Patterns of spatial genetic structure and diversity at the onset of a rapid range expansion: colonisation of the UK by the small red-eyed damselfly Erythromma viridulum

Abstract

Species’ geographic ranges may vary in size in response to a change in environmental conditions. The specific genetic consequences of range expansions are context dependent, largely depending upon the rate of colonisation as well as the origins and numbers of founders, and the time since colonisation. Like other “charismatic” taxa, such as birds and lepidopterans, the distributions of odonates (dragonflies and damselflies) are well-known through substantial monitoring programmes co-ordinated by various societies. The small red-eyed damselfly Erythromma viridulum (Odonata: Zygoptera) has undergone a substantial, northward range expansion in Europe in the last 30 years and has recently-colonised two distinct areas in the UK. We quantify the immediate genetic consequences of this rapid colonisation by genotyping more than 1,400 E. viridulum from 39 sites across the northwest margin of this species’ geographic range. Levels of genetic diversity and spatial structure are impacted by this species recent range expansion and non-equilibrium conditions that drive weak genetic divergence, even at regional spatial scales. Populations of E. viridulum become less diverse towards the edge of this species’ distribution, presumably as a consequence of colonisation through a series of founder events. Specifically, there is a significant reduction in genetic diversity in the smallest, most recent focus of colonisation in the UK; however, there are generally low levels of genetic diversity across this E. viridulum’s northern range margin. While most populations are generally poorly differentiated, E. viridulum nonetheless consists of two distinct lineages that broadly differentiate between eastern and western Europe. Genetic divergence between the two UK colonisation foci are indicative of distinct immigration events from separate sources; however a general lack of spatial structure prevents us from pinpointing the specific origins of these migrant damselflies.