Résumé
Biological invasions caused by accidental introductions often result in severe ecological impact. Revealing the pattern of spread can clarify processes which drive these types of invasions and therefore help mitigate their impact. Here, based on the genetic structure at five microsatellite loci, we reconstructed the invasion history of the topmouth gudgeon (Pseudorasbora parva). This small cyprinid fish was introduced from China in the early 1960s into countries around the Black Sea, spreading rapidly throughout Europe north, west and southward. Distribution of the genetic variation from 14 European and one Chinese founder population was analysed, revealing possible pathways of spread in Europe. Overall, the distribution of genetic diversity across populations is strongly correlated to the geographic and temporal spread of P. parva across its invasive range. Populations closest to the founder sites had high levels of genetic diversity and little genetic differentiation. Furthermore, a pattern of isolation-by-distance was found, which indicates human-mediated stepping-stone colonisation. The data also indicate that the invasive populations originate from recent admixture of divergent source populations with some recently established populations, although the admixture signal has been dampened by strong genetic drift. These results nevertheless show that despite being a human-mediated invasion, the pattern of spread resembles a natural stepping-stone colonization with a well-defined front of invasion.