Fig. 5. Direct unequal cell divisions at the two-cell stage cause chaotic mosaicism.

(A) Strand-seq karyogram of seven-cell embryo (E170) showing chaotic mosaicism after direct unequal cleavage divisions at the two-cell stage. The strand inheritance patterns detected by Strand-seq enable the identification of sister cells, the deduction of the preceding division, and the distribution of the chromosomal fragments. This analysis reveals that both blastomeres at the two-cell stage performed a multipolar division; a tripolar division generated the sister cells C830, C827, and C832; and a tetrapolar division generated the sister cells C833, C828, C826, and C831. This led to the random distribution of the tetraploid set of chromosomes over the sister cells. As a consequence, the DNA fragments distributed over the sister cells sum up to a 4n copy number state. (B) Karyogram of a seven-cell embryo (E167) analyzed by Strand-seq showing the results of direct unequal cell divisions at the two-cell stage. The strand inheritance patterns indicate that cells C809, C808, and C804 are sister cells, as are cells C807, C803, C802, and C806. The DNA fragments distributed over the sister cells sum up to a 4n copy number state. (C) Schematic reconstruction of the direct unequal cleavage divisions of a two-cell–stage embryo. The blastomeres of a two-cell–stage embryo [embryo 170 produced with 2.5 Gy–treated sperm, whose Strand-seq ideogram is shown in (A)] cleaved directly into three and four daughter cells. Sister cells are depicted with the same color.