Allozymic Variation in Four Indian Species of Genus Mus:A Comparative Analysis

# Indian Academy of Sciences Allozymic variation in four Indian species of genus Mus:a comparative analysis M. AWASTHI1,K.V.BHAT2andR.K.ANAND1Ã 1Experimental Animal Facility, National Institute of Immunology, Aruna Asaf Ali Marg, JNU Complex, New Delhi 110 067, India 2National Bureau of Plant Genetic Resources, Pusa Campus, New Delhi 110 012, India Abstract The present study focusses on allozyme variation in the commensal house mouse Mus musculus, the pygmy field mice M. booduga and M. terricolor, and the spiny mouse M. platythrix. Genetic heterozygosity was estimated using a set of 24 polymorphic biochemical genetic markers. The extent of variability present in M. booduga, M. terricolor and M. platythrix has been compared with that in the M. musculus complex. Levels of allozyme variation at species level indicate that M. musculus has the maximum heterogeneity, followed by M. booduga and M. terricolor, while M. platythrix shows comparatively homogeneous genetic make-up. Gene frequency data have been used to trace phylogenetic relationships among these four species. [Awasthi M., Bhat K. V. and Anand R. K. 1999 Allozymic variation in four Indian species of genus Mus: a comparative analysis. J. Genet. 78, 73±80] Introduction one of the lineages, are the Indian pygmy field mice, characterized by their small size. These are endemic The Indian subcontinent is considered to be the main hub of to the Indian subcontinent, and are morphologically very the genus Mus as evident from the large number of species similar. These mice also inhabit the same ecological fields. present in this part of the world. Marshall (1977) identified Like M. musculus, M. booduga has 40 all acrocentric many morphological groups, designated as `species' within chromosomes. M. terricolor on the other hand shows the genus Mus, and described 16 species from Asia alone variation in its karyotype. In different nonoverlapping based on quantitative differences in morphology and karyo- populations of M. terricolor, three divergent karyotypes type. These were grouped under three main subgenera: (types I, II and III) are found (Sharma 1996; Sharma and Pyromys, Coelomys and Mus. Sharma 1998). Mice belonging to subgenus Pyromys are commonly M. musculus is the true house mouse, showing a karyo- known as spiny mice because of the presence of trough- type of 40 acrocentric chromosomes. On the Indian sub- shaped spines mixed in the fur and are represented by M. continent a large number of species of the subgenus Mus platythrix and M. saxicola. Though sometimes included in exist which prevent the house mouse from occupying the subgenus Mus, they are quite divergent from Mus as outdoor habitats. The Indian house mouse is a purely they show distinct karyotypes (2n 22±26). commensal form found exclusively in association with man- The subgenus Mus has four lineages. M. booduga and made structures (Marshall 1977). M. terricolor (previously known as M. dunni till Musser and Since electrophoretic methods were first applied to deter- Carleton (1993) replaced the nomenclature), which form mine the extent of allozyme variation in population samples, a major confusion over the classification of Mus has been cleared. Genetic polymorphisms have contributed signifi- Ã For correspondence. E-mail: [email protected]. cantly to our understanding of intraspecific and interspecific Keywords. allozyme; heterogeneity; Mus musculus; Mus booduga; Mus terricolor; Mus platythrix. Journal of Genetics, Vol. 78, No. 2, August 1999 73 M. Awasthi et al. systematic relationships since they allow measurements of Table 1. Populations used in the study. the extent of gene exchange. The first phylogenetic tree of Locality Sample size the subgenus Mus was obtained from allozyme data (Sage 1981; Bonhomme et al. 1984). This was followed by Mus musculus sequence information and restriction fragment length 1. Coimbatore (CO) 3 2. Delhi (DEL) 10 polymorphism data for various genes (Jouvin-Marche et al. 3. Guwahati (GU) 10 1988), mitochondrial DNA analysis (Fort et al. 1984), Mus booduga and DNA/DNA hybridization of single-copy nuclear DNA 1. Chennai (CN) 10 (scn DNA) (She et al. 1990). All these data were used to 2. Coimbatore (CO) 5 draw a synthetic tree and to infer different levels of quasi- Mus terricolor III synchronous speciation events (Boursot et al. 1993). The 1. Chennai (CN) 10 first level corresponds to the separation of the subgenus Mus from other subgenera Nannomys, Pyromys and Coelomys. Mus platythrix Yet another level is the separation of the Indian pygmy mice 1. Coimbatore (CO) 4 2. Hosur (HO) 4 M. terricolor and M. booduga, which is correlated with the 3. Ooty (OT) 2 appearance of a 40-acrocentric-chromosome karyotype characteristic of the whole subgenus Mus. The final level of radiation corresponds to the individualization of the main subspecies of M. musculus which took place half a million Material and methods years ago. The house mouse is the most recent phylogenetic offshoot of the genus Mus. Animals: Mice were trapped live from six Indian localities The Indian M. musculus species has recently been the (Coimbatore, Chennai, Delhi, Guwahati, Hosur and Ooty; focus of interest primarily because of its high genetic table 1 shows sample sizes). Random samples from these heterogeneity on the basis of which it was proposed that the localities were identified on the basis of their morphology Indian subcontinent is the cradle from where the entire M. by Zoological Survey of India, Calcutta. In addition to this, musculus species originated and colonized the world aboriginal M. platythrix was distinguished on the basis of its (Bonhomme et al. 1994; Boursot et al. 1996; Din et al. large size, eyes, short tail, and presence of 26 acrocentric 1996). This hypothesis is supported by studies of mice from chromosomes. These animals shed off the entire caudal different localities of India which have shown high levels of integument when picked up by the tail. M. musculus and M. gene variation at 15 polymorphic biochemical genetic booduga showed 2n 40 chromosomes. M. terricolor type markers (Awasthi et al. 1998). III trapped from Chennai was identified on the basis of its Whereas M. musculus has been in the major limelight of large submetacentic X, large acrocentric Y, and three pairs research, its distant relatives have been relatively neglected. (1, 3 and 6) of biarmed autosomes with prominent hetero- There has been a solitary report on the Asian species chromatic short arms (Sen and Sharma 1983). platythrix and pahari, which were examined at 28 protein loci (Bonhomme et al. 1984). Allelic variation in ®ve Biochemical genetic analysis: Blood and tissues of freshly individuals of M. terricolor trapped from India was dissected animals were stored at 70C. RBC lysates, examined at 22 protein loci, and phylogenetic trees within plasma, kidney and liver homogenates were used for bio- the genus Mus were proposed (Bonhomme et al. 1984). chemical analysis. A total of 24 variable biochemical genetic Chatterjee et al. (1994) published the mitochondrial DNA loci were analysed. Electrophoresis was performed on restriction maps of M. booduga, M. terricolor and M. m. TITAN III cellulose acetate plates (Helena Laboratories, tytleri. Singh and Sharma (1996) reported the use of super- Beaumont, Texas, USA), with each marker having its own oxide dismutase (Sod-1) as a diagnostic marker for the unique buffer system (Langley and Roderick 1984). The identi®cation of M. booduga. Singh and Sharma (1997) grading of the alleles was done by comparing the electro- studied genetic variation in the pygmy mice M. booduga phoretic mobility profile with that of C57BL/6J inbred and M. terricolor complex and in the house mouse M. m. strain of mouse, whose alleles were considered as standard tytleri trapped from Varanasi and Delhi. allele 100, except for Hbb and Ldr-1 for which the conven- Whereas there are exhaustive reports on M. musculus tional alphabetic allele designation was used. from all over the world, there has not been much work done on the biochemical genetic structure of other closely related Data analysis: (i) Allelic frequencies at the polymorphic loci species. It would be interesting to examine them and to were calculated. (ii) Using the gene frequency data Nei's observe whether they also exhibit high genetic diversity. In genetic distance matrix was constructed (Nei 1978). (iii) this paper we report allozyme variation in different species PHYLIP package (version 3.5c, Felsenstein 1993) was used of Mus, compare their population structure and heterozygo- for phylogenetic analysis. One thousand replicates of the sity levels, and establish phylogenetic relationships linking original data set were constructed by bootstrap resampling them. and Nei's genetic distance was computed for all data sets 74 Journal of Genetics, Vol. 78, No. 2, August 1999 Allozyme variation in Indian Mus using the GENDIST program. These were further used to According to Marshall (1977) the karyotype of M. terricolor construct phylogenies using UPGMA and NEIGHBOR has evolved as a device to preclude interbreeding with M. JOINING implemented by NEIGHBOR program. CON- booduga. Compared to the Indian M. musculus populations, SENSE program selects the best possible phylogeny and these animals show a lower level of genetic polymorphism. this was used to construct an unrooted tree by DRAWTREE. Biochemical-genetic profile of M. platythrix Results On cellulose acetate electrophoresis these animals show distinct patterns that easily discriminate them from other Biochemical-genetic profile of M. musculus members of the subgenus Mus. Of the 24 loci studied, nine The Indian populations of M. musculus are highly hetero- loci, Amy-1, Apoa-1, Es-1, Es-10, Idh-1, Ldh-1, Pgm-1, geneous. This contributes to increased polymorphism. The Pgm-2 and Trf, were variable and the remaining 15 were three localities chosen for the study were Delhi, Guwahati invariant (table 2). Though these animals were trapped from and Coimbatore, which represent the north, east and south of three different localities, their level of polymorphism and India respectively.

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