(Commentary on a paper by Hannes Becher et al. (2020), recently published in Plant Communications. You can download the paper free of charge here.)
Fair Isle – famous for its knitted jumpers – is the most remote inhabited island in the United Kingdom, lying approximately mid-way between Orkney and Shetland. About 5 km long and 2.5 km wide, its flora comprises 331 plant species (Quinteros Peñafiel et al. 2017), eight of which are Eyebrights (Euphrasia species).
Eyebrights are facultative parasites that are notoriously difficult to tell apart, due partly to their tendency to produce intermediates as a result of hybridizing with each other. Nonetheless, they tend to occur in different habitats, sometimes only a few metres apart. This is especially true of three eyebrights on Fair Isle – Euphrasia arctica, E. foulaensis and E. micrantha – which occur in grassland, coastal and upland heathland habitats, respectively. Questions immediately arise as to the nature and causes of the morphological differences observed between these species in the field. Possibly, all of these differences could be environmentally determined and reflect phenotypic responses (plasticity) to contrasting environmental conditions. A way of telling if this is true, or whether the differences are partly due to inherited differences, is to raise plants from the different habitats in a common environment. If the morphological differences are maintained under the same environmental conditions, it can be concluded the eyebrights are genetically different from each other and there is a sound basis for treating them as different species.
A comparison by Becher et al. (2020) of these eyebrights raised from seed in a common environment at the Royal Botanic Garden Edinburgh, has shown there is a genetic basis to some of the morphological differences seen in the field. Consequently, it is correct to treat them as different species associated with ecologically contrasting habitats. Having demonstrated this, it is natural to ask how different genetically are these species from each other. Although impossible to do so a few years ago, this can now be done by comparing the genome DNA sequences of the three species, that is the sequence of the four nitrogen bases – Adenine (A), Guanine (G), Cytosine (C), Thymine (T) – along the length of their DNAs. When Becher and colleagues did this, it became clear that plants of the upland heathland species (E. micrantha) form a separate genetic group from those of the other two species, E. arctica and E. foulaensis, which are very similar genetically to each other. Further examination of the DNA sequences of the three species showed that rather than containing one genome, they each contain two different genomes. They are, in fact, hybrids which contain the genomes of two ancestral species. These two ancestral species crossed, yielding a hybrid which doubled its chromosome number to produce what is known as an allopolyploid species from which the three Euphrasia species on Fair Isle are derived. Remarkably, all of this information on the origins of the three species and how divergent they are from each other was garnered from their whole-genome DNA sequences.
The study of Euphrasia on Fair Isle demonstrates the power of genetic analysis in improving our understanding of the natural history of plants. By using the simple approach of comparing plants grown under the same set of conditions in a common garden, it is possible to distinguish difficult taxa from each other. Furthermore, by undertaking the most up-to-date forms of genetic analysis based on whole-genome DNA sequencing, it becomes possible to determine how species are related to each other and how they originated. DNA sequences can also provide clues on the genetic basis of adaptation to local habitats, although this form of analysis remains to be completed in the case of the Fair Isle eyebrights.
* E. micrantha is more commonly purple-flowered than white-flowered in the UK; however, flower colour is not diagnostic of the species.
Becher H, Brown MR, Powell G, Metherell C, Riddiford NJ, Twyford AD (2020) Maintenance of species differences in closely related tetraploid parasitic Euphrasia (Orobanchaceae) on an isolated island. Plant Communications DOI: https://doi.org/10.1016/j.xplc.2020.100105
Quinteros Peñafiel CV, Riddiford NJ, Twyford AD(2017) A floristic survey of Fair Isle. New Journal of Botany. 2-3:101-111.
(The research reported by Becher et al. (2020) was supervised by Alex Twyford, Edinburgh University, and funded by grants from the Natural Environment Research Council awarded to him.)
University of St Andrews