But what about the question?

It is all about the question. Without a solid question all the data in the world won’t make sense. There has even been recent debate as to whether or not the recent boom in DNA sequencing technology is actually helping the field of parasitology for similar reasons; its data but it is not helping to answer the fundamental questions surrounding parasite biology. I tend to disagree with this point of view but I am reminded of the importance of the question each time I am involved with the teaching of the University of Lethbridge field biology course.

In this course students spend a week in the Cypress Hills Interprovincial Park learning the basics of field biology/ecology. Mainly, how to take an observation made in the field and develop a question to test the basis of that observation in a scientifically sound manner, i.e. using replicates and randomization to ensure good data. There is also a great deal of” McGuiverness” also required because often the equipment that can be feasibly brought to and still function in the field is limited and fairly basic. One project carried out this year really shows how asking a simple question and devising a simple experiment can provide data and insight into some very relevant and interesting biological and ecological aspects of the Dicrocoelium manipulated behaviour system. From there we’ll speculate on how to further bring this into the lab, hopefully showing how the field and lab need each other despite what each may say of the other.

In this project the students asked the question of the Dicrocoelium infected ants found in the Cypress Hills Park, will they return to the same plant they were clinging to if given the choice of another plant. Check out the About page for more background on Dicrocoelium and its effect on its ant hosts but in brief the parasite causes ants to cling to vegetation when the temperature is cool. The parasites do this in order to put themselves in the path of grazing mammals that accidently ingests the infected ants clinging to the vegetation. The important thing here is that the parasite relinquishes control of the ant when the temperature gets to warm whereby the ant may die. Ok, so back to the question. If we raise the temperature and the ants stop clinging and then lower the temperature will the ant return to the same flower if given another plant of the same species to cling to as well? In order to test this, plants with a single infected ant were clipped and placed into a container with another flower head of the same species. The containers were then placed in a lit area until the ant inside released from the plant. The container was then placed in a shaded area to decrease the temperature and it was observed whether the ant returned to the plant, went to the new one or never returned to cling. If they do consistently return, this would indicate that they somehow know or are attracted to the original plant they clung to. Why would they do this? How do they know which plant they were on, good memory, a chemical cue perhaps? We can’t really answer how they would know here but this may lead us to a plausible explanation of why.

The ant containers

The ant containers

If infected ants left some sort of a clue as to where they had visited and where more likely to return, not only would they be more inclined to return to their original site of clinging but other infected ants may too be attracted to come and join them and this may be of advantage for the parasite. The parasite driving this clinging behaviour has the ultimate goal of getting into the final host that eats it off the plant. Once in that host it will then need to find a mate to reproduce with so, if there are lots of ants each with parasites inside all clinging in the same area, or flower it would seem likely that they would all get ingested at the same time. Then they would all end up in the same host with more potential mates. It was this line of thought that led to the second part of the field experiment.

Using the same set up as before, the containers (clinging arenas) with flower heads in them, the students set up an experiment to test the question: if four infected ants are made not to cling and then given a choice of four plants of the same species will they tend to cling in groups on the same flower or each choose a different one? By first asking if they would go to the same plant, indicating some sort of a memory or physical cue leading them back to the same plant a theory explaining why this may be biologically relevant was born, leading to a second experiment to further test that theory. Now I do not have the students’ data and will wait for their report in order to say if we got a reasonable answer to this question but let us imagine taking this to the molecular biology lab to find further justification for the nice little theory devised here.

A close up of one clinging arena.

A close up of one clinging arena.

Inside of each ant there are multiple parasite individuals, this number can vary from one to hundreds. From a parasite perspective then depending on how many other individuals are in the same ant the chances of finding a mate can be good or bad. Now this isn’t in the parasites control, so as we have mentioned, getting all the other infected ants to hang out and get eaten together would be in your best interest for this reason alone. However it may be of further importance from a genetic standpoint. Species that use sexual reproduction do so in order to recombine genetic material and create new and better combinations of genes. They are maximising genetic diversity. One of the ways to do this is by mating with individuals that are not genetically similar to you. This adds an extra level of consideration that now must be taken into account by the parasites within these ants. Are parasites infecting a single ant more or less likely to be related to each other than they are to parasites infecting another ant? This is a question that can be answered with molecular biology.

By using molecular biology techniques we can, in theory, determine the genetic relatedness of the individuals within each ant. This will not only provide evidence for or against the idea that the parasite wants to get as many ants as possible to be ingested together it can also shed light onto how the parasites are being transmitted to the ant, remember they get infected by snails, but we won’t even start speculating on that part of the life cycle.

Dicrocoelium life cycle

Dicrocoelium life cycle

The point of all this was to show firstly the importance of having a concise testable question not only for field research but all research and secondly how field work can lead to lab work. Using fancy lab techniques can often be regarded as the “better” of the two options but without observations and data from the field, sometimes the practical meaning behind the genetic data may get lost.  Plus a whole summer inside a lab can get quite dreary.

Brad van Paridon

Which manipulator is the baddest of them all?

 

Dicrocoelium is a stand out example of a parasite altering host behaviour when viewed alongside other similar parasites due to the level of sophistication in the control wielded by the parasite (for background info on Dicrocoelium click here).  In the most basic of examples of parasite induced altered behaviour there isn’t so much mind control as there is physical handicapping.  Take for instance the trematode parasite Curtuteria australis which burrows into clams. Once burrowed the trematodes form cysts in the foot (the part that sticks out of the clam shell) of the clam which, with enough accumulation, can start to impede the clam’s ability to dig in the sand.

This inability to dig efficiently makes them more vulnerable to one of their main predators, birds.  Which, you guessed it, are hosts for the clam dwelling parasites.  This is considered a basic form of manipulation because the case could be made that this is coincidental and not actually a deliberate act of manipulation that is being perpetrated by the parasite, it could be viewed as a by-product.  If one takes the stance that these parasites have evolved to specifically effect or control the host in a way that is of direct benefit to the parasite by increased transmission or fitness for example, then we must include any systems where morbidity due to infection could lead to increased predation as parasite altered behaviour.

Dicrocoelium shows higher levels of manipulation because it is not merely impeding function or health but rather; infection leads to the ant hosts abandoning any semblance of normal ant behaviour for a time and acting in ways that uninfected ants would never do.  There are a number of systems in which the manipulation shows this level of sophistication but I believe Dicrocoelium still has them beat. The first of these systems happens to be one of the most famous, the Cordyceps fungus.

This fungal parasite infects ants with airborne spores that float on the wind and land on the host. There are many species of insects each with its own arthropod host, insects and spiders mainly. The fungus then grows into the host brain and as it does it seems to make them go mad. Rightfully so too, I mean there’s only fungus growing in its brain.  The hosts start to not only twitch and move erratically but they also seem compelled to head up, climbing plants until the fungus has grown so large that it kills the host and bursts out of its head. Hopefully by this time the ant has climbed up to a good spot where the spores have the greatest chance of being carried farther on the wind.  This is an amazing sight and there are lots of cool videos and research on these killer mushrooms but in my mind it still doesn’t quite have the level of precision that Dicrocoelium or my next example exhibit.

The manipulation of the behaviour by the Cordyceps is clear but it is one way.  Once the infection is in the behaviour is changed and the effects increase with time culminating in death, unlike Dicrocoelium which does not kill its host.  Furthermore, this could be another example of impediment meaning the weird behaviour could merely be a side effect of the fungus destroying the ant brain.  As I’ve discussed in a previous article on Toxoplasma, figuring out the difference can be difficult.

The last example I want to bring up before I get to Dicrocoelium shows a highly evolved level of host manipulation but it too is one way. This last example is also different because it is an example from a parasitoid.  What’s this?  A parasitoid is an organism, many species of wasps actually, whose offspring obligatorily live off a host and which always causes death. These hosts tend to be eaten alive. The Emerald Cockroach Wasp is just one of these organisms and also has lots of cool videos on YouTube.

This wasp has evolved to out duel cockroaches and plant two deadly and precise stings to the brain of the roach.  These stings inject an amazing bit of venom that quickly acts on the cockroach nervous system, the first one freezing the roach for a small amount of time so the second sting can be made.  The second sting takes away the cockroach’s ability to make its own decisions making it completely at the whim of the wasp who then leads the roach, like a dog on a leash back to its lair where it will become the main course for the hungry wasp larvae.  The wasp lays the eggs on the roach, who just sits there, not moving, unable to regain control of its mind and body for up to TWO WEEKS while the eggs hatch and start eating it alive until there is nothing left.  This is one bad wasp.

There is one big reason for me though, as to why Dicrocoelium has all these others beat.  Dicrocoelium shows so such a high level of control in that it can somehow both take and restore control of the host; and it does so daily.  The infected ants climb up the plants and clamp on with their jaws remaining there, still alive, until they are accidentally eaten by grazing mammals, in which they grow to adults and reproduce.  The thing is though; they only do this when it’s cool out. In field observations from our lab the maximum temperature is around 19oC.

The ants can be witnessed clinging to plants in the early morning and in the evening but during the hot parts of the day they are nowhere to be found.   There is the odd one that apparently doesn’t get the message and ends up dying on the plant, but these aren’t that common.  This switching on and off, of the behaviour, shows a level of control unlike any of the other systems and is likely the way in which the parasite perfectly balances the payoff of increasing its exposure to the next host, with the cost of host death by exposure to heat and other elements.  It is sacrificing short term prolonged exposure, staying out all day, with intermittent exposures, going out when the host is less likely to die, that can then go on indefinitely.

There is also less evidence for the control to be a side effect of damage or poor health as the host is never killed by the parasite.  In fact it’s in the best interest of the parasite to keep this host alive.  This raises some interesting questions too about what the ants’ behaviour is like when it’s not clinging to plants.  Does it revert back to normal, meaning the parasite no longer has control and that the host is at risk of dying during its day to day ant life; which could be significant, ants naturally lead dangerous lives.  Or does the parasite retain control and compel the ants to return to the nest maybe, where they could be fed and protected, ready to go back out only to try and pass along their parasitic friends to unsuspecting hosts.  This would be an enormous level of control exhibited by this parasite if the latter turned out to be true, adding to an already impressive resume which also boasts the potential ability to detect temperature cues.  Let’s not forget that one, they seemingly know when the right time to stop and start their clinging is.  This is why the Dicrocoelium parasite is the finest example of host mind control and truly worthy of the Zombie Ants title.