Secondary Biology: What is Ash Dieback disease?

Ash dieback disease was discovered in 1992 in the remote Polish forest of Zabodny.  There are a variety of methods suggested in order to attempt to deal with the threat to Ash (Fraxinea excelsior).  This disease is associated with the fungus Chalara fraxinea which is thought to be a new species.  Spores of the sexual stage of the fungus are released from fallen leaves and these go on to infect other trees.  There is some evidence that the disease has a higher percentage of fatalities in forests with damper conditions, although in some areas 10-15% of trees do survive.

The methods suggested include:

• Destruction of trees which show symptoms of the disease
• Destruction of all trees in an area where the disease has been identified
• Restriction of transfer of Ash trees from region to region
• Avoidance of using Ash when replanting
• Removal of leaves and twigs that fall from established Ash trees
• Protection of trees by the use of fungicides

Now try these questions:

1. Why could these methods be effective?
2. For each method, state the conditions (for example forest/urban location) necessary for the method to be most effective; show your reasoning.
3. Some scientists have called for restrictions on felling trees; give reasons why this policy could be successful.
4. Design suitable experiments to demonstrate that resistance to Ash dieback is inherited and indicate the results you would expect if the gene for resistance is:

      i.      Dominant
      ii.      Recessive
      iii.      Not dependant on one gene alone

5. Indicate the likely proportions of resistant and susceptible F1 and F2 plants with respect to the crosses that you set up in your experiments.
6. Why should the virulence of Ash dieback disease indicate that the causative organism has recently arisen?

Reading around the subject

Background to the disease:

http://www.telegraph.co.uk/news/worldnews/europe/poland/9669165/Ash-dieback-the-ruined-Polish-forest-where-deadly-fungus-began.html

There is an interesting Wiki article on sex in fungi here:

http://en.wikipedia.org/wiki/Sex#Fungi

and in more detail here:

http://en.wikipedia.org/wiki/Fungus#Reproduction

On evidence for some resistance to the disease:

http://www.academicjournals.org/jaerd/PDF/Pdf%202012/14MayConf/Kirisits%20and%20Freinschlag.pdf

Some aspects of the odd-ball fungi

As you will have found in the articles Fungi are classified on the morphology of their sexual apparatus, i.e. how they do sex.  There are 3 groups:

Zygomycetes (also called Glomeromycetes).  These small fungi produce gametes which fuse inside a thick-walled gametangium thus producing a zygospore (2N).  Under suitable conditions the zygospore germinates, divides by meiosis (now N) and produces new haploid fungal threads which can produce many more spores by mitosis.  The spores are distributed (wind, water animal) and spread the fungus.

Ascomycetes.  These are typically dikaryotic in that nuclei from their parents do not fuse until just before reproduction i.e. they are N but carry genes from BOTH parents.  During reproduction a special hook shaped hypha is formed called an ascus, the two nuclei fuse (now 2N) and meiosis takes place to form ascospores (N).  Pressure builds in the ascus which bursts to distribute the spores.

Bascidiomycetes.  These, like the Ascomycetes, have dikaryotic hyphae, but the control of nuclei in each segment of the hyphae is more rigorous through the use of “Clamp Connections” which allow the movement of nuclei from segment to segment as necessary to maintain the dikaryotic state.  The sexual apparatus is different too, with the production of many sac-like bascidia, although the fusion of nuclei and subsequent meiosis to form bascidiospores is similar to the Ascomycetes.  The bascidia are often arranged underneath large specialised structures familiar to us as Toadstools and Mushrooms where the bascidia cover the gills, or in some bascidiomycetes, tubes.

Unfortunately, the method of reproduction of many fungi is not known and such species are traditionally put in a “holding pen” called the Fungi imperfecti until such time as research indicates which of the above groups they should belong to.  This can lead to the same fungus having two names, one (possibly older name) applied before the sexual stage was known and another given to the same fungus as a “newly” discovered species with evidence of its method of reproduction.  Chalara fraxinea is just such an example, with a sexual phase confusingly called Hymenoscyphus pseudoalbidus (see the pictures in this blog which show the white fruiting bodies of H pseudoalbidus).  There is a good article on this “Dual naming” problem here:

http://en.wikipedia.org/wiki/Teleomorph,_anamorph_and_holomorph

and nice pictures of the sexual stage of the fungus here (remember the notes above regarding dual naming):

http://www.academicjournals.org/JAERD/PDF/Pdf%202012/14MayConf/Kirisits%20et%20al.pdf

Some examples of syllabus content for which the above exercise is relevant:

John Giles

John Giles is an educational consultant and author specialising in IT and computing. He works closely with exam boards, and has written syllabuses and exam papers.