constructed (α, θ)-interval partition evolutions for α ∈ (0, 1) and θ ≥ 0, in which the total sums of interval lengths ("total mass") evolve as squared Bessel processes of dimension 2θ, where θ ≥ 0 acts as an immigration parameter. These evolutions have pseudo-stationary distributions related to regenerative Poisson-Dirichlet interval partitions. In this paper we study symmetry properties of (α, θ)-interval partition evolutions. Furthermore, we introduce a three-parameter family SSIP (α) (θ1, θ2) of self-similar interval partition evolutions that have separate left and right immigration parameters θ1 ≥ 0 and θ2 ≥ 0. They also have squared Bessel total mass processes of dimension 2θ, where θ = θ1 + θ2 − α ≥ −α covers emigration as well as immigration. Under the constraint max{θ1, θ2} ≥ α, we prove that an SSIP (α) (θ1, θ2)-evolution is pseudo-stationary for a new distribution on interval partitions, whose ranked sequence of lengths has Poisson-Dirichlet distribution with parameters α and θ, but we are unable to cover all parameters without developing a limit theory for composition-valued Markov chains, which we do in a sequel paper.