Newcomers and Old Friends: Long-Distance and Bridgehead Introductions Both Contribute to the Recent Invasion of the Little Fire Ant in Southern Europe

Abstract

Aim: Biological invasions result from the combination of (i) population dispersal opportunities and (ii) adaptations to the recipient environment. Identifying complex migration histories, made of long-distance dispersal from the native range and secondary introductions, or genetic patterns indicative of adaptation is crucial to build coherent management efforts of problematic invasive species. We here aimed at determining the routes of introduction and describing the genetic peculiarities of recent introductions of Wasmannia auropunctata, a destructive invasive ant species with a polymorphic clonal/sexual reproduction system. Location: We focused on three recently established European populations (Cyprus, France and Spain) and their relationship with earlier worldwide introductions and the native South American range of W. auropunctata. Methods: We used a combination of cytochrome c oxidase subunit I (COI) sequencing and genotyping of a total of 686 European individuals at 12 microsatellite loci (from 76 nests and seven localities), together with previous worldwide datasets totalling 503 COI sequences and 6963 genotyped individuals. Phylogenetic reconstruction and genetic differentiation analyses were used to infer the origin, reproduction systems and genetic diversity of the three European populations. Results: We show that the history of the invasion of Europe is a mix of secondary introductions from a Mediterranean bridgehead population (Israel), and a novel long-distance introduction from a climatically similar area of northeastern Argentina. All newly introduced populations reproduce clonally and display an outbred genotypic pattern, consistent with all prior introductions and with anthropised areas of the native range. Main Conclusions: This study confirms that preventing novel introductions is complex and requires adequate surveillance tools to simultaneously monitor areas of the native range with potential prior adaptation and previous introductions that could act as bridgeheads. Because of their ecological and genetic similarities, the fate of more ancient introductions could help us foresee what the future holds for European introductions.