The impact of predation on phenotypic diversity is a topic that has only been studied recently. Prey use predator-specific correlated sets of morphological and behavioral traits to deter predators, and depending on the selection regime, these traits can be differently correlated with one another. For instance, trait compensation occurs when only one of the traits is beneficial to the organism and as a result, the traits are negatively correlated with one another. Conversely, trait cospecialization refers to the simultaneous application of two traits that are each helpful to the organism; these traits will be positively correlated with one another.
Ancestral Leucorrhinia dragonfly species are hypothesized to have inhabited lakes with fish as top predators (fish lakes) and possessed large abdominal spines to deter predation; fish often spit out spined dragonfly nymphs. However, with a shift into ponds with dragonflies as the apex predator (dragonfly lakes), these spines have undergone a reduction in length. Mikolajewski et al. (2010) sought to study the antipredator defenses of Leucorrhinia against the backdrop of these different habitats and predation regimes. More specifically, the researchers studied the covariance of abdominal spine length, burst escape swimming speed, and arginine kinase (Ak) activity, the mechanism responsible for providing energy for short-lasting, highly consumptive muscle activity (as occurs in burst escape swimming). The researchers hypothesized that burst escape speed is under weaker selection pressure in dragonfly lakes and may in fact be costly to maintain. As such, they anticipated higher burst escape swimming speeds and Ak activity in Leucorrhinia species inhabiting fish lakes than those species inhabiting dragonfly lakes.
Five Leucorrhinia species were considered: L. albifrons, L. caudalis, L. dubia, L. pectoralis, and L. rubicunda. L. dubia and L. rubicunda are traditionally considered dragonfly-lake specialists, although L. dubia’s phenotypic plasticity allows it to survive in both habitats. Nymphs’ abdominal spines were measured, and those lacking spines were subjected to a sham measurement. Burst escape speed was then measured for each nymph; a predator attack was simulated and burst escape swimming was videotaped. Three trials were completed, and the mean burst escape swimming speed was calculated. Directly after the swimming trials, nymphs were individually frozen, weighed, ground and assayed for Ak activity, which was expressed at absorbance per mg wet mass.
As expected, fish-lake specialists L. pectoralis, L. caudalis and L. albifrons had higher burst escape swimming speeds than dragonfly-lake specialists L. dubia and L. rubicunda. Although fish-lake L. dubia had lower burst escape speeds than true fish-lake specialists, they had higher burst escape speeds than that of dragonfly-lake L. dubia. Evolutionary contrast analysis regressions reported positive correlations between spine length and burst escape speed (R^2 = 0.84), spine length and Ak activity (R^2 = 0.76), and Ak activity and burst escape speed (R^2 = 0.98).
This work broaches a number of questions that could be addressed with follow-up studies. First and foremost, common garden experiments would be invaluable in determining the basis of population differences in L. dubia; are these differences the result of genetic differentiation or phenotypic plasticity? Furthermore, are these traits influenced by independent selection, or are they controlled by pleiotropy?
Citation: Mikolajewski et al. 2010. Predator-driven trait diversification in a dragonfly genus: covariation in behavioral and morphological antipredator defense. Evolution 64: 3327–3335.
Figure: Mikolajewski and Johansson (2004)
Taking into account the discussions that have occurred this semester, I found it interesting that this kind of study has only recently been done. I feel like we have hit this topic a few times within other discussions, but that is one of the reasons this study was interesting. It had a simple design and it was easy to follow, giving relatively straight forward results. The move to a fishless lake was accompanied by a partial loss and reduction of larval spines along with burst swimming speed and AK activity. These results supported their predictions, that in the presence of a fast predator, actively searching for prey, a stronger selection for protection will occur. Longer spikes led to fish spitting the dragonflies out while increased burst speed and AK activity allow the dragonfly to flee for safety.
They mention that Enallagma (a damselfly larvae) shows an increase in burst speed and AK activity when shifted from fish lakes towards dragonfly lakes. This allows the larvae to escape predation from dragonflies but is not readily used with fish since they lack the dorsal spines and would not be spit out. I like this point because it opens up doors to new directions the study can take. Specifically, what makes certain behavioral and morphological antipredator defenses useful or not? What I would have liked to see in further detail (yes, it would have been a completely new study) is how the populations would respond to the introduction of fish to a dragonfly lake. Take a dragonfly lake and introduce a top fish predator, how many generations will it take for the defensive characteristics to return?
I thought this paper raised a lot of interesting points…the authors’ hypothesis that, in the genus Leucorrhinia, “…reduced selection for high burst speed and the costs of burst speed may underlie the reduction in burst speed in species that have invaded dragonfly lakes,” really made me think. I remember someone questioning this statement in our class discussion, saying that it was odd that they viewed selection occurring in the Leucorrhinia genus from fish lakes to dragonfly lakes when dragonflies existed before the relatively new fish. I can definitely understand the reasoning behind this logic, but, from what I found online, dragonflies haven’t been around as long as fish (350 million years ago compared to 530 million years ago, unless my sources are wrong).
My second point is that I agree with the authors’ point of view that population differences for burst escape speed and Ak activity in L. dubia could be the result of phenotypic plasticity, rather than genetic differentiation. I was confused about exactly what mechanisms lie behind phenotypic plasticity, but then I thought back to the seminar on November 30th with Dr. Mathews and Dr. Janusek and their talk on epigenetics. It does seem that, since various predator escape mechanism phenotypes of L. dubia are being influenced by the environment, that perhaps the control of what genes are being transcribed is occurring due to epigenetic mechanisms, such as an acetylation of a terminal lysine group on a histone tail to unwind the DNA and allow transcriptional access. I began to do some research on the internet to see if epigenetics is a possible explanation of phenotypic plasticity in ecology, and sure enough, it is…I found many papers from notable ecological and biological journals examining the connection between phenotypic plasticity and epigenetics, which I think is an extremely interesting area of research.
This was the second paper that had to do with phenotypic plasticity this week! I wonder why the authors did not do the “common garden experiments” as I believe they could have had much stronger results with such findings. From my research on phenotypic plasticity, it seems that much of the literature on this topic is in the field of entomology. This fact makes me wonder if insects in general have higher phenotypic plasticity compared to vertebrates. If so, could this fact explain the evolutionary history of the extremely diverse and wide spread arthropods? Unfortunately, my Invertebrate Zoology course in undergrad did not take an evolutionary approach to the topic. I believe this would be useful in any Biology course, not just in Evolution classes.