in Iberia, whereas its presence in both elms in Italy indicates that this lineage has an Italian origin.

Credit: S. DOMÍNGUEZ

Eighteen field elms with the same haplotype (from lineage C), collected from across Spain, Britain and central Italy, as well as five lineage-D individuals from Spain and Britain, were genotyped using seven nuclear microsatellites⁵ and two amplified fragment-length polymorphism (AFLP) primer combinations. The results revealed a widely distributed clone within lineage-C elms, represented here by five Spanish and three British samples (see supplementary information). We detected slight differences among the AFLP fingerprinting profiles of these eight samples, attributable to somatic mutations. The closest genetic similarity was observed between the clone and samples from Latium in Italy, which reinforces the idea that its origin was Roman.

These eight individuals were classified as English elm (U. minor var. vulgaris², commonly called U. procera Salisb. by British botanists²). In Britain² and in Spain⁶, English elm rarely sets seeds, but produces pollen normally and is very effective in vegetative propagation. These reproductive features, together with the deliberate plantation of this variety of elm by humans, could explain the maintenance of this genotype and its spread over Spain and Britain. But when, whence and why was it transported?

Although it has been suggested that the English elm was introduced during the Bronze Age by Celtic tribes², our results support a hypothesis⁷ that it corresponds to the Atinian elm, which was used for vine-training by the Romans. In his treaty De Re Rustica⁸ (written in about AD 50), the Roman agronomist Columella advocates the use of elm for this purpose, recommending in particular a barren tree that was vegetatively propagated — the Atinian elm.

Columella owned three farms in Latium⁹, where most Italian lineage-C samples are found, and a fourth vineyard in Xerez¹⁰ (Andalusia, Spain). He and other farmers may have introduced different Italian elms to the Iberian peninsula, including the Atinian elm. Columella's writings influenced the subsequent establishment of vineyards to such an extent that the Roman emperor Domitian prohibited the plantation of new vines in Italy in AD 92 and ordered half of the vineyards in the provinces to be cut down¹¹. In the meantime, the Atinian clone spread across Iberia and was probably transported to Britain in the form of root suckers, as indicated by the presence of elm pollen in a Roman vineyard in Britain¹² and by the coincident distribution of suspected Roman vineyards¹² and U. procera in Britain².

The identification of the English elm with the Atinian clone was first proposed in the nineteenth century⁷. Our findings provide molecular support for this proposal and indicate that the English elm originated from the massive propagation of the Atinian clone by the Romans. This large-scale transformation of the elm's natural diversity became critical in the twentieth century, when most English elms succumbed to Dutch elm disease¹, and should be taken into account in current European elm breeding and conservation strategies.