Genetic divergence, admixture and subspecific boundaries in a peripheral population of great tit, Parus major (Aves, Paridae)

Secondary contact zones have been formed between several pairs of avian
species and subspecies in northern and northeastern Iran during the
post-Pleistocene and Holocene times. Three subspecies groups out of the
four in the great tit (Parus major), namely, major, bokharensis and
cinereus are supposed to have come into local or regional secondary
contact in northeastern Iran. Parus major intermedius known from this
region, has long been suggested to have a hybrid origin, stemming from
hybridization between the yellow western (major) subspecies group and the
grey eastern (bokharensis or cinereus) subspecies group, based on
intermediate plumage coloration. Here, we investigated the evidence of
intergradation between subspecies groups and the evolutionary origin of P.
m. intermedius using mitochondrial DNA sequences (236 individuals, control
region) and microsatellites (73 individuals, 11 loci) and used Approximate
Bayesian computation to test competing scenarios of demographic history of
the studied populations. Our analyses indicate a divergence origin for
intermedius, resulting from the expansion of major subspecies group. The
low mitochondrial diversity (p = 0.00065, q = 0.00170) and high genetic
differentiation in comparison with central populations suggest that
intermedius is a peripheral population. Microsatellite results show no
signs of nuclear admixture between bokharensis and major subspecies
groups. However, one phenotypically intermedius specimen had bokharensis
mtDNA and major nuclear DNA in the assumed hybrid zone (Lotf Abad),
supporting past introgression.