Germline-soma relationships in ciliated protozoa: the inception and evolution of nuclear dimorphism in one-celled animals

Unique features of ciliates are reviewed in the framework of a speculative series of evolutionary transitions: a uninucleate protozoan gave rise to a multinucleate unicell and then a nuclear dimorphic unicell, with a germline micronucleus and a differentially amplified somatic macronucleus, possibly before the divergence of ciliates and heterokaryotic foraminiferans. Ciliates evolved to proliferate only in the diploid state. Macronuclear DNA fragmentation and amitotic karyokinesis arose multiple times. Variability arises in the macronucleus, a potential source of selectable diversity, and probably the cause of clonal senescence. Transposons may have played a role in micronuclear silencing, and proliferate rapidly in hypotrichous ciliate germlines due to their precise elimination from the developing macronucleus before its genes are expressed.