Loss of Pol32 induces chromosome breaks in post-embryonic cells.

A. Mitotic figures showing the different classes of DSBs in the mutant pol32L27/L30; polDl10/+. In the two wild-type nuclei chromosomes are individually identified. In the mutant nuclei, only the chromosome with a DSB of interest is denoted and the approximate location of the DSB marked with either an arrowhead (centric DSB) or an arrow (non-centric DSB). Aa, Ab: two wild-type nuclei with no broken chromosomes. Ac: a non-centric X chromosome break. Ad: a DSB at the centromere-proximal rDNA region on X. Ae: a DSB at the distal rDNA region. Af: a DSB at the DAPI-bright block of X. Ag: a non-centric DSB on chromosome 2. Ah: a centric DSB of 2. Note the rest of chromosome 2 is missing in this nucleus. Ai: a non-centric DSB on chromosome 3. Aj, Ak: two nuclei each with a centric DSB on 3. Al; a DSB at the rDNA region of the Y chromosome. Am: multiple DSBs in a nucleus. B. DSB frequencies. The top chart quantifies the percentage of nuclei with at least one chromosome break in three different genetic backgrounds. The bottom chart quantifies the two classes of breaks on each major chromosome from neuroblasts of the genotype pol32L27/L30; polDl10/+, with the numbers indicating the percentages of centric DSBs. C. FISH identifies broken sites in X peri-centromeric region. In each triplet of images, to the left is a DAPI-stained chromosome figure; in the middle is FISH image of a nucleus double labelled with rDNA (green) and 359 satellite (red) probes; and to the right is the merged product. Ca, a normal female nucleus. Cb, a nucleus with a DSB at the distal region of rDNA where the sister chromatids remain synapsed. Cc, a nucleus with a DSB splitting the rDNA arrays into approximate halves. Cd, a nucleus with a DSB splitting the DAPI-bright 359 satellite arrays into approximate halves. Ce, severe rDNA instability. Numerous acentric chromosomal fragments are visible with rDNA at one end (arrow), and rDNA fragments are present that do not appear to be attached to any major chromosomes (arrowhead). Cf, a possible case of rDNA expansion. An X chromosome with a greatly expanded rDNA array (arrow), when compared with another X chromosome with a normal appearance. In the same nucleus, the 359 satellite is attached to an aberrant chromosome proximally (arrowhead). Scale bars indicate 10μm.