The breakage is not specific to the Plk4 transgenic insertion. (A) Oocytes coming from a Plk4 line A^flox/wt; Cre⁺; MyoX^flox/wt cross display both Plk4 line A type of break (12.5% of oocytes, upper panels, see blue arrows) and a new form of break producing a fragment with no centromeres (15.6% of oocytes, middle panels, see orange arrows). DNA is in gray levels and CREST is in magenta. The two lower panels show control oocytes (Plk4 line A^flox/wt; Cre⁻; MyoX^flox/wt and Plk4 line A^wt/wt; Cre⁺; MyoX^flox/wt). The percentage of oocytes without broken bivalent from these controls is indicated on the pictures of the two left lower panels. Scale bar is 5 µm; n is the number of oocytes. (B) Percentages of oocytes with the different types of break in the Plk4 line A^flox/wt; Cre⁺; MyoX^flox/wt cross (on n = 32 oocytes). (C) Quantitative analysis of the volume of the smallest chromosome fragment in oocytes from Plk4 OE lines A (blue squares) and oocytes from the Plk4 line A^flox/wt; Cre⁺; MyoX^flox/wt cross (orange triangles) observed at NEBD + 6h00; *, P = 0.0121 (Mann–Whitney test); n is the number of oocytes. Error bars correspond to SD. (D) Examples of oocytes with the new type of broken bivalent in the Plk4 line A^flox/wt; Cre⁺; MyoX^flox/wt cross observed at NEBD + 6h00. Broken bivalents have been artificially colored in orange. Scale bar is 5 µm. (E) Scheme representing the position of the MyoX LoxP sites (orange) on chromosome 15. (F) Proposed model of the new bivalent breakage occurring in the Plk4 line A^flox/wt; Cre⁺; MyoX^flox/wt cross. MTs are in green. Kinetochores are in pink. The LoxP sites insertion region appears in orange.