This in turn leads to the formation of two separate nuclei in each cell of the two cell embryo, called the ‘paired nuclei’ phenotype ( Figure 1-figure supplement 1A Rahman et al., 2015). Consequently, the maternal and paternal chromosomes segregate into two separate DNA masses at each pole of the spindle.

If the NE between the two pronuclei persists through anaphase, as is the case in mutant embryos with delays or defects in NEBD, the two sets of parental chromosomes remain physically separated during the first mitosis. NEBD is thus spatially regulated in the early embryo but the underlying mechanisms are still unclear. Complete disassembly of the NE occurs later at the metaphase-to-anaphase transition in C. Accordingly, gap formation depends on proper chromosome alignment at the metaphase plate ( Rahman et al., 2015). The breakdown of the envelope located between the two pronuclei starts with the formation of a membrane gap ( Audhya et al., 2007), most likely triggered by egg and sperm chromosomes that come into contact with the NE. This allows the capture of the chromosomes by the microtubules and the mixing of the parental chromosomes ( Figure 1-figure supplement 1A). After rotation and centration of the nucleo-centrosomal complex, the NE breaks down in the vicinity of the centrosomes and between the juxtaposed pronuclei. After fertilization, the egg and sperm pronuclei meet near the posterior pole of the embryo. elegans embryo provides a dynamic developmental context for the investigation of the mechanism regulating nuclear envelope breakdown (NEBD) ( Figure 1-figure supplement 1A) and for review ( Oegema, 2006 Cohen-Fix and Askjaer, 2017 Pintard and Bowerman, 2019).

This step is essential to permit the mingling of the maternal and paternal chromosomes and their capture by the microtubules of the developing spindle. For this mitosis to occur properly, the NE of the pronuclei must be disassembled in a timely manner ( Rahman et al., 2020). In the zygotes of humans, but also in Caenorhabditis elegans, parental chromosomes are initially in separate pronuclei each surrounded by a nuclear envelope (NE) and meet for the first time during the first mitotic division.

After fertilization, the haploid gametes of the egg and sperm have to come together to form the genome of a new diploid organism.

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