PS 11-157 - Spatial variation in Wolbachia frequencies predict the invasion origins of an insect pest recently introduced to North America from Europe

Abstract: The introduction of non-native species across the world represents a major global challenge. Retracing invasion origin is an important first step in understanding the invasion process, often requiring detailed sampling of the native range. Insect species frequently host Wolbachia, a widespread endosymbiotic bacterium that manipulates host reproduction to increase infected female fitness. Here, we draw on the spatial variation in infection frequencies of an actively spreading Wolbachia strain wCer2 to investigate the invasion origin of the European cherry fruit fly, Rhagoletis cerasi, introduced to North America in the last decade. First, we tested the introduced fly population for the presence of Wolbachia and determined that it lacks the wCer2 strain and the associated mitochondrial haplotype, indicating that the fly likely originated from a wCer2-uninfected population in Europe. We then estimated spatial population variance of the wCer2 infection frequency across the native range and predict the infection frequency in unsampled regions by using an extensive dataset covering more than 230 populations collected across 25 years, complemented with an additional 14 populations sampled for this study. We found the wCer2 uninfected regions of the Mediterranean, and/or the eastern R. cerasi range as the most likely candidates for the invasion origin. Finally, using mathematical models of Wolbachia frequency dynamics, we explore an alternative scenario and infer that initial wCer2 frequency below 5%, imperfect maternal transmission, and positive fitness effects could also explain the observed wCer2 frequency in these initial stages of introduction. Given the prevalence of biological invasions, uncovering invasion origins gives fundamental insights into how invasive species adapt to new environments and improves biocontrol of agriculturally important pest species.