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Insect herbivores and the drivers of plant speciation

This is a guest post by Christina Baer, a Ph.D. candidate in Ecology, Evolution, and Systematics at the University of Missouri-St. Louis

Fifty-two years ago, two biologists published a huge paper proposing that caterpillars and their food plants evolve in response to each other (Ehrlich and Raven 1964). They presented quite a bit of evidence for this: closely related caterpillars usually eat closely related plants and can tolerate the same defensive chemicals produced by plants. And in the last fifty years, biologists have found many examples of plants changing their chemical defenses in response to insects, or insects evolving new ways to get around plant defenses. But Ehrlich and Raven went one step further and proposed that insect herbivores could cause new plant species to evolve. Unfortunately, they didn’t suggest any step-by-step scenarios for how insects could cause plant speciation, which made it difficult to test the hypothesis. So far, no one has.

Several years ago, the Marquis lab decided that we would try to come up with ways that herbivorous insects could cause new plant species to evolve. To do this, we had to think of ways that insect herbivory could affect different groups within a plant species differently. If the entire species responds the same way, then the species’ characteristics will change, but it will still be one species. So we brainstormed all the different things that could cause individuals to respond differently (location? soil? timing?) and looked for previous research that illustrated each step.

The simplest scenario we developed would apply to plants that use the same chemicals to defend their leaves and attract pollinators. In both cases, simple chemical changes can have dramatic effects on how the animals respond. If some plants evolve new defensive chemicals in response to herbivores, then those same chemicals could attract different pollinators to the flowers. If the original pollinator is not attracted, then the plants with the new chemicals would be reproductively isolated from the rest of the population. Given time, the two groups would separate into different species. To see the other scenarios we came up with, you can check out our new article, “Ode to Ehrlich and Raven or how herbivorous insects might drive plant speciation”.

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Scenarios by which herbivores may coevolve with host plants: a) the reciprocal arms race model, and b) the speciation arms race model. From Marquis et al. 2016.

Writing this article was interesting for me because it was very different from writing up an experiment. With an experiment, you usually start in the middle of the article (the methods and results) and work out to explain why you did the experiment (the introduction) and what it means (the discussion). It’s fairly clear from the beginning what needs to be in the article and how it should be organized. With a conceptual article, it’s much less obvious what needs to be included and it takes even more work than usual to make your ideas clear and organized. I think the article got pulled apart and put together in a completely different order two or three different times while we writing it. At one point, I made a list of each paragraph’s main ideas so that I could arrange them in different orders and see what made the most sense. In the end, it took about as long for us to write the article as it did to come up with the possible scenarios.

Conceptual articles are important for scientists because they rearrange what we know and help us see how we can test ideas in new ways. In this case, we’ve outlined ways of testing for herbivore-driven speciation and pointed out a couple of systems that already have evidence for several steps. We’re hoping that other ecologists will use these ideas to finally test Ehrlich and Raven’s hypothesis. Since most of the scenarios we came up with are complicated, it will be interesting to see how common herbivore-driven speciation is.

Cited papers:

Marquis, R. J., D. Salazar, C. Baer, J. Reinhardt, G. Priest, and K. Barnett. 2016. Ode to Ehrlich and raven or how herbivorous insects might drive plant speciation. Ecology 97:2939-2951.

Ehrlich, P. R. and P. H. Raven. 1964. Butterflies and plants: a study in coevolution. Evolution 18:586-608.

About the author:

cbaersnapshot

Christina Baer is a PhD candidate in the Marquis Lab at the University of Missouri-St. Louis. Her research interests include plant-insect interactions, natural history, and community ecology, so she’s doing her dissertation research on how tropical caterpillars build shelters to protect themselves from predators and parasitic insects. She wants to be a professor when she grows up.

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Coming back with a 3-Min competition

It’s been a while since we have posted, we know. In the past year, one of us had two offspring (Maria is officially Dr. Pil, mother of Sebastião), and I’m in homestretch of my one and only offspring, my dear dissertation. The truth is: I miss the blog, I miss writing without peer-review manuscript prep or grant purposes, I miss talking about science that fascinates me, and I miss rambling about academia.

3MT-logoMy ‘return post’ will be about a recent experience I had – a 3-MIN thesis competition! I signed in almost instantaneously after I read the email from grad school about the competition. Don’t get deceived, I was prompt to take part in it not because I love competitions, but because I did a 2-min ‘lightening talk’ on a meeting in the past, and it was an absolute disaster. I needed to try again.

I won’t say my performance was successful, but it was definitely better than the first time I tried giving these types of very (very) short talks. I started preparing my 3-MIN talk by watching the presentations of  the past winners of the worldwide 3-MIN Thesis Competition. Yes, there’s such a thing, and one can learn a lot by seeing the strategies people come up with to tell a complex story of years of research within such a short amount of time. The four strategies I noticed were:

1st. Impactful opening and closing statements. You need to start with something that catches people’s attention and end with a statement that will leave the audience thinking about your take home message.

2nd. Clear presentations very often use numbering/listing to organize ideas – this makes the content very easy to follow.

3rd. You need to make people relate to your research. No matter how much you think your research topic is important (and I bet it is), you lose your audience if you don’t articulate it on a way that people from different backgrounds relate to.

I wrote a speech with these three observations on mind. The core of my dissertation is to use historical biogeography to determine turnover rates of avian haemosporidian assemblages in the West Indies. “Say what?,” you may think, if you are not the N=very limited number of people who work with what I work. Below, I share how I adapted my speech. I didn’t win the competition, but I’m happy with the fact that I made it to the finals.

Time traveling with bird malaria parasites in the West Indies islands

by Letícia Soares

Time traveling is not a privilege of science fiction characters. In my research, I used the geologic history of islands in the West Indies as a time machine to determine how the distribution of bird malaria parasites changes over evolutionary time scales. World wide, 3 billion people live at risk of malaria, and a half million people die of it every year. Bird and human malaria parasites are closely related through evolution. In fact, malaria parasites are also a threat to bird populations, and the disease has lead to the extinction of at least 10 unique bird species in Hawaii. With that said, my research 1) determines how rapidly these parasites can jump from one population to another, and 2) how the disease spreads over time scales of millennia. Bird populations in islands are the perfect study model to understand the evolution of host and parasite interactions, because birds canʼt go to the doctor to get treated, and we can see how parasites and hosts evolve in a natural system. 2 thousand years ago, the Earth was going through a glaciation, and sea levels were very low, causing some islands in the West Indies to be connected by landbirdges that were once covered by sea water. During that time, birds could move across islands that, today, are isolated by sea. I used this history of past connection as a natural isolation experiment to determine how long it takes to observe changes in 1) the type and 2) the frequency of parasites infecting these populations. I searched for the parasite DNA on the blood samples of birds, and 5 thousand blood samples across 21 islands later, I found that within 2 thousand years, there is a complete turnaround on the malaria parasite strains and the frequency they occur in these bird populations. This fast turnaround indicates that within 2 thousand years, birds may evolve resistance to parasites and parasites may evolve alternative ways to exploit birds. My research shows that birds and malaria parasites in nature are like Alice in Wonderland, in the sense that for one of them to get somewhere else, it needs to run at least twice as fast than the other.

Herpes viruses got a friend: Helminth parasites can promote the reactivation of latent viral infections

In a fascinating story about co-infections and co-evolution, helminth parasites play a role in a two-signal reactivation pathway of latent infections of herpes-like viruses. 

The helminth Heligmosomoides polygyrus can re-activate latent herpes viruses through the modulation of transcriptior factors and inhibition of anti-viral cytokines. Photograph by Constance Finney.

The helminth Heligmosomoides polygyrus can re-activate latent herpes viruses through the modulation of transcriptor factors and inhibition of anti-viral cytokines. Photograph by Constance Finney.

We all know at least one person who has exhibited the signs of an infection by herpes viruses, as well as their complaints about how this inconvenient infection might re-occur after long dormant periods. In fact, more than 90% of the human population is accounted to latently carry viruses of the herpes family. Although most research on disease mechanisms and host immunity have focused on one-host-one-parasite systems, most vertebrates are known to carry a vast community of parasites, that can behave much like herpes viruses do, alternating between latent and active phases. There is evidence that parasites can interact when in co-infection, however little is known about the precise mechanisms through which these organisms deal with each other when exploring a common host.

In a study published this month in the Science magazine, researchers investigate how helminth parasites influence the end of the latency stages of herpes viruses in murine rodents. The researchers experimentally infect rodents with a herpes-like virus modified to express luciferase – a bioluminescent enzyme that can be used to track the viral replication inside the host. Then, they challenged the same rodents with infections of two different types of helminths, Heligmosomoides polygyrus and Schistosomiasis mansoni, and found out that both parasites promote viral reactivation. Interestingly, the helminths elicit viral ‘awakening’ through a cascade of cell-mediate immunity that starts with the activation of lymphocytes Th2. Once activated by helmintic infections, Th2 cells produce IL-4, which is the crucial factor on the re-activation of herpes viruses. The exit from the latency state is dependent on the expression of one viral gene (gene50), and such expression relies on the bond of a single signaling molecule to gene50. The misfortune of the host and the beauty of co-evolution come from the fact that IL-4, which synthesis is a product of the helminth presence, is the the activator of this one signaling molecule that promotes the expression of the gene necessary for the ‘awakening’ of the herpes virus. Also, IL-4 not only promotes viral gene expression, but also blocks the activity of anti-viral cytokines. Hence, the viruses only exit the latent state when the host immune system provides an ideal medium for their proliferation, by both stimulating viral re-activation and inhibiting anti-viral immunity – all thanks to helminths parasites. What a fine example of co-evolution and organismal adaptation! 

How helminths go viral: Helminth infection activates TH2 cells to release IL-4 and IL-13, both of which ligate the IL-4 receptor (IL-4R) on M2 macrophages. In M2 macrophages harboring latent herpesvirus, the IL-4R activates host cell STAT6, which then acts directly on the key viral gene that initiates viral replication. Figure and caption adapted from Maizels and Gause 2014.

How helminths go viral: Helminth infection activates TH2 cells to release IL-4 and IL-13, both of which ligate the IL-4 receptor (IL-4R) on M2 macrophages. In M2 macrophages harboring latent herpesvirus, the IL-4R activates host cell STAT6, which then acts directly on the key viral gene that initiates viral replication. Figure and caption adapted from Maizels and Gause 2014.

Reese et al, 2014. Helminth infection reactivates latent γ-herpesvirus via cytokine competition at a viral promoter. Science Vol. 345 no. 6196 pp. 573-577.

How blogs/blogging can add to your career

Last year, Inger Mewburn  & Pat Thomson published a paper (Studies in Higher Education 2013, DOI:10.1080/03075079.2013.835624) on a relatively small-scale research to analyze what academics blog about and why. Blogging has become increasingly common over the last few years, especially among young researchers, and this paper introduced a good summary of the current reasons why. According to their research based on 100 academic blogs “academics most commonly write about academic work conditions and policy contexts, share information and provide advice”. I read their paper when I was on the verge of deciding whether to go ahead and start writing a blog, and decided to do some searching of my own about what it is that academics blog about and if it was worth doing it. So, here is what I found.

Most of what I looked for was related to biology, obviously, because that is what I do. There are some pretty good ones out there, and for me, the best I get from them is information on topics that are a little outside of my scope and therefore would normally not read anywhere else. That is, it is a great way of being on the loop about other research topics that are not relevant to your own exactly, but that are fun to know about! Just a few of my favorite ones, in case you’re interested: The Molecular Ecologist, Dynamic EcologyEEB and Flow, and The Loom. For me, there is nothing like a Sunday morning browsing through the lire app where all my feeds are combined.

There are people who really go all in and you can find inspiring stories out there of graduate students that started blogging about their own research and got so involved with it, they are developing part of their carriers though it. Jeremy Yoder, for example, tells the story of when he began blogging about things he found interesting while still in grad school and nowadays is the manager for the aforementioned blog, the Molecular Ecologist.

But, really, why do most academics blog? Does it help with readership of the articles that are being blogged about? I found that the answer to that seems to be yes, at least according to this experiment done by Melissa Terras with her own articles, where she introduces this interesting formula: If (social media interaction is often) then (open access + social media = increased downloads). However, Thompson and Mewburn suggest that what really gets this world spinning is the global meeting place it provides. It is not always about outreach. Research now has the potential to be collaborative in ways never achieved before, and blogging appears to play a significant role in this. There is a lot of discussion going on (yes, in the form of blogging, but also in journals like this one) about how social media is changing the way we do science. My personal opinion is that the change has indeed started, if not entirely revolutionizing yet, certainly a major force to be aware of. Social media is how a lot of us communicate, where we look for things that interest us.

Interested? Now, there are different scales to this. If you’re looking for a place to start, there are manuals on how to start microblogging through Twitter (“How to start tweeting”), but if you’re feeling brave, check out this A to Z guide on social media for academia. All in all, social media can certainly open up new forms of communication and dissemination.

Poster describing the results of a study examining Twitter use among a sample of 8,826 academics from 5 universities. Authors Jason Priem, Kaitlin Costello and Tyler Dzuba

Poster describing the results of a study examining Twitter use among a sample of 8,826 academics from 5 universities. Authors Jason Priem, Kaitlin Costello and Tyler Dzuba. From figshare http://dx.doi.org/10.6084/m9.figshare.104629 .