Figure 1. Comparative analysis of somatic mutations in mice and flies.

(A) Schematic depiction of the LacZ-plasmid model in Mus musculus and D. melanogaster. Note that the mouse line 60 contains about 10 head-to-tail organized plasmid copies per integration site, but only two plasmid copies are depicted. For the transgenic flies, each line harbors only 1 copy of the pUR288 plasmid. Individual plasmids can be rescued by excision of genomic DNA with Hind III (H) or Pst I (P). After purification from the mouse or fly genomic DNA, self-ligation and transformation into Escherichia coli C (ΔlacZ, galE ^-) host cells, individual plasmids are recovered in the form of ampicillin-resistant colonies. A small amount of transformants is plated on medium containing X-gal, to determine the total number of plasmids rescued (titer plate). The remainder is plated on media supplemented with the lactose-analogue p-gal, to select only the cells harboring a mutant lacZ (selective plate). The mutant frequency is the ratio of the colonies on the selective plate versus colonies on the titer plate (times the dilution factor). Location and direction of the integration site of the pP[CaSper]vector is shown for line 11. For the mouse, the location and direction of the integrated pUR288 concatamers is shown for line 60. LacZ =  lacZ reporter gene; P5′ = 5′ end of (pP[CaSper]); P3′ = 3′ end of (pP[CaSper]); white =  the white selection marker. (B) Direct comparison of spontaneous somatic mutation frequency in mice (3-month old; liver) and flies (1–2-days old; whole body). White bars represent the frequency of point mutations and black bars the frequency of DNA rearrangements. The frequency of all mutations in the fly is greater than that of mouse liver (one-tailed Welch Two Sample t-test, p = 9.04e−05). Error bars are Standard Deviations.