News and blog

Introduction to parasitism in Euphrasia

Max Brown. 7th March 2018.

Euphrasia species are parasitic plants, which means that they steal water and mineral nutrients from host plants. More specifically they are considered hemiparasitic plants, and thus also contain chlorophyll in their leaves and can photosynthesise. This does not necessarily mean that hemiparasitic plants can survive without host association. Indeed, no parasitic plant has been documented in the wild that has survived without attachment to a host plant (Joel et al., 2013). This is in contrast to the potentially more familiar, yet strange, holoparasitic plants, which cannot produce any of their own sugars, and poach sugars from the host plant too. A good example of this are the Broomrapes (Orobanche sp).

 

   

[Left shows Euphrasia vigursii x tetraquetra from a coastal loaction in Cornwall; right shows Euphrasia in association with other hemiparasitic plant species, Rhinanthus minor and Odontites vernus in Sussex]

The mechanism of how hemiparasites such as Euphrasia parasitize a host is fairly well understood. Lateral branches of the roots attach to a host plants’ root through a structure called the haustorium. The haustorium is the hallmark of all parasitic plants that acts as the mediator by which products from the host can be taken by the parasite. Hemiparasitic plants (in general) can only produce lateral haustoria, as opposed to terminal haustoria. Lateral haustoria are produced on young lateral roots and an individual plant can produce thousands of lateral haustoria. This makes it possible for an individual to parasitise many hosts in the immediate vicinity. Holoparasitic plants produce terminal haustoria and is the main functional haustorium through the life of the plant. Euphrasia and related taxa in the Rhinantheae are purely xylem feeders and do not attach to the phloem.

Euphrasia plants are thought to be generalist parasites with a broad host range. Some hosts however, allow Euphrasia plants to grow taller and produce more seed than others (for example some grasses and legumes). Other hosts appear to be poor hosts for Euphrasia and even harmful. Research on the related genus Rhinanthus has shown that some hosts (Plantain and Oxeye daisies) actually have defences against parasitic attack by either killing their own cells which come into contact with a parasitic plant or by strengthening the walls of the roots (Cameron et al., 2006).

In other parasitic species, notably the hemiparasitic Yellow Rattle (Rhinanthus sp), there have been implications for community wide effects of hemiparasitism. The argument is that hemiparasitic plants act to suppress dominant species and allow less competitive species to grow. Despite the logic of this argument, there have been very variable effects of host suppression by hemiparasitic plants on plant diversity. Some studies have reported increases in plant diversity, others a decrease (Fibich et al., 2017). The latter study cited however finds significant associations between hemiparasites and high species richness across a number of root hemiparasitic taxa.

The first question in my PhD aims to address the effect of different host species on the fitness of a single Euphrasia species. At the Royal Botanical Gardens Edinburgh I planted 3000 Euphrasia arctica seeds from a single population in the winter of 2016. The resultant germinants (around 1500 plants) were given one of nearly 50 host species across a wide phylogenetic range to parasitise. Provisionally, the data suggests that the number of reproductive nodes that a Euphrasia produces is extremely variable across the range of hosts available to parasitise. Only very few hosts give any strong positive fitness benefits, potentially suggesting that the functional range of hosts is narrower than once suspected.

References:

Cameron, D. D., Coats, A. M. & Seel, W. E. 2006. Differential resistance among host and non-host species underlies the variable success of the hemi-parasitic plant Rhinanthus minor. Annals of Botany, 98, 1289-1299.

Fibich, P., LEPS, J., Chyrty, M. & Tesitel, J. 2017. Root hemiparasitic plants are associated with high diversity in temperate grasslands. Journal of Vegetation Science, 28, 184-191.

Joel, J., Gressel, J. & Musselman, L. 2013. Parasitic Orobanchaceae, Berlin, New York, Springer.

 

An introduction to Euphrasia ID: diploids vs tetraploids

Alex Twyford. 29th January 2018.

Many botanists I know refuse to identify Euphrasia as they consider them too difficult. I can’t think of another plant group of a similar size that attracts such negativity from field botanists. But while individual species identification can be difficult, learning the basic species groupings and the informative characters is surprisingly easy, and can get you a long way towards understanding the plant in front of you. Once you’ve finally got your eye in, Euphrasia are an extremely rewarding group of plants to work with.

The simplest and most important division of Euphrasia species in Great Britain and Ireland is between diploids and tetraploids. While there are other ploidy levels elsewhere in the world, in the UK we only have two. Fortunately for us, these different ploidy groups are divergent, and likely represent different waves of colonization from ancient continental ancestors, and as such they’re surprisingly distinct.

Diploids, with 2n=22 chromosomes, possess long glandular hairs, and the stalk of the hairs are often 10-times longer than the head of the hair. You can see these hairs in the field with a hand lens, and even without a lens these species still appear distinctly hairy. These hairs may be retained on dry specimens, but may also become damaged over time.

Tetraploids, with 2n=44 chromosomes, don’t have long glandular hairs. They may however, have some eglandular short hairs or bristles, or are glabrous.

Starting with this single, relatively easy character, you can reliably distinguish between diploids and tetraploids. In terms of species, the tetraploids include the widespread species E. arctica, and the often purple-flowered moorland E. micrantha, while the diploids include species such as the rather robust looking downland E. anglica. Getting to the species-level is usually relatively easy for the diploids, while identifying the tetraploids needs more care and attention.

Are there exceptions to the diploid vs tetraploid hair character? Unfortunately, yes, but these are rare. Some of these may be cases where diploids hybridise with tetraploids, giving rise to some fascinating plants that combine characters from the two ploidy levels. These warrant their own blog post.

I commonly get asked why chromosome number is associated with different hair types. The answer isn’t clear, but is almost certainly nothing to do with chromosome number per se. While chromosome duplication can change morphology, it tends to increase the size of characters rather than introducing new characters. What’s more likely is that different hair types evolved in species with different ploidies in continental Europe before they invaded the UK.

I’d encourage you to get out next season and look for diploids and tetraploids, even if you can’t get to species-level just yet.

1st November 2017

Our Euphrasia research goes from strength to strength, with Chris Metherell finishing his handbook (out 2018), Max Brown completing his first common garden experiment and fieldwork, and an early release of Xumei's DNA barcoding paper now avaliable on BioRxiv. With this momentum, we're hoping to use the website to give more frequent updates. Please email me (Alex.Twyford [at] ed.ac.uk) with any Euphrasia stories you'd like to share. 

Archive

18th June 2015

Earlier in the spring, I launched a project to sequence Euphrasia populations from across the UK (and Northern Europe), to better understand the relationships between species in this taxonomically complex group. To achieve this broad sampling, I contacted vice-county recorders, local botanists, land-owners, and natural history enthusiasts to ask them to send me samples from their area. After lots of hard work preparing materials and contacting participants, I have just sent out the first 100 collection kits (below, left). Each kit contains collection notes, 20 silica bags, seed envelopes and return postage (below, right). If you are interested in participating and haven't been in contact, please do send me an email. I'm really looking forward to seeing what everyone finds in their local patch.