Prevalence of two endosymbionts in range-expanding <i>Zodarion</i> spiders (Araneae: Zodariidae)

Abstract Major changes in species geographical ranges are usually explained by climatic changes or human activities. Intrinsic causes, such as endosymbiotic agents, have not yet been considered thoroughly, despite their potential to affect profoundly the biology of organisms such as arthropods and their aim to maximize their own transmission, e.g. by altering the dispersal abilities of their host. Here, we describe on a large scale the prevalence of two widespread endosymbionts, Wolbachia and Cardinium, in the rapidly range-expanding spider Zodarion rubidumSimon, 1914. Using standard PCR, we screened for the presence of both endosymbionts in specimens collected from 40 populations across Europe. We interpolated the prevalence of the endosymbionts over the study area using ordinary kriging, looking for spatial patterns corresponding to the putative expansion pathway of Z. rubidum. We found that both endosymbionts are widely present throughout Z. rubidum populations, with Wolbachia showing low prevalences with a few patches of high incidence, and with Cardinium being almost at fixation across the study area. Furthermore, in seven additional Zodarion species we investigated the relationship between the prevalence of endosymbionts and spider geographical ranges. We failed to find any significant relationship between these two parameters. We discuss possible explanations for such wide endosymbiont distributions and question their potential impact on Zodarion populations.