Nuclear differentiation and nuclear heteromorphism in the protozoa]

This is a review based upon a lecture given at the Developmental Biology School near Moscow in November 1991. Cases of successive and simultaneous nuclear differentiation are defined, and examples from various groups of protozoa are considered. Three cases of simultaneous nuclear differentiation, leading to the nuclear dualism phenomenon (heteromorphism) are analysed in detail. In heterokaryotic agamonts of some Foraminifera, nuclear differentiation occurs at the diploid level, proved to be irreversible and caused by either deletion or stable repression of some genes. In the Karyorelictid ciliates, the somatic nuclei (macronuclei) are paradiploid, metabolically active but unable to divide. They are irreversibly differentiated due to deletion of part of the initial (micronuclear) genome. Differentiation occurs in every cell cycle from the generative nuclei (micronuclei), which retain omnipotency and reproduce by mitosis. In most (higher) ciliates the differentiation of macronuclei occurs at an early stage of replication and involves a more or less drastic reorganization of the micronuclear genome, including both deletions and transpositions. Thereafter, the reorganized (and generally reduced) genome is strongly amplified to provide a high dose of active genes. The chromosomes of the majority of ciliates are fragmented in their macronuclei into either subchromosome-sized or gene-sized molecules, both being acentric. Systems regulating the differential replication of these molecules are likely to exist in macronuclei to keep the respective gene doses within certain limits.