Here we report our study on two new karyotypes and C-bandings of M. m. faeceus . Our study shows that there are polymorphisms in the X chromosome of M. m. faeceus (Rodentia), which exibitsubtelocentric X ST , metacentric X M and submetacentric X SM patterns, respectively. One new karyotype formula is: 2n ϭ 49 ϭ 4sm ϩ 42t ϩ 1m ϩ XX (sm,sm), and the sex chromosomes are: X M X SM ; The other new karyotype formula is: 2n ϭ 50 ϭ 4sm ϩ 43t ϩ 1m ϩ XX (sm,sm), and the sex chromosomes are: X SM X SM . One C-banding karyotype formula is: 2n ϭ 49 ϭ 4sm ϩ 44t ϩ X(st)0, and the sex chromosome is: X ST O and the other C-banding karyotype formula is: 2n ϭ 50 ϭ 4sm ϩ 44t ϩ XY(m,t), and the sex chromosomes are: X M Y.
Chromosome preparationOne hundred mg ml Ϫ 1 colchicine was injected into the mandarin voles at the dose of 0.01 ml g Ϫ1 body weight. When the animals were dead (2 to 3 h after injection), their gonads were examined to identify their sex. Chromosome specimens were prepared by the air-dry method (LI 1985) and stained conventionally with 10% Giemsa for 30 -50 min.
C-banding patternsC-banding patterns were obtained by using the BSG method (SUMNER 1972).
Chromosome analysis and statisticsSixty to eighty metaphase cells of every individual, in which the chromosomes were dispersed well, were chosen to count the chromosome numbers and to analyze the structure of chromosomes. Representative metaphase cells were photographed with a Leica DMRA2 microscope.
RESULTS
Polymorphic X chromosomes of Microtus m. faeceusOur chromosome analysis revealed two chromosomal types for Microtus m. faeceus , XX/XY or XO/XY. We measured the arm length and calculated the arm ratio of X chromosomes. The results show that the X chromosomes were polymorphic, showing three types with the longest being a subtelocentric X chromosome (X ST ), the middlesized being a submetacentric chromosome (X SM ), and the shortest being a metacentric X chromosome (X M ) (Fig. 1, 2). Heteromorphic X chromosomes also exist in karyotypes of M. mandarinus captured from Russia and Lingbao and Wendeng in China (KOVALSKAYA et al. 1974;ZHU et al. 2003;WANG et al. 2003).A new karyotype 2n ϭ 50 ϭ 4sm ϩ 43t ϩ 1m ϩ xx (sm,sm) with the sex chromosomse X SM X SM Our analysis of the chromosomes of M. m. faeceus revealed a new karyotype. The second pair of autosomes (no. 2) was H type autosomes in this new karyotype (Fig. 1c). The fi rst and the third pair of autosomes were submetacentric chromosomes (SM, SM), and the chromosomes were commonly telocentric autosomes from no. 4 to no. 24, and no. 23 autosome had only one chromosome. The sex chromosomes were X SM and X SM , and this was also a new polymorphism of X chromosome in M. m. faeceus , which was not reported in Zhu's article (Fig. 1a -b).