COS 255-5 - Parallel changes in morphology, sex ratio and parasite load along four range shift transects of the blue-tailed damselfly, Ischnura elegans

Abstract: Poleward range shifts are a crucial response to climate change for many species. Recent work has shown that range expansion can have profound evolutionary consequences for traits across an organism’s life history, feeding back on the range expansion itself. For example, traits promoting population spread (dispersal, fecundity) are expected to be under selection at the range edge, as well as adaptations to the local abiotic and biotic environment. However, the predictability of these changes in natural systems consisting of varying environmental gradients is not well understood.

The aim of our study was to explore key phenotypic, genomic, and ecological changes along four parallel range expansions of the blue-tailed damselfly, Ischnura elegans. We collected morphometric and life history data from several thousand individuals originating from >200 sites across range expansion transects in Scotland, Norway, Sweden, and Finland. We field-caught adult damselflies in waterside vegetation using sweep nets in timed catching bouts. We counted the number of parasites on each individual and took images by positioning each individual in a petri dish on a flatbed scanner. We then measured damselfly morphological traits (thorax, abdomen and wing length; abdomen and thorax width) using ImageJ.

We found that damselfly overall body size and wing size consistently increased with latitude along the 3 parallel transects in Scandinavia, leading to increased dispersal capacity and reproductive potential at the expanding range fronts. The proportion of males captured at a site increased poleward along each transect in Scandinavia, consistent with male-biased dispersal at the range expansion front. Finally, high-latitude populations experienced lower parasite load. Comparing Scotland and Scandinavia, variation in body size appears to follow the direction of range expansion rather than latitude or climate per se.

In conclusion, our results show consistent changes in key morphological traits, sex ratio and host-parasite interactions across parallel range expansion transects of Ischnura elegans. These changes will feed back on demography, social interactions, and dispersal at the range edge and should be taken into account when forecasting range shift dynamics in natural systems.