Interview with the authors: testing the role of ecological selection on color pattern variation

The color variation that exists among individuals has lent itself to the study of selection since Darwin. Recently, Zaman, Hubert, and Schoville (2019) investigated the effects of selection on the diversity of the wing color pattern in the butterfly Parnassius clodius across a large portion of its range. These researchers found evidence supporting the idea that coloration may serve as a warning signal to predators, providing some predator avoidance benefits to individuals. In addition, the variation of solar radiation and precipitation observed geographically across sites was negatively correlated with the amount of melanin observed at each site. This suggests that the occurrence of melanin may provide a selective advantage in the form of thermoregulatory function. For more information on how selection influences butterfly wing coloration, please see the full article and the interview with Dr. Schoville below. 

Parnassius clodius mating, male located below female. Photo by Sean Schoville.

What led to your interest in this topic / what was the motivation for this study? Khuram and I were both interested in butterfly color pattern variation, and in particular, cases where there might be competing selective pressures acting on wing pattern phenotypes. Most work on butterfly wing patterns focuses on predator-prey interactions and aposematic colors, but butterfly wings are essential to flight performance and important in thermal regulation. A number of recent papers have shown that butterfly color pattern appears to be responding to climate warming, and then there are well-known cases (such as alpine Colias, thanks to Ward Watt) where thermoregulation has been linked to basking behavior and the pigment on wings. Thus, in examining variation in Parnassius clodius (which occurs over a broad elevation and latitudinal range), we hoped that we could decouple environmental signals that might act on different wing color elements. 

What difficulties did you run into along the way? Sampling our butterflies across this large region was a major challenge, particularly as adult flight times are rather short (two to three weeks). And then we were surprised by the strong difference in adult phenology across sites (some adults are active in May, others in late July). This is evident regionally (Utah versus Washington), and across elevation within a region. In the end, it required three years of effort, with some very long road trips from Wisconsin.

What is the biggest or most surprising innovation highlighted in this study? After some initial efforts to link aposematic variation (red eyespots) indirectly to predator communities (through climate variables that might covary with predator abundance), we realized this was too tenuous. So, we were delighted to discover publicly available data on bird abundance. While this did not solve the problem (perhaps due to lack of spatial resolution in the bird data), I think using this data to analyze butterfly wing patterns was one of the more innovative aspects of our paper. We had a much easier time linking spatial climate data to melanization (dark pigmentation). As an aside, this raises the important point that some data, i.e. abiotic environmental data, is much easier to come by than biotic data. This is unfortunate, as we expect biotic selective forces to be equally or more important drivers of microevolution in some cases.

Lead author on this study, Khuram Zaman, after a day of collecting samples. Photo by Sean Schoville.

Moving forward, what are the next steps in this area of research? We’d like to extend this work in two directions. First, we’d like to connect color pattern traits to underlying genes and measures of heritability. Although the genes controlling butterfly color pattern are well studied, to date no representative of the snow Apollo subfamily Parnassinae have been included in these efforts. Members of the family are tremendously variable and quite stunning in their dramatic contrasts of color. Second, experiments are needed to link our inferences of ecological selection to fitness differences, as well as performance in the field. Physiological assays of melanic variants, coupled with mechanistic thermodynamic models, have been developed for Colias butterflies (see Joel Kingsolver and Lauren Buckley’s work). This type of modeling could provide important connections to conservation of Parnassius clodius populations under changing climates, and might perhaps extend to conservation work on other highly threatened Parnassius species.

What would your message be for students about to start developing or using novel techniques in Molecular Ecology? The development of novel approaches is a key part of advancing biological knowledge, but it can be a daunting endeavor given the breadth and scope of the scientific literature nowadays. Integrating multiple approaches, on the other hand, can equally help to advance our knowledge and provide opportunities to address long-standing questions. This is the direction we took in this paper. My personal view is that students should to try to master multiple techniques (assemble a toolkit, so to speak) and apply those techniques to fundamental problems. Hopefully, it’s a lot of fun in the process and leads to interesting collaborations!

What have you learned about methods and resources development over the course of this project? We are entering a golden age of data-rich resources, in terms of spatial environmental data and genomics data. These increasingly provide the power to test refined hypotheses about evolutionary and ecological processes, and are becoming more accessible to all researchers. One of my favorite accomplishments in the paper is using genetic covariance data among populations (relatedness data) as a covariate in fitting morphology ~ environment models. The use of such population contrasts is important in controlling for non-independence in the data due to ancestry. While we have known about the importance of genetic covariance in hypothesis testing for some time (thanks to Joseph Felsentstein’s work), it is only recently possible to use genome-wide data. This provides very precise measures that are highly informative, and enabled us to rule out the role of genetic drift as a driver of wing pattern variation. 

Describe the significance of this research for the general scientific community in one sentence. Our research demonstrates that butterfly wing color patterns evolve in response local climate conditions, as a way to regulate body temperature.

Describe the significance of this research for your scientific community in one sentence. Our work demonstrates that elements of butterfly wing pattern phenotypes respond independently to different sources of selection, with climate variation acting on thermoregulatory ability as an important driver of butterfly color pattern.

Parnassius clodius basking to thermoregulate. Photo by Sean Schoville.

Zaman K, Hubert MK, Schoville SD. 2019. Testing the role of ecological selection on color pattern variation in the butterfly Parnassius clodius. Molecular Ecology 28:50586-5102.

Summary from the authors: Selection at behavioural, developmental and metabolic genes is associated with the northward expansion of a successful tropical colonizer

Link to paper: https://onlinelibrary.wiley.com/doi/10.1111/mec.15162

Picture: Green Anole Lizard (Anolis carolinensis) on railing in Hilo, Hawaii. Author: Paul Hirst. CC-BY-SA-2.5

The green anole (Anolis carolinensis), also called the American chameleon due to its ability to change color, is a common species in South-East USA. It has been studied for decades to understand how reptiles adapt to their environment.  Unlike other species of its genus, its range encompasses territories outside tropical climate, reaching the winter-exposed flanks of the Appalachians. The green anole colonized these colder regions from Florida in the last 300,000 years. We used DNA variation covering the whole genome and contrasted populations having recently colonized colder territories with the ones from tropical Florida. We compared multiple approaches to detect which segments in DNA sequences harbored variation compatible with selection. Since these signatures can also be produced by past demography, we took the latter into account to limit the detection of false positives. We then identified the most likely function of genes overlapping with candidate regions for selection, and observed that many of those were involved in exploratory behavior, immunity and response to cold. This suggests that the success of green anoles may have been due to changes in both physiology and behavioral shifts, a hypothesis that could be further tested experimentally.

Yann Bourgeois and Stephane Boissinot

Bourgeois, Y., & Boissinot, S. (2019). Selection at behavioral, developmental and metabolic genes is associated with the northward expansion of a successful tropical colonizer. Molecular Ecology. 2019. 28-15. 3523-3543