C. elegans pronuclei fuse after fertilization through a novel membrane structure

Abstract

After fertilization, parental genomes are enclosed in two separate pronuclei. In Caenorhabditis elegans, and possibly other organisms, when the two pronuclei first meet, the parental genomes are separated by four pronuclear membranes. To understand how these membranes are breached to allow merging of parental genomes we used focused ion beam scanning electron microscopy (FIB-SEM) to study the architecture of the pronuclear membranes at nanometer-scale resolution. We find that at metaphase, the interface between the two pronuclei is composed of two membranes perforated by fenestrations ranging from tens of nanometers to several microns in diameter. The parental chromosomes come in contact through one of the large fenestrations. Surrounding this fenestrated, two-membrane region is a novel membrane structure, a three-way sheet junction, where the four membranes of the two pronuclei fuse and become two. In the plk-1 mutant, where parental genomes fail to merge, these junctions are absent, suggesting that three-way sheet junctions are needed for formation of a diploid genome.

Molecular genetics of maternally-controlled cell divisions

“…Surprisingly, this phenotype persisted in the subsequent mitotic divisions during early embryogenesis [36]. More recent studies have demonstrated that wild-type Plk1 is required for the formation of a novel three-way membrane junction that promotes pronuclear fusion in C. elegans [37]. Interestingly, this structure does not normally form during subsequent somatic division in wild-type embryos, explaining the persistence of dual nuclei in temperature-sensitive plk1 mutants [37].…”

Membrane fusion drives pronuclear meeting in the one-cell embryo

“…To better understand how pronuclei fuse, in this issue, Rahman et al (7) used focused ion beam–scanning electron microscopy (FIB-SEM) in Caenorhabditis elegans zygotes, to study pronuclear fusion. The FIB-SEM approach allowed Rahman et al (7) to generate three-dimensional reconstructions of nuclear membranes from both wild-type and mutant embryos at prophase, prometaphase, and metaphase. These images confirmed early EM findings and revealed other novel observations leading to a new, multistep model for pronuclear fusion.…”

“…Rahman et al (7) also examined pronuclear membranes in mutant embryos. It was previously shown that plk-1 is necessary for the mixture of parental genomes.…”

“…Overall, Rahman et al (7) have successfully used FIB-SEM to uncover novel three-way sheet junctions between the membranes of pronuclear envelopes. This implies a novel mechanism in which membrane fusion, and not NEBD, drives pronuclear fusion before the first mitosis.…”

“…Since similar two-membrane interfaces between pronuclei have been hinted at in budding yeast, sea urchin, and algal systems (4, 5, 6), the findings described here from C. elegans are likely conserved. Thus, Rahman et al (7) have made a significant advance in delineating the mechanisms of pronuclear fusion in the presence of persisting membranes. Further mechanistic understanding of this new paradigm may inform studies on aneuploidy and fertilization.…”