Monthly Archives: May 2014

a new paper, a new hypothesis

Recently my PhD supervisor, Mark Forbes, and I have published a paper “A hypothesis to explain host species differences in resistance to multi-host parasites” in Ideas in Ecology and Evolution outlining a new hypothesis in testing the evolution of resistance of host-parasite associations (Forbes and Mlynarek 2014, find it here). This hypothesis outlines the reasons why certain hosts can kill their parasites while their close evolutionary relatives can’t.

Through observations of dragonflies and damselflies, we noticed that sometimes two closely related species differ in whether they are able to resist their parasites. We note three cases in this paper where one of the host species cannot kill (resistant) the parasitic mite whereas the other is a lot more resistant:

1- Sympetrum obtrusum and Sympetrum internum infected by a mite, Arrenurus planus (Forbes et al 1999) where S. internum almost always kills the infecting mites.

Sympetrum obstrusum. The more widespread of the two Sympetrum dragonflies

Sympetrum obstrusum. The more widespread of the two Sympetrum dragonflies


2- Leucorrhinia frigida and Nannothemis bella infected by a mite, limnochares americana (Lajeunesse et al 2004) where N. bella kills more their mites.

Nanothemis bella

Nanothemis bella. The little dragonfly that can kill its parasites


3- Nehalennia gracilis and Nehalennia irene infected by a mite, Arrenurus sp (not yet described) (Mlynarek et al 2014) where N. gracilis always kills all the infecting mites

Nehalennia irene

Nehalennia irene. The very widespread damselfly that cannot kill its parasites


Each of the host species that kill their mites more readily are also the host species that are less common. By less common I mean that they cannot be found in a few locations.

The obvious question is ‘why would it be that the less common species is better at getting rid of their parasites than the more common one?’ Logically, it should be the other way around. The common species, because it comes into contact with more parasites species, should be better at generally recognizing what would hurt them rather than the uncommon species that has not had the same experience. But clearly in several cases of dragonfly hosts parasitized by water mites it is not the case. But the uncommonness of the species may be where their success lies.

This paper therefore puts forth a reason for the pattern that we see. I mentioned before that each of the species that kills their parasites is present in fewer locations. When a species is in fewer locations, it probably cannot disperse or travel to other locations. So it can only breed with the other individuals of that species present at that location and this leads to less genetic mixing between populations for this species. This may benefit that species because certain individuals are successful at killing their specific parasites and then those individuals pass those traits to their off-spring and resistance can evolve with time. Whereas the common species, that can easily mix individuals between locations, may swamp out those resistance traits because there is just so much of that species everywhere.

So this hypothesis proposes that it may be good to be rare! At least it may be good to be rare to a certain extent.


Forbes, M.R. and J.J. Mlynarek. 2014. A hypothesis to explain host species differences in resistance to multi-host parasites. Ideas in Ecology and Evolution 7: 17-24.

Forbes, M.R., Muma, K.E. and B.P. Smith. 1999. Parasitism of Sympterum dragonflies by Arrenurus planus mites: maintenance of resistance particular to one species. International Journal of Parasitology 29: 991-999.

Lajeunesse, M.J., Forbes, M.R. and B.P. Smith. 2004. Species and sex biases in ectoparasitism of dragonflies by mites. Oikos 106: 501-508.

Mlynarek, J.J., Knee, W. and M.R. Forbes. 2014. Explaining resistance in a multi-host parasite. Evolutionary Biology 41: 115-122.

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Grad School Done!

I have successfully defended my PhD thesis “Explaining Interspecific Variation in Susceptibility and Resistance to Parasitism in Damselflies”! My program took me 4 years and 3 months to complete; it was worth every minute. My defense was difficult, I could have done so much better and I know it, but I did my best. Going into the defense I felt theoretically and practically ready, I was just too nervous. But even through my nerves, my examining committee saw that I knew my stuff. I’m glad they did.

Because I’m very fresh out of graduate school, so I thought I’d highlight a couple keys points that I noticed every one of my graduate student friends/colleagues have.

First, patience and perseverance! For the students thinking about graduate school, it is a slow process with lots of highs and quite a few lows. You have to be able to persevere! You will persevere if you like what you’re doing and if you are passionate about what you are studying.

Second, a hobby! This one may surprise you considering that graduate studies are so time consuming that they take up most of your time and are pretty much considered a lifestyle. But you need a hobby to take your mind away from your project at least for an hour a day. Mine is my dog (Maya), maybe I shouldn’t call her a hobby. But she makes me happy. Taking her on walks/hikes and playing/training her gives me the necessary breaks. So whatever you do, find something that pulls you away from your graduate projects. It should be something that doesn’t make you feel guilty that you are not working on your thesis.

Remember to have fun!

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