Nuclear dimorphism is a term referred to the special characteristic of having two different kinds of nuclei in a cell. This feature is observed in protozoan ciliates and some foraminifera. Ciliates contain two nucleus types: a macronucleus that is primarily used to control metabolism, and a micronucleus which performs reproductive functions and generates the macronucleus. Nuclear dimorphism is subject to complex epigenetic controls.
The micronucleus is globally repressed during the vegetative state, and serves as the diploid germline nucleus, whereas all known vegetative gene expression happens in the macronucleus, which is a polyploid somatic nucleus.
The ciliated protozoan Tetrahymena is a useful research model for studying nuclear dimorphism; it maintains two distinct nuclear genomes, the micronucleus and the macronucleus. The micronucleus genome contains five chromosomes that undergo mitosis during micronuclear division and meiosis during conjugation, which is the sexual division of the micronucleus. The macronuclear genome is broken down and catabolized once per life cycle during conjugation, allowing it to be site-specific, and a new macronucleus differentiates from a mitotic descendant of the conjugated micronucleus.