Thursday, 17 January 2013

Secondary Science - 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
  1. Indicate the likely proportions of resistant and susceptible F1 and F2 plants with respect to the crosses that you set up in your experiments.
  2. Why should the virulence of Ash dieback disease indicate that the causative organism has recently arisen?
Copyright Jon Bunting
Reading around the subject

Background to the disease:
There is an interesting Wiki article on sex in fungi here:
and in more detail here:
On evidence for some resistance to the disease:

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:

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

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

Exam boardSyllabus nameSection referenceComment
AQAA level Biology3.1.1Fungi as examples of pathogens; Disease
3.2.6Cell differentiation
3.3Analyse, Interpret, Explain and Evaluate
3.4.1Some aspects of variation in population size
3.4.7Some aspects of conservation of species
3.4.8Some aspects of selection
3.6Drawing valid conclusions; discuss and assess the relative effects of limitations in experimental procedures.
3.7Contribution made by scientists in respect of decision making
CIEBiology (9700)HDisease
PNatural selection
QProtecting threatened species
EdexcelA level Biology 8B101/9B1012.3Meiosis
2.4Some aspects of structure of plants, natural selection, conservation
3.3Factors affecting numbers and distribution of a species
3.4Mechanisms of infection
OCRBiology H021 H4212.2.2Some aspects of infection and transmission of disease
2.3.4Some aspects of biodiversity and conservation
5.1.2Some aspects of the effect of the environment on evolutionary changeMeiosisPhenotypic ratiosContinuous and discontinuous variation
5.3.1Factors affecting an environment
5.3.2Some aspects of factors which affect ecosystems (particularly with reference to timber in a temperate climate)

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.

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