Parasite of the month

European mistletoe (Viscum album)

All around North America and Europe, mistletoe is used as a Christmas decoration. Yet, this evergreen plant is one of the strangest, from a morphological and a biological point of view.

The European mistletoe is a hemi-parasitic plant (i.e. parasitic with some photosynthetic abilities) that survives by driving its roots into the bark of a host tree and sucking water and nutrients off of it. It can be found over 200 different trees and shrubs in the Northern Hemisphere and can easily take down a healthy host in case of large infestation.

During the winter, it produces white berries that contain a green seed surrounded by a viscous substance – the viscine. In the spring, the seeds, stuck to the host tree by the miscine, will germinate. A cylindric organ, the hypocotyle, will emerge and will orientate within a few weeks towards the host plant through negative phototrophism (the hypocotyle basically flees the sunlight, which is quite rare because most plants use positive phototrophism). Once contact has been made with the bark, the tip of the hypocotyle turns into a fixation cone that will reinforce the fixation of the seed, already attached to the host plant by the viscine. At this stage (in July), the mistletoe is no longer free-living.

During the growth, the hypocotyle presses the host tissues so that the cone literally pierces the bark of the host and reaches the layer of so-called living wood – the xylem (used to transport water and nutrient in vascular plants). The penetration of host tissues is facilitated by the production of hydrolytic enzymes by the cone and will take several months. During this process, the cone turns into a primary sucker that divides itself into secondary suckers that siphon water and nutrients from the xylem.

After this first winter, the mistletoe will grow and the suckers will keep contact with the xylem of the host without being compressed by the growth of the host. The mistletoe undergoes a dichotomic growth, a growth process quite widespread in algae but very rare in embryonic plants (i.e. plants that produce seeds). Actually the growth of mistletoe – symporadic growth – differs from typical dichotomic growth in that the terminal bud always dies within a year, leaving two auxiliary buds develop. It will take four to five years for the mistletoe to produce flowers and berries.

In the Anglo-Saxon paganism, the mistletoe is associated to Freya, the goddess of love, beauty and fecundity. The custom tells that a man had to kiss any maiden who is under a mistletoe sheaf placed under the roof. In the Gaelic world, druids considered mistletoe as a sacred plant. They believed the plant had miraculous properties that can be used to cure disease and protect humans against witchcraft. In these times, if enemies were to encounter under mistletoe in a forest, they had to disarm and observe a truce until the following day. This would have led to the custom of suspending mistletoe to the roof and to kiss under it as a sign of friendship and goodwill. Since the age of the druids, the medicinal properties of the plant have been uncovered and many drugs made from mistletoe are used in Europe to treat hypertension, to prevent atherosclerosis and to slow down the progression of cancer.

source for picture: https://www.ces.ncsu.edu/wp-content/uploads/2014/12/mistletoe.jpg

Parasite of the month

Cyanophages

I visited relatives in France last August and, when my plane took off, I photographed the archipelagos surrounding Helsinki. When we passed the shore of the neighbouring city, Espoo, the reed line suddenly became quite evasive. The colour and structure were somehow different from usual reeds and it was only when the plane flew over the open sea that it became clear to me: these were cyanobacterial blooms (also known as blue-green algae). Later on, I could see large blooms offshore. Although I had observed these in small lakes before, I had only seen pictures of such large cyanobacterial blooms that occur in the Baltic Sea. The boat on the picture looks very tiny...

Among the diversity of cyanobacteria that can contribute to the bloom, Nodularia spumigena is the most common and troublesome in the Baltic Sea. Nitrogen and phosphorus concentrations have rised in the last century in the Baltic Nutrient as a result of human activities that increased productivity (i.e. generation of organic matter) - i.e. eutrophication - in marine ecosystems. High availability of resources for primary producers favours the growth of fast-growing organisms, such as cyanobacteria. In addition to directly competing for light and nutrients with other organisms, N. spumigena produces toxins that can accumulate through the food-web either when the cyanobacteria is consumed by grazers (e.g. copepods, daphnia) or in their dissolved form, when the bloom decays.

Cyanophages are important players in bloom termination. Although most of our knowledge is restricted to marine phages, virus of cyanobacteria have also been detected in freshwaters but their role in freshwater ecosystems remains unclear. Current work aims at controlling blooms with grazers (e.g. great pond snail, Lymnaea stagnalis) and cyanophages. Viral lysis has been notably discussed to affect dissolved nitrogen concentration not only by limiting its consumption by regulating cyanobacteria populations but also by inducing the release of nitrogen from lysed cells. One of the best examples of how parasites can alter whole ecosystems.

Behavioural manipulation: 10 bedtime stories!

Behavioural manipulation is one of the most impressive consequences of parasitic infections. In order to increase their transmission rate, some parasites alter the behaviour of their host, sometimes leading to its death.

Suicides of hairwom-infected crickets (Thomas et al. 2002) and of mice infected with protozoa (Berdoy et al. 2000) are among the best-documented examples where host death is necessary for parasite reproduction or transmission. Such manipulations can be very simple, such as for the eye fluke Diplostomum pseudospathaceum, which encysts in the eyes of its fish host to cause blindness and increase predation by the fish-eating bird final host (Karvonen et al. 2003). Other parasites, such as the trematode Schistosoma mansoni (Kavaliers et al. 1999), use more sophisticated manipulation by secreting of neuroactive substances to alter host behaviours.   

Ten bedtime stories of behavioural manipulation by parasites. 

Among these is notably mentioned the tapeworm Schistocephalus solidus, which is known to deeply affect the behaviour of its secondary intermediate host - the threespine stickleback Gasterosteus aculeatus. More about this parasite here!

Parasite of the month

Bracoviruses

These polydna viruses replicate in parasitic wasps that infect lepidopterans. Viral replication takes place in the ovaries of the wasp in calyx cells, allowing the vertical transmission of the virus. As such, female wasps are responsible for viral transmission but the virus is also found in male wasps.

Bracoviruses are not known to cause diseases in wasps. Instead, the virus facilitates infection of caterpillars by wasp eggs. When female wasps lay their eggs inside caterpillars, a quantity of virus is injected as well and prevents the immune system of the caterpillar from killing wasp eggs.

In a recent study, Gasmi et al. (2015 PLoS Genetics) found recurrent insertions of bracovirus DNA into lepidopteran genomes. This indicates not only that the virus can enter the germlin cells of the caterpillar by using parasitic wasps as vectors but also that this third-party might play an important role in the coevolution of braconid parasitic wasps and their hosts.

source for picture: https://burkelab.files.wordpress.com/2014/01/lifecycle2.png?w=625

Looks like a nice blog on parasites!

In spite of the summer break, here is a nice blog I came across today, "Parasite Ecology":

https://parasiteecology.wordpress.com/

Filled with cool parasite stories and funny cartoons for the young and the not so young scientists :)

Parasite of the month

Pleistophora mulleri

This microsporidian is a specialist parasite that replicates in the cells of the abdominal muscle of the freshwater amphipod Gammarus duebeni. Although it was initially found in the Irish sub-species G. d. celticus (Terry et al. 2003), an extended survey evidenced its presence in other sub-species of G. duebeni in Europe (Ironside et al. 2008). Infections by P. mulleri were found to reduce foraging abilities of G. d. celticus on smaller amphipod species and to increase predation risk by larger amphipods. Accordingly, the microsporidian can affect invasion process by reducing the predation of G. d. celticus on smaller invading species such as Crangonyx pseudogracilis, and to increase predation of infected individuals by larger invaders, such as G. pulex (Fielding et al. 2005).


It is only known to be transmitted between individuals through cannibalism (MacNeil et al. 2003). By comparing selectivity of cannibalism between infected and non-infected individuals, Bunke et al. (2015) showed the parasites to alter host behaviour. Indeed, healthy gammarids avoid preying on infected conspecifics while this is not the case for infected individuals. Although it is not clear if this alteration of host behaviour is adaptive or not for the parasite, the authors suggest it to be a by-product of infection, rather than a manipulation. Yet, the effects of cannibalism and of its alteration by P. mulleri on the invasion of C. pseudogracilis and G. pulex remain to be explored.

source for picture: http://cdn4.sci-news.com/images/2015/03/image_2618-Gammarus-duebeni-celticus.jpg

Parasite of the month

Nosema spp. 

These microsporidians parasitise invertebrates. Most nosema infect insects and the best-known parasitize bees. These get infected by ingesting spores of the fungus. Multiplication in the epithelial cells leads to the lysis of the infected cells and the release of new spores within 10 days. Transmission of nosema between individuals is facilitated by prophylaxis in eusocial bees. Nosema can prevent colonies to thrive in the spring, following overwintering, and are thus considered as a pest by beekeepers.

Nosemosis has become an emerging disease in the last decades and is now listed by the World Organization for Animal Health (Office International des Epizooties - OIE) because of its role in the worldwide decline of bees. A growing number of studies thus reveal the influence of pesticides in nosema outbreaks. For instance, Pettis et al. (2013) investigated the interaction between pesticide exposure and the susceptibility of the honeybee Apis millifera to the gut pathogen Nosema ceranae. The results indicate that bees exposed to pesticides were more likely to become infected.  Interestingly, fungicides had the strongest effect on bee's susceptibility to infection, confirming the high resistance of nosema spores to extreme environments. Moreover, the researchers found that in many samples, bees carried a significant proportion of pollens from weeds rather than from cultivated crops in the sampled areas. It is thus likely that the exposure of foraging bees to pesticides is currently underestimated.

source for picture: http://www.athbui.com/Nuachtlitir%20pics/California%20Beacha/Nosema%202012%20%2811%29.JPG

Parasite of the month

Anguillicola crassus (Anguillicoloides crassus)

 

This nematode infects the Japanese eel, Anguilla japonica. Free-living larvae settle in the substrate to get ingested by an intermadiate host, typically a copepod. Young eels get infected by feeding on infected copepods. Interestingly, when infected copepods are eaten by other fishes, the parasite remains alive and can be transmitted from these paratenic hosts to larger eels feeding on them. In the eel, the parasite migrates to the swim bladder where it sexually produces eggs and causes pathological damages (see Muñoz et al. 2015 and references therein). These eggs then pass through the digestive tract and are released into the water.

Importation of Japanese eels to many places of the world for aquaculture is believed to have facilitated the spillover of A. crassus to native eels (Dangel et al. 2013).  Within 30 years, most populations of European eels A. anguilla became infected and the parasite is now specializing to infect its novel host (Weclwaski et al. 2013). This rapid spread to native species is connected to the high plasticity of the parasite's life-cycle. Larval infections have been reported in a range of organisms, including aquatic insects and amphibians (Moravec &Skoríková 1998). Recent evidences suggest that the use of paratenic hosts might be facilitated by hyperparasitism of native parasites by A. crassus (Emde et al. 2013). As a consequence, very few measures can be installed to protect native species from this invader and the spillover continues to cause high mortality in native eel populations (Kirk 2003, Barry et al. 2014).

source for picture:http://web.abo.fi/instut/fisk/Fin/Parasiter/anguillicola.htm

Joensuu here we come!

Ecologists and evolutionary biologists will meet in Joensuu (Finland) on 9-11 February 2015. The meeting is organised under Oikos Finland and supported by Nordic Society Oikos, Suomen Biologian Seura Vanamo and by Societas Pro Fauna et Flora Fennica.

During two days, the Finnish national meeting for ecologists and evolutionary biologists will gather over 215 participants to the Joensuu Campus of the University of Eastern Finland.

Plenaries include Paula Harrison (Univ. Oxford), Anssi Karvonen (Univ. Jyväskylä), Elizabeth Borer (Univ. Minnesota) and Toni Laaksonen (Univ. Turku).

Here is a more detailed program: https://www.jyu.fi/bioenv/en/divisions/eko/ecology_meeting/Programme

source for picture: https://www.jyu.fi

28:06:42:12

The end is near!

Feels weird to finally see my name here (but good weird!): http://www.helsinki.fi/bio/tutkimus/vaitoskirjat/  

Here is a link to the electronic version of my doctoral dissertation: https://helda.helsinki.fi/bitstream/handle/10138/152906/swimming.pdf?sequence=1

I'll defend my doctoral dissertation on the 30th of January at 12 o'clock noon at lecture hall 2, Infocentre Korona (Viikinkaari 11, Helsinki).

Comment on the image: from what I have heard, it is not given to you actually - you just get the right to buy it.