This week’s BEACON Researchers at Work blog post is by MSU graduate student Michael DeNieu.
The Dworkin lab is like a certain popular energy drink…it gives you wings. There’s a wide range of topics ongoing in the lab spanning functional and quantitative genetics, evolution, and development, but the common thread is that the Drosophila wing is used as a model to understand these processes. It is a multivariate trait. It’s easy to measure, and there is an abundance of natural variation. There are powerful tools for elucidating genetic and development mechanisms underlying phenotypic traits. What could possibly be lacking? In a word: ecology. Very little is known about the natural history of the fruit fly Drosophila melanogaster. For a lot of questions, that isn’t an issue. However, a major goal of the research in the lab is to gain a better understanding of the processes underlying adaptive evolution, and it is hard to infer evolutionary implications without knowing the functional significance of the traits we study. We want to know what variation exists in natural populations, what forces maintain it, and what that variation means for future evolutionary trajectories.
As an undergraduate student, I was always interested in how traits are shaped by natural selection. I was particularly interested in behavioral evolution because of the complex integration of various functions required to produce an appropriate behavioral response. The organism must detect relevant stimuli from the environment, process the information, and then initiate the response. The organism must also possess the morphological traits required to execute the behavior. It’s clear Dr. Ian Dworkin, my adviser, was having the same thoughts around the same time. He had devised a system in which flies were exposed to predators so that he could measure selection for natural variation in wing morphology.
In addition to providing the ability of flight, the wing plays an important role in many behaviors. The rapid “beating” of the wing produces the male courtship song, and is also used in aggressive displays between males. Yet it is still just one part of the fly. We wanted to take the approach even further to discover what traits might be involved in anti-predator behavior in order to begin to understand the selective forces present not just on the wing but on the whole organism in natural environments. When I joined the lab as a PhD student we did just that. The predation assay was adapted in order to initiate populations experimentally evolved under predation pressure. Our predation populations undergo selection by nymphs of the Chinese mantis (Tenodera aridifolia sinensis). Only surviving flies are able to pass their genes to the next generation. Left behind are the half-eaten remains of flies that were unable to avoid or escape the quick strike of the mantids. Comparing the predation populations to control populations – free from selection by the predator – allows us to identify behaviors and morphology as they evolve. Because we save the surviving populations, we have a morphological and more importantly a genetic record of the adaptive process.
One of the advantages of actively evolving populations is that they are free to adapt along any available trajectories. As experimenters, we are not forced to hypothesize and guess at the traits we predict might be important. We allow selection to identify them for us. We have performed over 70 generations of experimental evolution, and the predation populations have evolved morphologically and behaviorally, but not always in ways we would have predicted. Both replicate predation populations have evolved to increase their survival in the presence of the mantids. Surprisingly though, the populations have different wing morphology – both in size and shape – suggesting that despite the same selective pressure, they are evolving along different trajectories. Observations of their behavior during predatory selection show that they behave differently as well. Flies in one population favor ambulatory locomotion while the other is more apt to fly from place to place. Preliminary tests of the startle-induced behavior of these populations supports these observations.
This might mean that escape may not be the primary trait responsible for the increase in survival. There is a large body of research demonstrating the profound effects predation has on populations. In particular, many studies have focused on risk effects of predation. Risk effects result from modifications of behavioral patterns by prey organisms in order to reduce the likelihood of encountering and potentially being eaten by a predator. Because much of our data point to the conclusion that avoidance behaviors play a significant role in the evolutionary response, we investigated changes in potentially risky behaviors, including aggression and foraging.
Mantids are ambush predators. They sit still waiting for prey to move in front of them and strike quickly before it can escape. Any behavioral change that reduces the probability of encountering a mantis is likely to evolve. Consequently, we predicted that foraging and aggression levels would be reduced in our predation populations. When we measured foraging behavior we saw the predicted decrease. When we measured aggression, we were surprised yet again. The predation populations have increased their aggression level. Our current hypothesis is that the increase in aggression is a plastic response to predation risk. As you will see in the first part of the video below, aggression is measured in the absence of the predators. We think the flies are reducing aggression when predation risk is high, and increasing it when risk is low.
Whatever the answer turns out to be, one thing that I enjoy most about this project is that evolution always has the ability to surprise you. Part of the excitement of experimental evolution is that you get to watch it happen. The challenge is to make sure that you’re paying attention to the right things and asking the right questions.
For more information about Michael’s work, you can contact him at denieumi at msu dot edu.