The origin of the genetical diversity of Microtus mandarinus chromosomes

Here we describe our comparative studies on two types of X chromosomes, namely XM and XSM, of the mandarin vole (Microtus mandarinus). By chromosome G- and C-banding analysis, we have found that two different types of X chromosomes exist in mandarin voles. The two types of X chromosomes present two different G- and C-banding patterns: the XM chromosome is a longer metacentric X chromosome which is C-band negative; and the XSM is a shorter submetacentric X chromosome which has one C-band at the centromere and another one at the middle part of the short arm. The XSM has 6 G-bands including one on the kinetochore, one in the middle of the short arm, and four on the long arm. The XM has 7 G-bands including one on the kinetochore, two on the short arm, and four on the long arm. We have further found that female voles can be grouped into three types based on the composition of the X chromosome but the male voles have only one type. The three female groups are: (1) female voles (XMXSM), in which the two X chromosomes are different, the longer one is metacentric and the shorter is submetacentric; (2) female vole (XSMXSM), in which the two X chromosomes are both submetacentric; (3) female vole (XMO), in which there is only one X chromosome that is metacentric. Surprisingly, we have never found female voles with XMXM, females with XSMO or males with XMY. We hypothesize that the XSM chromosome is derived from the XM through its breakage and re-joining. The paper also discusses the formation of XMO females.