  We have carried out extensive Monte Carlo simulations of the fusion of tense
apposed bilayers formed by amphiphilic molecules within the framework of a
coarse grained lattice model. The fusion pathway differs from the usual stalk
mechanism. Stalks do form between the apposed bilayers, but rather than expand
radially to form an axial-symmetric hemifusion diaphragm of the trans leaves of
both bilayers, they promote in their vicinity the nucleation of small holes in
the bilayers. Two subsequent paths are observed: (i) The stalk encircles a hole
in one bilayer creating a diaphragm comprised of both leaves of the other
intact bilayer, and which ruptures to complete the fusion pore. (ii) Before the
stalk can encircle a hole in one bilayer, a second hole forms in the other
bilayer, and the stalk aligns and encircles them both to complete the fusion
pore. Both pathways give rise to mixing between the cis and trans leaves of the
bilayer and allow for transient leakage.
