We karyotyped six species of Microtus voles collected along the southern edge of their range in northern and western Iran. Diploid and fundamental numbers were as follows: M. socialis and M. paradoxus 2n = 62, FNa = 60,
M. qazvinensis 2n = 54, FNa = 54, M. transcaspicus 2n = 52, FNa = 52, and
M. mystacinus (= M. rossiaemeridionalis) 2n = 54, FNa = 54. Two cytotypes were retrieved in M. irani from its type locality: 2n = 48, FNa = 46 and 2n = 64, FNa = 62. While our results confirmed an early report of 2n = 64 for this vole, the 2n = 48 cytotype remains unexplained. Karyological variability is relatively low in social voles and chromosomal data contribute little to individual species recognition. We argue that Arvicola mystacinus
De Filippi, 1865, described from Lar Valley (north-east of Tehran) is the oldest available name for 2n = 54 voles with the following synonyms:
M. subarvalis Meyer, Orlov & Skholl, 1969, M. epiroticus Ondrias, 1966, and
M. rossiaemeridionalis Ognev, 1924.
Two species of brush-tailed mice, genus Calomyscus, are known from eastern Iran: Calomyscus hotsoni has been reported from southwestern Pakistan and southeastern Iran, and C. elburzensis ranges through northeastern and central Iran. Based on molecular studies of two mitochondrial genes, all the specimens from eastern Iran examined herein belong to either of these two species. Furthermore, our data expand the northern distribution limits of C. hotsoni to just south of Birjand and the southern limits of C. elburzensis to east of Birjand. Morphometric analyses conducted on three geographic groups of C. hotsoni within Iran, contained specimens from Birjand (group 1), Zahedan and Khash (group 2) and Saravan (as group 3) revealed a north-to-south cline of decreasing body and cranial size, such that the most significant differences were between the northern and southern most groups. Karyological studies also showed differences in autosomal arms between the two geographical groups in Iran. Although the phylogenetic analyses separated these two groups into distinct clades, along with a third clade containing most of the C. hotsoni from Pakistan. The morphometric and molecular partitioning of geographic populations of C. hotsoni were not concordant. We consider the north and south groups of C. hotsoni as distinct Evolutionary Significant Units. There is evidence of introgression between the two forms across a broad geographic area presented by individuals of group 2 resulting in a clinal pattern of variation.