Cryptic genetic diversity in Rattus of the San Francisco Bay region, California

Invasive species can have complex invasion histories, harbor cryptic levels of diversity, and pose taxonomic problems for pest management authorities. Roof rats, Rattus rattus sensu lato, are common invasive pests of the San Francisco Bay Area in California, USA. They are a significant health risk and pest management efforts impose a large financial investment from public institutions and private individuals. Recent molecular genetic and taxonomic studies of black rats in their native range in Asia have shown that the species is a complex of two karyotypic forms and four mitochondrial genetic lineages that may represent four distinct species. We used mtDNA sequences and nuclear microsatellite variation to identify which mitochondrial lineages of the R. rattus group are present in the San Francisco Bay Area and to test for gene flow among them. We recovered specimens with mtDNA sequences representing two of the major mtDNA lineages of the R. rattus group. Microsatellite variation, however, was not structured in concordance with mtDNA lineages, suggesting a more complex history involving hybridization and introgression between these lineages. Although Aplin et al. (2011) and Lack et al. (2012) reported R. rattus Lineage II in North America, this is the first detailed examination of possible gene flow amongst lineages in this region.