Electron tomography of two mating yeast nuclei reveals multiple steps to fusion.
Before human gametes fuse, their nuclear membranes break down to allow the nuclear contents to mix. But when yeast mate, the inner and outer membranes of the two nuclei have to join, as do the spindle pole bodies (SPBs). Spanning the inner and outer membranes, SPBs anchor the microtubules that winch the nuclei together. Whether the components merge simultaneously or in three separate steps has been debated, due to the difficulty of catching nuclei in the act.
Melloy et al. used electron tomography to capture 3D images of nuclei at different stages of fusion. They found that nuclei retained separate SPBs even after the inner and outer membranes had joined, indicating that the SPBs are the last to fuse.
To determine which membranes linked up first, the team turned to light microscopy. They filled the nucleus with one marker and the lumen between the inner and outer membranes with another. The lumen marker started moving from one partner to the other about 30 seconds before the nuclear marker, indicating that the outer membranes merge first. Researchers suspect that outer membranes are drawn together by SNARE proteins. The mystery now is which proteins join the inner membranes. 
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