The earth-colored mouse Mus terricolor is a common field mouse of the Indian sub-continent infesting paddy and wheat fields and was known as Mus dunni Wroughton, 1912 until Musser and Carleton (1993) synonymised it with the former name. This Mus species co-exists in the same habitat with the sibling species Mus booduga Gray, 1837. Both the species have 2n=40 chromosomes like other species of the subgenus Mus. In contrast to all acrocentric chromosomes of Mus booduga, Mus terricolor is characterized by a large submetacentric X and a large acrocentric Y chromosome (Matthey and Petter 1968, Sharma and Garg 1975, Markvong et al. 1975, Manjunatha and Aswathanarayana 1979). The short arm of X and the entire Y chromosome are heterochromatic (Sharma et al. 1986, 1990). Compared to the conserved karyotype of Mus booduga throughout its distribution range, Mus terricolor shows divergent karyotypes due to a variable number of heterochromatic short arms established in homozygous condition (Sen and Sharma 1983, Sharma et al. 1986, 1990) which indicates that Mus terricolor is in an active phase of evolutionary differentiation. Cytogenetic studies by Sharma and his associates revealed that Mus terricolor is differentiated into three distinct karyotypes (2n = 40) designated as chromosome types I, II and III. Chromosome type I has a wide distribution throughout the subcontinent except the southern peninsular region and has all acrocentric autosomes with C-band positive minute perceptible short arms. Chromosome type II, characterized by two pairs of submetacentric autosomes 1 and 3 with heterochromatic short arms, is found in Mysore and Erode in the peninsular India, while Chromosome type III distributed in Chennai, Tirupati and Madurai is characterized by three pairs of submetacentric autosomes 1, 3 and 6 with heterochromatic short arms.

Karyotype differentiation in Mus terricolor is due to acquisition of varying amount of constitutive heterochromatin in and around the centromere on specific autosomes. Different studies have been carried out in Mus terricolor chromosome types and their populations covering vast regions of southern, central and western part of India (Sharma and Garg 1975, Sen and Sharma 1983, Sharma et al. 1986, Sharma 1996) but populations from West Bengal in eastern India were not included. The northern part of West Bengal, characterized by hills (Darjeeling District) and the Terai and the Dooars regions in the foothills, is well known for biodiversity and diverse ecological features.

In view of the aforesaid situation, this study has been conducted to know the extent of intra- and inter- population heterochromatin variation in Mus terricolor chromosome type I from the Terai and the Dooars regions of foothills of Darjeeling in West Bengal.

All the individuals of Mus terricolor analyzed from ten populations of the Terai and the Dooars demonstrated the diploid number 2n=40 with all acrocentric autosomes and a large submetacentric X and a large acrocentric Y chromosomes in the complement as characteristic. No chromosomal polymorphisms like inversion and Robertsonian translocations were observed. Chromosomes prepared from each individual showed C-band staining, however, few metaphases in each slide either did not show C-band staining or has weak stain. Analyzable metaphase spreads always showed C-bands shown in the representative karyotypes from each population (Figs 1–3).

The mouse major satellite DNA, largely present as pericentromeric constitutive heterochromatin blocks in all chromosomes except Y, is highly repetitive (Jones 1970, Pardue and Gall 1970, Dev et al. 1973). This region has been found to be highly variable andfast evolving indicating its role in early stages of evolution (Shaw 1994). Constitutive heterochromatin has been shown to be highly polymorphic between and within species of Mus (Akeson and Davisson 1991, Forejt 1973, She 1990, Piálek 2005, Mitsainas et al. 2009). The studies on different rodents of the genera Peromyscus, Mastomys, Oryzomys, Sigmodon, Rattus, Apodemus and Mus revealed a common C-band pattern, i.e. large sized centromeric C-bands in the autosomes and X- chromosomes, and a completely heterochromatic small Y-chromosome (Modi 1987 and references there in).

Mus terricolor is an actively speciating incipient species complex in which constitutive heterochromatin is playing a major role in karyotype differentiation (Sen and Sharma 1983, Sharma et al. 1986, 1990, Bahadur 1995). Variation in autosomal C-positive heterochromatin in the range of populations studied, suggest that Mus terricolor is in a dynamic state of speciation. Variation and accumulation of heterochromatin have been shown in rodents by many workers. They have agreed that the accumulation of C-heterochromatin represents a recently evolved trait in rodents (Baverstock et al. 1976, Greenbaum and Baker 1978, Gamperl 1982a, Gamperl et al. 1982, Sen and Sharma 1983, Qumsiyeh et al. 1988, Gallardo 1991). C-band polymorphisms in terms of size variation in wild derived inbred strains of mice have also been reported by Akeson and Davisson (1991). In our study the presence of population specific and/ or chromosome specific large blocks of C-bands, either in homozygous or in heterozygous condition suggest an increase or accumulation of C-positive heterochromatin which is consistent with above findings.

C-band polymorphism in X chromosomes of Mus terricolor populations revealed interesting features. Two discrete heterochromatic blocks on short arms of X chromosomes in NXL and BDN (Fig. 1a, d) suggest segmental localization of heterochromatin. Balajee and Sharma (1994) have also shown the same result in Mus terricolor by digesting the metaphase chromosomes with Alu I and staining with Giemsa which produces C-band like features.

The large size of the Y chromosome in Mus terricolor is due to accumulation of C-positive heterochromatin (Sharma 1996) which shows population wise variation in banding intensity. In our study the cause of staining differentiation is not clear, though C-band polymorphism and apparent absence of C-positive chromatin in the Y chromosome has been shown in different species of rodents (Phenacomys intermedius Merriam, 1889, Microtus californicus Peale, 1848, Microtus orchogaster Schreber, 1842, Clethrionomys californicus Merriam, 1890 and Microtus oregoni Bachman, 1839, Microtus arvalis Pallas, 1778) by different workers (Zenzes and Voiculescu 1975, Gamperl 1982a, b, Vorontsov et al. 1984, Modi 1987) which suggests compositional heterogeneity of heterochromatin (Peacock et al. 1981, Patton and Sherwood 1982, Gallardo 1992) or unusual DNA sequences with different staining properties. (John and Miklos 1979, Peacock et al. 1981).

Populations of Mus terricolor showed prominent telomeric C-band on the long arm of X and also on acrocentric Y, but telomeric C-bands were not observed in autosomes in any population. Large prominent autosomal telomeric C-bands have been shown in the common wood mouse, Sylvaemus sylvaticus Linnaeus, 1758 by Nadjafova et al. (1993) and Nadjafova (2008) who implicated its role in differentiation of the species. The evolution of telomeric heterochromatin has been suggested to occur due to transposition and amplification of the centromeric satellite component (Hirning et al. 1989) in case of Sylvaemus sylvaticus. The situation in Mus terricolor needs to be intensively investigated for conclusive inferences.

Intra- and inter-specific karyotype evolution involving heterochromatin has been studied and discussed in many species but the evolutionary significance of heterochromatin is not established due to simultaneous involvement of chromosomal rearrangements, like inversions and Robertsonian translocations (Duffey 1972, Bradshaw and Hsu 1972, Pathak et al. 1973, Robbins and Baker 1981, Patton and Sherwood 1982, Davis et al. 1985, Modi 1987). However, evolutionary classification of the European wood mice of the subgenus Sylvaemus and genus Apodemus is based on chromosomal markers, like species specific C-positive heterochromatin (Orlov et al. 1996, Nadjafova 2008). Comparative FISH analysis of C–positive blocks of centromeric heterochromatin in different species of wood mice, Sylvaemus (Rubtsov et al. 2011) and three chromosomal forms of Sylvaemus uralensis Pallas, 1811 (Karamysheva et al. 2010) revealed variation in copy number and the level of homology of repetitive sequences as well as their localization. In the present study overall centromeric heterochromatin variation in size and intensity of bands in autosomes and heterochromosomes in Mus terricolor populations is also suggestive of quantitative as well as qualitative variation. Chatterjee et al. (2003) have commented on the basis of their southern hybridization experiments that Mus terricolor types differ in satellite DNA organization from that of Mus musculus Linnaeus, 1758, an allied species, and Mus booduga, the sibling species.

It can be concluded that very large to minute C-bands and even absence of C-bands in centromere of autosomes within and between populations of Mus terricolor indicates presence of differential amount of heterochromatin which might have evolved by non-reciprocal DNA turnover mechanisms in wild populations that has also been suggested by many workers (Dover 1982, Redi et al. 1990), however, this needs more extensive studies.