NIH Research Festival
In a fertilized embryo, the nuclear envelopes (NEs) of the maternal and paternal pronuclei must break down to allow mixing of the two paternal genomes. To date, only the fate of NE proteins has been examined. In C. elegans, these dissociate from the NE after pronuclear meeting, leaving behind the double membranes of each pronucleus. The process that breaches these membranes is unknown. To address this, we followed nuclear membrane architecture in the one-cell embryo from prometaphase to anaphase using Focused Ion Beam - Scanning Electron Microscopy (FIB-SEM) at 9 nm resolution. After the two pronuclei meet, they are separated by four nuclear membranes. We found that in prometaphase, the two pronuclei become linked through membrane junctions that connect the outer nuclear membranes of the two pronuclei, generating 4-way junctions. As the cell reaches metaphase, the interface between the two pronuclei is composed of only two nuclear membranes. A reduction in the number of membranes between the two pronuclei from four to two, possibly achieved by movement of membrane material away from the pronuclear interface through multiple junctions. These form a flat interface surrounded by three-way junctions, wherein four pronuclear membranes are reduced to two. This flat interface is fenestrated by multiple holes ranging from tens of nanometers to several microns in diameter. These membrane junctions and the multiple fenestrations were not previously detected in any system. At metaphase, the chromosomes of the two pronuclei mix through one of the large fenestrations, which then expands to take up most of the interface. At anaphase, the membrane interface disappears. Thus, in C. elegans, the two pronuclei do, in fact, fuse. The presence of membrane junctions suggests that parental genome mixing requires a membrane fusion machinery, the nature of which is under investigation.
Scientific Focus Area: Cell Biology
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