Figure 6.

Fertilization processes and variants of heterogoneic divisions. (A–C) Sequence of events and their outcomes following the first post-zygotic division. (A) Normal fertilization followed by post-zygote mitosis, resulting in two diploid biparental daughter blastomeres. (B) Dispermic fertilization followed by ectopic metaphases or the extrusion of the paternal pronucleus, leading to the segregation of one paternal genome in an androgenetic cell line. (C) Normal fertilization followed by the segregation of the maternal and the paternal genomes into distinct cell lineages (see D). (D) The different hypothetical models, which might cause the first zygotic cleavage into heterogoneic cell lineages in E19_BRP012: (i) One pole of the gonomeric spindle loses its integrity and becomes tripolar; subsequently the zygote divides directly in three cells; (ii) the maternally and paternally derived spindles operate separately in the zygote (Van Blerkom et al. 2004) and segregate the paternal and the maternal genomes into four haploid blastomeres; and (iii) following failure of cytokinesis, the zygote contains four nuclei, which are allocated stochastically into daughter blastomeres following a noncanonical first zygotic division (direct cleavage into three or four cells), or the maternal and the paternal pronuclei are allocated in two different daughter cells following a noncanonical zygotic division.