Spatiotemperal variation in butterfly wing pattern colouration – are there latitudinal, regional or temporal variation in female wing colouration in Polyommatus icarus?

Abstract

This study explores the roles of genetic variation and phenotypic plasticity for explaining patterns of local wing colour variation in the common blue butterfly, Polyommatus icarus. Polyommatus icarus is one of the most widespread species of butterflies in Europe. Across its range, the female varies a lot in its dorsal colour, from all brown to almost totally blue. It has been suggested that, in Sweden, the blue colouration is latitude-correlated, with more blue coloured females further north. However, this hypothesis has never been formally tested. Here, I utilise museum collections as well as a citizen-science driven online database (artportalen.se), to test the anecdotal hypothesis of latitude-correlated blue colouration in female P. icarus. Additionally, I explore regional variation, as well as temporal variation among spring and summer generations. Even though the effect of latitude was found to be only marginal, regional differences showed to be large. Despite these large differences, all colour morphs were found in most regions, suggesting that genetic drift is not a main factor in creating the variation. Furthermore, the spring generation was generally bluer than the summer generation. The generational difference suggests that a plastic response to the environment explains the colour variation. Yet, the generational differences were still smaller than the regional. The result of this study is a step towards unravelling the roles of genetic variation and phenotypic plasticity on the spatial and temporal variation in this species. This in turn is important for the understanding of the processes that create and maintain biodiversity.

Popular Abstract

Great variation in wing colouration in the common blue butterfly – but how and why?

Common blue butterflies, Polyommatus icarus, are special in that they vary a lot in wing colour. The males are, as the name of the species implies, simply blue. The females on the other hand are browner, but may show a varying degree of blue colouration. What is remarkable is the extent of the variation in the blue colour in the females. Some females are completely brown, while others have a partial coverage of blue colour, stretching from just a hint to almost complete coverage. Why do we see this variation in the female wing colour, and what processes generate and sustain the variation?

The variation seen in the female P. icarus is an example of intraspecific variation, i.e. variation among individuals within a species. The question of how the variation has arisen basically goes back to the classical debate of how nature (i.e. the genes) and nurture (i.e. the environment) influence an individual. That means that the variation can reflect genetic variation among individuals, which in turn can be generated either by genetic drift (i.e., the genetic variation has arisen due to chance) or natural selection (i.e., the genetic variation has arisen due to that certain genes provide a better chance of survival and reproduction). Alternatively, the variation can be caused by a phenomenon called phenotypic plasticity (i.e. when individuals with the same genes can develop different features due to environmental influence). A first step to disentangle the importance of these different processes in this species is to examine how the wing colour variation is distributed geographically and temporally (i.e. how it changes over time).

It has been stated in literature that the variation in the females have a geographical pattern. Blue females are believed to be more common in the north parts of Sweden, and brown females to be more common in the south. This hypothesis has, however, never been formally tested. In this thesis, I have used museum collections and online databases to investigate the geographical variation in female colour pattern in P. icarus within Sweden. I have also explored the temporal variation by comparing different generations (in large parts of Sweden this species appears as adults both during spring and summer).

The variation in blue wing colour did not show a clear correlation with latitude. The blue females were not more common further north, and the brown females were only slightly more common further south. On the other hand, the regional differences were remarkable; different areas had highly different frequencies of brown and blue females. What is also interesting is that despite this large regional variation, all different colours were found in all areas, which indicates that the variation is not caused by genetic drift.

Polyommatus icarus has, in the south of Sweden, two generations each year. A comparison of the variation in blue colour showed that the spring generation was bluer than the summer generation. Since the summer generation is the direct offspring of the spring individuals, this temporal difference likely reflects an environmental effect on colouration. However, the differences between regions were larger than the differences between generations, implying that the colour variation is due to both genetic and environmental variation. Although this study does not fully answer the question of how the variation is created in this species, it does contribute important pieces of information. In the end this helps our understanding of the processes that generate and maintain biodiversity.

Bachelor thesis in biology, 30 credits
Institution of Biology, Lund University
Supervisor: Magne Friberg, Erik Svensson

• author: Olofsson, Kristoffer
• supervisor: Magne Friberg; Erik Svensson
• organization: Degree Projects in Biology
• course: BIOY01 20181
• year: 2018
• type: M2 - Bachelor Degree
• subject: Biology and Life Sciences
• language: English
• id: 8962354
• date added to LUP: 2018-10-25 14:31:56
• date last changed: 2018-10-25 14:33:17