The partners in crime that could solve the case…

One of the strongest ecological structuring forces the world has ever known, that has influenced the genetic make-up of every organism on earth, is that of the partnership between parasite and host. The term parasite may evoke images of tapeworms being pulled theatrically out of patients, of wasps laying eggs in a caterpillar’s body cavity or of a poor male crab being irreversibly denied its gender. A parasite is defined as an organism that feeds on a host without killing it on its first encounter and may require that host, or a number of host organisms, in order to complete its life cycle. It is therefore a term that when used in its widest sense includes viruses, bacteria, protists (microparasites) and eukaryotic organisms (macroparasites) the world over.

It is not surprising that scientists estimate that the first immune systems arose in near synergy with the emergence of protozoans 2.5 billion years ago (Ellis et al., 2011). The arrival of invading bacterial species provided the selective pressure to induce the development of effective immune systems and thus sparking the greatest arms race of all.

Over evolutionary time parasites have had to continually develop mechanisms to avoid and overcome host immune systems while hosts have developed increasingly intricate ways to prevent parasitism. So that ‘any advancement made by either host or parasites creates a selective pressure on the counter part; there is no winner, only continual co-evolution’ (Dawkins and Krebs, 1979). In fact it was theorised by the esteemed Leigh Van Valen in the 1970s that sex itself is a mechanism brought about by the need to diversify genes and outsmart those pesky parasites (The Red Queen Hypothesis; reviewed recently by Moran et al., 2011).

Knowledge and understanding of the immune systems of terrestrial vertebrates such as cows, dogs and us has provided human society with ways to manage and control diseases. In some cases eradication has even been possible e.g. Polio. This branch of science has been key to the success of breeding livestock and managing fisheries for an ever increasing population and it has the potential to do the same for two other sectors of importance to Britain. The immune systems of plants and invertebrates have been poorly studied in comparison to vertebrates and new evidence in both fields is breaking down some old assumptions and potentially paving the way to a brighter future for Britain’s seas and wildlife.

Invertebrates
Disease in crustacean aquaculture is the biggest restraint to this industry (Stentiford et al., 2012). New evidence is revealing the long hidden and ignored immune mechanisms of invertebrates and in particular crustaceans. Research in this field throughout the world, including that occurring in the UK, has the potential to revolutionise our understanding and improve disease prevention in this key sector (Hauton, 2012). It was an invertebrate gene, from the common fly (Drosophila drosophila) that displayed the unprecedented degree of variation and reinvigorated research into invertebrate immune systems (Schmucker et al., 2000). The defensive gene (DSCAM) in question has the ability to produce upwards of 20,500 different isoforms (species dependant) which is draws parallels to the diversity of antibodies that we ourselves produce. It has been isolated in eight invertebrate species, most of which are marine crustaceans (including model and commercial species; signal crayfish, pacific white shrimp and the giant tiger prawn). The precise immune-role of the gene and its associated proteins is unknown but it is believed it may bind directly to invading bacteria and viruses or alert the immune system to their presence.

Aquaculture is the fastest expanding food industry in the world, producing 10M tonnes per year, and is one of the few marine resources in the UK with the potential for growth. However as crustacean species are being reared in ever increasing densities, in order to supply this huge demand, aquaculture facilities are breeding grounds for viruses and bacteria. In the 1980s when this situation began to arise, legislation controlling the testing and transport of crustacean products was slow to react and subsequently novel diseases, like White Spot Syndrome Virus (WSSV), spread across the globe. Nations heavily invested in aquaculture have sustained substantial losses since then. To date the entire Asian continent loses 40% of production year on year to viruses (60%) and disease (20%). It was in Asia that investment in the industry was stemming rural-urban migration through the economic development of rural areas and communities.

With the world fisheries in a bad state and the potential for growth remaining in this sector, immune research concerning commercially important crustacean species could improve the UK’s food security. A transatlantic report published earlier this year by a member of CEFAS highlights this issue in relation to lobsters.

Trees and Plants
Trees are a vital part of our heritage and economy as well as making huge impacts on both rural and urban landscapes. Trees and woodlands provide invaluable services to our civilisation in Britain; from naturally filtering water so that it is potable (drinkable) to preventing flooding and possibly reducing urban impacts. In particular it was highlighted in the ‘Read Report’ the vital role UK woodlands could play in carbon storage and reducing UK CO2 emissions. This is a topical issue considering the legally binding targets set out in the Kyoto Protocol (a cut in GHG emissions, 12.5% below 1990 levels) which have not been met and the target set out in the Climate Change Act 2009 (80% cut in GHG emissions below 1990 levels by 2050).

As the climate changes parasite geographic ranges will extend and enter new regions to encounter new hosts and this could threaten plans to restore UK forests and mitigate rising CO2 levels. In the UK we are experiencing warmer and wetter winters due to the action of the North Atlantic Oscillation, which has been in a predominantly positive state in recent years. This allows existing parasites in the UK to behave more virulently and spread more readily (Harvell et al., 2002). The problem is also amplified by the common issue of increased international trade of plants (as stated above for aquaculture).

There are two issues with the arrival of non-native parasites; 1- is that if not immediately recognisable it takes between 10-20 years for the damage to be recognised both in the field and statistically; 2- invasive parasites (of all kinds) cause high mortalities and infection rates as internal biological systems of both the host (Trees) and the parasite have not evolved and adapted in synergy. This also has an impact on the way we conserve our trees in Britain and the iconic species that must be prioritised (Kirby et al., 2010). Immunological research into the mechanisms that govern the plant immune system will form the foundation for more effectual management and control of disease.

The mechanisms that are employed within plant immune systems are still not fully understood (a good review of current theories was given by Jones and Dangl, 2006; Coll et al., 2011). As plants and trees are sessile organism they are continuously integrating biotic and abiotic signals from the environment (Jones and Dangl, 2006). Deciphering these from the parasite-host interplay takes time and expertise, but this research is extremely worthwhile as it will undoubtedly lead to effective disease control and forestry management. Even once they are understood in a general sense; nuances between classes and orders are likely as some diverged earlier in their evolutionary history.

The discipline of comparative immunology falls under the umbrella term of taxonomy and systematics. This umbrella subject was highlighted as a critical skill gap in NERCs 2011 skills report and given the number of Universities committed to offering PhD’s and postdoctoral positions within these two fields you can see why (summary entitled ‘MOST WANTED’). Last week in our review of the highly successful Policy Lunch Box on Research Careers the points made about ‘making the case for scientific research’ in the lead up to the 2015 spending review ring very true for this field. The pioneering research within these two fields of immunology have a great potential to mitigate these and other key issues facing the UK and the world. It also highlights the importance of funding bodies to horizon scan for future problems and invest in research that may seem ‘blue skies’ at the time but will contribute to the advancement of a country’s economy and social wellbeing.

It is encouraging to see the new ‘Tree Health and Plant Biosecurity’ initiative has been set up to ensure funding is being targeted at tree parasites and management. This is under the LWEC partnership between the BBSRC, Defra and the Forestry commission and they will be accepting research proposals in April of next year.

At the BES Annual Meeting in December we will be running a BRAG/BES Special session on the Sustainability of Aquaculture and Agriculture and if this article sparked your interest in that field event details can be found here.

References
Behringer, D.C., Butler, M.J., Stentiford, G.D. (2012) Disease effects on lobster fisheries, ecology, and culture: overview of DAO Special 6. Diseases of Aquatic Organisations. 100: 89-93

Coll, N.S., Epple, P., Dangl, J.L. (2011) Programmed cell death in the plant immune system. Cell Death and Differentiation. 18(8): 1247-1256

Ellis, R.P., Parry, H., Spicer, J.I., Hutchinson, T.H., Pipe, R.K., Widdicombe, S. (2011) Immunological function in marine invertebrates: Responses to environmental perturbation. Fish Shellfish Immunol. 30: 1209-1222

Dawkins, R., Krebs, J.R. (1979) Arms Races between and within Species. Proceed. R. Soc. Biol. Sci. Lon. 205: 489-511

Harvell, C.D., Mitchell, C.E., Ward, J.R., Altizer, S., Dobson, A.P., Ostfeld. R.S., Samuel, M.D. (2002) Climate warming and disease risks for terrestrial and marine biota. Science. 296: 2158-2162

Hauton, C. (2012) The scope of the crustacean immune system for disease control. Journal of Invertebrate Pathology. 110(2): 251-260

Jones, J.D.G., Dangl, J.L. (2006) The plant immune system. Nature. 444: 323-329

Kirby, K.J., Perry, S.C., Brodie-James, T. (2010) Possible implications of new tree diseases for nature conservation. Quarterly Journal of Forestry. 104: 77-84

Locker, E.S., Adema, C.M., Zhang, S., Kepler, T.B. (2004) Invertebrate immune systems – not homogenous, not simple, not well understood. Immunol. Revs. 198: 10-24

Moran, L.T., Schmidt, O.G., Gelarden, I.A., Parrish, R.C., Lively, C.M. (2011) Running with the red queen: Host-parasite coevolution selects for biparental sex. Science. 333: 216-218

Schmucker, D., Clemens, J.C., Shu, H., Worby, C.A., Xiao, J., Muda, M., Dixon, J.E., Zipursky, S.L. (2000) Drosophila Dscam is an axon guidance receptor exhibiting extraordinary molecular diversity. Cell. 101: 671-684

Schulenburg, H., Kurtz, J., Moret, J., Siva-Jothy, M.T. (2009) Introduction. Ecological Immunology. Phil. Trans. R. Soc. Biol. Sci. 364: 3-14

Stentiford, G.D., Neil, D.M., Peeler, E.J., Shields, J.D., Small, H.J., Flegel, T.W., Vlak, J.M., Jones, B., Morando, F., Moss, S., Lotz, J., Bartholomay, L., Beringer, D.C., Hauton, C., Lightner, D.V. (2012) Disease will limit future food supply from global crustacean fishery. J. Invert. Path. 110: 141-157

Watthanasurorot, A., Jiravanichpaisal, P., Liu, H., Söderhäll, I., Söderhäll, K. (2011) Bacteria-induced Dscam isoforms of the crustacean, Pacifastacus leniusculus. PLOS Path. 7(6):e1002062