Origin of unitary symmetry and charge conservation in strong interactions

We discuss the relation of the existence of multiplets of (strongly) interacting particles and the possible unitary symmetry of their interactions. We present here a dynamical principle which concerns the one-particle propagators (two-point functions) but yielding the existence of a (unitary) symmetry group for their trilinear interactions. We derive, as a by-product, electric charge (and hypercharge) conservation in the interaction of these particles