Since late 1997 I have been coordinating DNA studies of the thymes in my National Plant Collection®. The first study was carried out by Dr. Madan Thangavelu, who at that time was at Wye College and is currently at the Medical Research Council, Cambridge.
The intention is to study the DNA of all the thymes in my Collection. This is a long term project, possibly over several years. The best results are obtained when the plants are growing fast, in late spring and late summer, using small amounts of the youngest leaves. Older leaves give too much contamination from the oils present and the results are poor. Many thymes have been assigned to the wrong species and DNA studies will help sort out the present muddle.
DNA profiles for the study carried out in 1997. Those on the right delineate at specific level and those on the left show differences within a species. The names in parentheses refer to the former invalid names
Group C are demonstrated to be cultivars of T. vulgaris (Group A), with identical profiles in both studies, confirming that all samples are the same plant, which has now been renamed Thymus vulgaris 'Golden Pins'.
In 1999 Dr. Madan Thangavelu and I carried out the Golden Thyme DNA Study, as a result of which our native golden leaved thymes were placed in T. pulegioides rather than T. citriodorus in which they had mistakenly been placed because the leaves were lemon scented. (In May 2007 it was established that T. citriodorus is a synonym for T. pulegioides.)
DNA profiles for the Golden Thyme DNA Study, demonstrating that the golden leaved lemon scented thymes (Group 4) belong in Thymus pulegioides (Group 6) and not in Thymus citriodorus (Group 3).
Tree based on DNA profiles from the Golden Thyme DNA Study, demonstrating that Thymus citriodorus and its cultivars are distinct from the species T. pulegioides and T. vulgaris and that the lemon scented, golden leaved thymes belong in the species T. pulegioides.
Dr. Madan Thangavelu and I are about to commence the Creeping Thyme DNA Study, funded by a grant from the John Spedan Lewis Foundation. This will enable us to resolve the confusion over the relationship between the two species of wild creeping thyme native to north west Europe. We intend using plant material from the thymes which I have collected during my field trips to sites in Britain and in Europe.
When Dr. Madan Thangavelu and I carried out the Golden Thyme DNA Study we used two samples each, of Thymus vulgaris and T. pulegioides, together with one sample of T. citriodorus hort. and one sample of each cultivar. Dr. Alan Leslie, chairman of the RHS Advisory Panel on Nomenclature and Taxonomy, has asked us to repeat the study using a larger number of samples. In addition to confirming that the golden leaved bushy cultivars, which had been wrongly assigned to T. citriodorus, are cultivars of T. pulegioides, we also have a problem with the silver variegated cultivars, T. vulgaris 'Silver Posie' and T. citriodorus 'Silver Queen'. These cultivars are practically identical apart from smell, the former smelling of thyme and the latter of lemon, not a satisfactory characteristic for determining species!
We propose to take DNA from T. vulgaris, T. pulegioides, T. citriodorus hort. and their cultivars, to give us a reliable baseline to determine species delineation. There are a considerable number of named wild collected T. vulgaris available in the nursery trade, particularly in Germany and France and I have collected T. pulegioides both in Britain and in Austria, which will be used in the study. In addition to these species and cultivars there are several other bushy thymes which we intend to include in this study. I am not entirely happy about the species into which certain thymes are currently assigned and it would be expedient to include them in this study into the bushy thymes. Thymus herba-barona is the species to which the lemon scented thyme which was collected by Beth Chatto in Corsica has been assigned, but I feel there are too many differences between them, which could mean they belong to separate species. There are several thymes native to Spain with very large bracts and flowers which are very different from those in other species, as well as T. capitatus from the Mediterranean region, which is also very different from other thymes. The cultivar known as T. 'Valerie Finnis' is more like plants in the genus Micromeria. These will all be used in the study.
The results of DNA studies are now becoming regarded as an essential tool to be used in conjunction with the more established methods of species delineation. It is therefore important that we have a complete DNA profile of Thymus species and cultivars which are readily available. Our previous studies have used different chemicals so we have not been able to compare the two groups. Once we have completed the Creeping Thyme DNA Study and the Bushy Thyme DNA Study we intend to produce a DNA tree to determine the relationships between the various species of thyme, as well as their cultivars.
I am pleased to announce that I have now been awarded an RHS bursary which will enable us to carry out the Bushy Thyme DNA Study in the summer of 2007.
Introduced November 2001, last updated May 2007