Tandem repeat units are only detected in the left domain of the mitochondrial DNA control region in sika deer. Previous studies showed that Japanese sika deer have more tandem repeat units than its cousins from the Asian continent and Taiwan, which often have only three repeat units. To determine the origin and evolution of these additional repeat units in Japanese sika deer, we obtained the sequence of repeat units from an expanded dataset of the control region from all sika deer lineages. The functional constraint is inferred to act on the first repeat unit because this repeat has the least sequence divergence in comparison to the other units. Based on slipped-strand mispairing mechanisms, the illegitimate elongation model could account for the addition or deletion of these additional repeat units in the Japanese sika deer population. We also report that these additional repeat units could be occurring in the internal positions of tandem repeat regions, possibly via coupling with a homogenization mechanism within and among these lineages. Moreover, the increased number of repeat units in the Japanese sika deer population could reflect a balance between mutation and selection, as well as genetic drift.
Keywords: Cervus Nippon; control region; evolution; tandem repeats.