Introgressive hybridization offers a unique platform for studying the molecular basis of natural selection acting on mitogenomes. Most of the mtDNA protein-coding genes are extremely conserved; however, some of the observed variations have potentially adaptive significance. Here, we evaluated whether the evolution of mtDNA in closely related roe deer species affected by widespread mtDNA introgression is neutral or adaptive. We characterized and compared 16 complete mitogenomes of European (Capreolus capreolus) and Siberian (C. pygargus) roe deer, including four of Siberian origin introgressed into European species. The average sequence divergence of species-specific lineages was estimated at 2.8% and varied across gene classes. Only 21 of 315 fixed differences identified in protein-coding genes represented nonsynonymous changes. Only three of them were determined to have arisen in the C. pygargus lineage since the time to the most recent common ancestor (TMRCA) of both Capreolus species, reflecting a decelerated evolutionary ratio. The almost four-fold higher d N /d S ratio described for the European roe deer lineage is constrained by overall purifying selection, especially pronounced in the ND4 and ND5 genes. We suggest that the highly divergent C. capreolus lineage could have maintained a capability for genomic incorporation of the well-preserved and almost ancestral type of mtDNA present in C. pygargus. Our analyses did not indicate any signs of positive selection for Siberian roe deer mtDNA, suggesting that the present widespread introgression is evolutionarily neutral.

Extremely widespread in the Palaearctic, roe deer are heterogeneous in many ways including their morphology. At present, two roe deer species and four basic subspecies are acknowledged. The taxonomic system of the Siberian roe deer is poorly developed compared to that of the European roe deer. One of the principal points of roe deer systematics is the determination of the boundaries of the ranges of the roe deer subspecies requiring a thorough study of the species' diversity. By using craniometry, one of the most adequate systematics techniques, we have analyzed geographical distribution of the Suberian roe deer in the Russian Far East and adjacent areas. All studied specimens were separated into three groups, the distributions of craniometric parameters of two of which did not overlap. One of the groups corresponds to Capreolus pygargus pygargus Pallas, 1771, while the other two do not correspond to any of the Siberian roe deer subspecies presently acknowledged. We have shown that the C. pygargus pygargus range extends to the east to the Northern Primorye, i.e., it is 2,000 to 3,000 km wider than previously believed. The subspecies status of Capreolus pygargus tianschanicus Satunin, 1906 was not confirmed.The Southern Primorye and left bank of the Amur River are thus a part of the range of Capreolus pygargus melanotis Miller, 1911, penetrating here from the Northeastern China.

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