Phylogenetic relationship between Callosciurus caniceps and C. inornatus (Rodentia, Sciuridae): Implications for zoogeographical isolation by the Mekong River

To test the riverine barrier hypothesis in small arboreal mammals, we examined phylogenetic relationship between two Callosciurus squirrel species (C. caniceps and C. inornatus) geographically isolated in the northeastern part of the Indochina Peninsula by the Mekong River. We did this by comparing complete mitochondrial DNA cytochrome b gene sequences of these two squirrels and five other Callosciurus species: C. erythraeus, C. finlaysonii, C. nigrovittatus, C. notatus, and C. prevostii. Phylogenetic trees showed that there are three major lineages: (1) a lineage containing C. caniceps, C. erythraeus, C. finlaysonii, and C. inornatus; (2) a C. notatus lineage; and (3) a lineage containing C. nigrovittatus and C. prevostii. Callosciurus caniceps was most closely related to C. inornatus. Based on transversional divergence rate at the third codon position of cytochrome b gene, they diverged about 1.2 million years ago. The drastic eastward shift of the Mekong River may have isolated ancestors of C. caniceps and C. inornatus. Then, these two species might have evolved in the different environments of each side of the Mekong River. This phylogeographic history suggests a typical case of allopatric speciation.