This week’s BEACON Researchers at Work post is by MSU postdoc Idelle Cooper.
If damselflies were painters, they would surely be watercolorists, and probably impressionists, too. As soon as the morning sun strikes the vegetation along the riverbank, the damselflies are fluttering and basking, displaying their colorful bodies and dusky wings to potential mates, and ovipositing in the reflective surface of the water. For anyone who pauses along a stream in the summertime, these colors and behaviors stand out as lovely and interesting traits, and while they are the intriguing results of an evolutionary history of selection, they are also currently playing a role in future adaptation and speciation.
Damselflies are in the same order as dragonflies, but smaller and hold their wings behind them at rest. Males and females are often different colors, and if there are multiple species sharing the same stream, they are often distinct in their body and wing pigmentation. As an evolutionary biologist, I am interested in what selection pressures may lead to variation within and between species. Sexual dimorphism (which are differences between the sexes) can arise from selection differences between males and females in what traits enable them to mate (sexual selection) and differences that enable them to survive (ecological selection).
Some questions that I’m pursuing include: what traits are under sexual and ecological selection within a species? How do multiple species arise and then remain separate, without interbreeding? What determines the geographic distribution of species and how might they respond to climate and habitat changes? To explore these questions as a BEACON postdoc, I am working with folks at MSU (Tom Getty, Muraleedharan Nair, and Chris Klausmeier) and UT-Austin (Molly Cummings and Eben Gering). We are investigating these topics at many levels within nature, from the cellular and biochemical level within individuals to dynamics within populations and to interactions between species.
Color is often important for survival and mating success, but is dependent on the ecology of the organism. Think of the camouflage of mice in sand dunes and rabbits in snow, or the warning coloration of unpalatable butterflies that is effective only at certain frequencies in the population. There are many cases of color being used as a signal to predators or mates, but the pigment that causes the coloration may also be important under abiotic conditions and function in thermoregulation and UV protection. The effect of high UV levels may be particularly dire, as the anthropogenic depletion of stratospheric ozone and the resulting rise in ultraviolet radiation exposure are well documented, but we don’t yet understand the evolutionary consequences. Changes in UV levels may affect direct selection on organisms by affecting their survival, as well as indirect selection through changes in species ranges, species recognition, and mate choice. We are addressing both of these processes by studying the Megalagrion damselflies of Hawaii and the Calopteryx damselflies of the upper Midwest and southern Canada.
In Megalagrion, males of most species in this endemic genus are red in color and live in open, exposed habitats. Females are often green and live in the shade of the forest except when they are mating around water (see the photo of a male and female in copulation). What we know so far is that red pigmentation is prevalent under high solar radiation and functions as an antioxidant. Free radicals produced under high UV can cause tissue damage and mutation, making antioxidant production an important adaptation in plants and animals under stressful abiotic conditions. In humans, oxidative stress is thought to contribute to diseases such as Alzheimers, Parkinson’s, diabetes, and cardiovascular diseases. Foods rich in antioxidants, including tomatoes, red peppers, red wine, pomegranates, and others (note the red coloration) had help sequester the damaging effects of free radicals. While I certainly don’t advocate eating damselflies, it is interesting to consider how these tiny creatures have evolved a powerful antidote that allows them to live and mate in exposed habitat.
Males need the red pigmentation because they are constantly in the sun, defending territories around water. Females don’t need the extra protection because they are in the shade, except in some cases: at high elevation, above tree line. Amazingly, females at high elevation express the red coloration of the males, and at first glance look just like the opposite sex. This color switch doesn’t confuse the males, however, which readily find the females. What appears to be mimicry of males in this case is simply the result of females making use of a sort of sunscreen in the same way the males do. While we understand this pattern most thoroughly in one species, Megalagrion calliphya, we are finding that the same pattern in other species. Red coloration in high UV environments is prevalent throughout this endemic radiation, and the switch from green to red pigmentation in females evolved multiple, independent times. Protection from UV radiation is important in these damselflies, is related to body color, and affects their species ranges. But what will happen as UV levels rise?
While we are interested in how species may persist through trait changes or distribution shifts, these responses will also affect species interactions, particularly because color is often used as a sexual signal for mating or species recognition. A recent range overlap of the Calopteryx species C. maculata and C. aequabilis provides an opportunity to measure selection on body and wing coloration. Where the species don’t overlap, C. maculata and C. aequabilis look similar, especially in the dark coloration of female wings. However, in sympatry (where the species overlap), female C. maculata have dark wings and C. aequabilis have light wings. This example of character displacement is thought to be important for reinforcing speciation because it prevents mistaken identity during mating.
This system was well studied by Waage in the 1970s, and our current surveys indicate that since that time, at least C. aequabilis has moved northward. The southern boundary in Michigan is now several hundred miles north of where it was 35 years ago. Such northern movement of organisms is predicted as a response to global climate change, and though we haven’t verified the cause of this shift, changes it the area of species overlap may influence rapid evolution of those characters important in sexual selection. In our recent mating trials, male C. maculata mistake the dark-winged C. aequabilis females as mates much more than the light-winged C. aequabilis from areas of overlap. Our research will continue to explore the species distributions and mating preferences of these species. As for their artistic preferences, these damselflies would probably admire Monet’s lily pads, though perhaps instead they would favor Renoir’s nudes.
For more information, see www.msu.edu/~cooperi or email Idelle at cooperi at msu dot edu.