Recostructing the phylogeography of an invasive species: tracing invasions routes of norway rats (Rattus norvegicus) using mtDNA control region

The norway rat, Rattus norvegicus, is a cosmopolite invasive species that moves especially following human routes. It is considered one of the worst pest, overall it may be dangerous for native species, agriculture and foodstuff. The norway rat reached Europe during XVIII century from Asia, but little is known concerning its invasion history and population structure in this area. Here we used the mtDNA control region to study pattern of invasion of Rattus norvegicus by a phylogeographic approach. We used 385 rat sequences, 174 collected from Europe, Africa, Thailand and Middle East and 211 retrieved
from GenBank. Despite the very recent invasion of Europe, the results show an unexpected high genetic diversity. Twenty-six haplotypes were recognized and we built a Maximum Parsimony (MP) tree to identify the relationship between them. In the MP tree we identified three aplogroups that are also recognizable in the Statistical Parsimony (SP) network. Two of them show a star-like structure in the SP network suggesting populations expansion. This structure, corroborated by mismatch distribution, Tajima's D and Fu's Fs neutrality indices, suggest a recent sudden expansion of the populations in Europe and Africa.
In addition, the different geographical distribution of the haplogroups suggests the occurrence of two different invasion events in Europe possibly along two different colonization routes: an African route and a Middle East one. Furthermore, some haplotypes are shared between Europe and other invasive areas. This suggests to consider Europe as a possible new source area for Rattus norvegicus. Among the European countries, Italy has the highest genetic diversity, probably due to a recent introductions from different sources. For 117 specimens, 18 selected from our samples and 99 retrieved from GenBank, we performed an additional phylogeographic analysis using the cytochrome b as molecular marker. We built a network to identify the geographical distribution of the haplotypes according to the Statistical Parsimony method. The structure of the cytb network supports the hypothesis of at least two separate routes of colonization of Europe.