Contact zones are usually considered meeting areas of pairs of closely related species and represent major geographical discontinuities. The species complex Parus major is a small passerine, which is widely distributed across Eurasia. Traditionally, this complex species is classified into four subspecies groups: major, bokharensis, cinereus, and minor, each with independent evolutionary histories that have been in secondary contact since the Pleistocene glaciation. Northeastern Iran has long been considered as one of these secondary contact zones between major, bokharensis and cinereus subspecies groups. Also, Parus major intermedius which is known from this region has long been suggested to have a hybrid origin from hybridization between the yellow western (major) and the grey-colored eastern (bokharensis or cinereus) subspecies groups based on its intermediate plumage coloration. In this study, we sequenced and assessed the complete mitochondrial genome of 67 individuals to elucidate the evolutionary origin of Parus major intermedius. Our findings demonstrate a pattern of low mitochondrial diversity and high genetic differentiation in P. m. intermedius in comparison with other studied populations. Furthermore, two specimens from the assumed hybrid zone, northeastern Iran (Lotfabad), with a similar phenotype to that of P. m. intermedius had bokharensis mtDNA, suggesting potential introgression between bokharensis and major groups.

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