Commensalism facilitates gene flow in mountains: a comparison between two Rattus species

Small mammal dispersal is strongly affected by geographical barriers. However, commensal small mammals may be passively transported over large distances and strong barriers by humans (often with agricultural products). This pattern should be especially apparent in topographically complex landscapes, such as mountain ranges, where valleys and/or peaks can limit dispersal of less vagile species. We predict that commensal species would have lower genetic differentiation and higher migration rates than related non-commensals in such landscapes. We contrasted population genetic differentiation in two sympatric Rattus species (R. satarae and R. rattus) in the Western Ghats mountains in southern India. We sampled rats from villages and adjacent forests in seven locations (20-640 km apart). Capture-based statistics confirmed that R. rattus is abundant in human settlements in this region, whereas R. satarae is non-commensal and found mostly in forests. Population structure analyses using ~970-bp mitochondrial control region and 17 microsatellite loci revealed higher differentiation for the non-commensal species (R. satarae F-statistics=0.420, 0.065, R. rattus F-statistics=0.195, 0.034; mitochondrial DNA, microsatellites, respectively). Genetic clustering analyses confirm that clusters in R. satarae are more distinct and less admixed than those in R. rattus. R. satarae shows higher slope for isolation-by-distance compared with R. rattus. Although mode of migration estimates do not strongly suggest higher rates in R. rattus than in R. satarae, they indicate that migration over long distances could still be higher in R. rattus. We suggest that association with humans could drive the observed pattern of differentiation in the commensal R. rattus, consequently impacting not only their dispersal abilities, but also their evolutionary trajectories.