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)