Evolution of an octacentric isochromosome in mouse L-cells

A sub-line of mouse L-cells exhibits a rather long biarmed chromosome which shows eight C-bands. Incorporation of BrdU for less than one cell cycle results in lateral asymmetry in the pericentromeric region as well as in the two arms. These regions of asymmetry correspond to four C-banding regions in each arm. Before it is fully condensed at metaphase, and particularly upon treatment with Hoechst, this chromosome expresses eight primary constrictions. Hence it is an octacentric. Presumably, it originated from tandem rejoinings between the short arms and the long arms of four chromosomes followed by the formation of an isochromosome. Since it is present in almost 100% cells, this octacentric chromosome must divide equationally at every cell division.