Quasi-invariance and reversibility in the Fleming-Viot process

Reversible measures of the Fleming-Viot process are shown to be characterized as quasi-invariant measures with a cocycle given in terms of the mutation operator. As applications, we give certain integral characterization of Poisson-Dirichlet distributions and a proof that the stationary measure of the step-wise mutation model of Ohta-Kimura with periodic boundary condition is nonreversible.