Trifolium dubium Sibth. (Fabaceae)


Lesser hop trefoil

Illustration of Trifolium dubium from Sowerby's English Botany (1864, t.366).  Type specimen of Trifolium dubium collected by John Sibthorp, probably around Oxford (Oxford University Herbaria).

In 1794, Professor John Sibthorp, third Sherardian Professor of Botany, described a new sort of clover; he was not the first to spot it, as he admitted, but he was the first to give it its formal botanical name, Trifolium dubium. Sibthorp found this annual, yellow-flowered, prostrate clover growing in meadows and pasture around Oxford. This clover is a native British species, commonly found across Europe and eastwards as far as northern Iran. Furthermore, it is found as a weed species in many areas outside Eurasia, presumably having been transported in crop seeds and animal fodder. Trifolium dubium also occupies a wide range of habitats ranging from grasslands through rock outcrops to waste places.

The genus Trifolium, which comprises approximately 250 species distributed throughout the world's temperate and subtropical regions, has enormous agronomic and ecological importance because of its value as a nitrogen fixer, fodder and green manure. Trifolium dubium is frequently confused with another important fodder legume, yellow-flowered black medick (Medicago lupulina). However, the two species may be readily distinguished; black medick has black, coiled fruits and leaflets that have tiny points at their tips, lesser hop trefoil has pale, oblong fruits and leaflets without sharp points.

Polyploidy is very common in plants and is important evolutionarily and economically; many of our food plants are polyploids. The cells of polyploids contain more than two sets of chromosomes. Most clovers have two sets of chromosomes, they are diploids. A few clovers mingle the genomes of two species and have four sets of chromosomes, they are allotetraploids. Trifolium dubium is an allotetraploid with 30 chromosomes that combines the chromosomes of two parental species with similar geographical distributions and habitat requirements. Using pieces of DNA that are unique to each parental species, and labelled with fluorescent dyes, it is possible to identify the 14 chromosomes that come from hop trefoil (Trifolium campestre) and the 16 chromosomes that come from slender trefoil (Trifolium micranthum) in the nuclear genome of lesser hop trefoil. Trifolium dubium is morphologically intermediate between the two parental species.

Culturally, Trifolium dubium is one of two clovers most frequently associated with the name Irish shamrock, the other is white clover (Trifolium repens). This illustrates one of the problems of trying to link common names with scientific names. Although the shamrock is thought to be uniquely Irish, the botanical species with which the name is associated are not endemic to Ireland.

Further reading

Ansari HA et al. 2008. Molecular and cytogenetic evidence for an allotetraploid origin of Trifolium dubium (Leguminosae). Chromosoma 117: 159-167.

Nelson EC 1991. Shamrock: botany and history of an Irish myth. Boethius Press.

Stephen Harris