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The C-band and G-band patterns of Microtus agrestis metaphase chromosomes are described. The C-band pattern reveals constitutive heterochromatin as uniformly intenselystained areas but cannot aid in identifying autosomal pairs. The G-bands revealed by a heat renaturation method (ASG) were compared with those revealed by treatment with three proteolytic enzymes. All procedures yield apparently the same specific pattern of crossbands on the autosomes, and each autosomal pair can therefore be identified by its characteristic pattern. The constitutive heterochromatin of the sex chromosomes stains uniformly with the heat renaturation method but is subdivided into regions with different staining intensities with each enzyme treatment. A G-band karyotype for Microtus agrestis metaphase chromosomes is presented.

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A cell line from a female Microtus agrestis has been established in vitro, with most cells showing the normal female complement with two entire X chromosomes. Cloning of this line yielded stable cell strains, some of which showed deletions of the X chromosomes. Karyotypically, the deletions include (1) loss of the long arm of one X chromosome, (2) loss of one entire X chromosome, and (3) loss of one entire X chromosome together with loss of the long arm of the other X chromosome. In the last case, only the “functional” portion of the presumably active X chromosome remained. Radioautography has confirmed the actual loss of both constitutive and facultative heterochromatin from the X chromosomes in these clones. In addition, the parent cell line yielded a clone tolerant to 5-bromodeoxyuridine (BUdR) at 5 µg/ml. This cell strain retains thymidine kinase activity, however, as evidenced by uptake of thymidine, and by incorporation of BUdR into the DNA.

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