  We simulate the decay dynamics of an isolated monopole defect in the nematic
vector of a spin-1 Bose-Einstein condensate during the polar-to-ferromagnetic
phase transition of the system. Importantly, the decay of the monopole occurs
in the absence of external magnetic fields and is driven principally by the
dynamical instability due to the ferromagnetic spin-exchange interactions. An
initial isolated monopole is observed to relax into a polar-core spin vortex,
thus demonstrating the spontaneous transformation of a point defect of the
polar order parameter manifold to a line defect of the ferromagnetic manifold.
We also investigate the dynamics of an isolated monopole pierced by a quantum
vortex line with winding number k. It is shown to decay into a coreless
Anderson-Toulouse vortex if k = 1 and into a singular vortex with an empty core
if k = 2. In both cases, the resulting vortex is also encircled by a polar-core
vortex ring.
