Figure 1.

Nuclear dimorphism and the paramecium life cycle. (A) Life cycle. Left, vegetative cycle. During vegetative growth, paramecia divide by binary fission. The two micronuclei (MIC) undergo mitosis in the absence of nuclear envelope breakdown, while the macronucleus (MAC) elongates and divides by an amitotic process. Right, sexual cycle. P. aurelia species have two mating types, and sexually reactive cells can conjugate with a partner of the opposite mating type. In the absence of an appropriate partner, an auto-fertilization process (autogamy) occurs, illustrated here. Autogamy begins with meiosis of the two MIC to yield eight haploid products, seven of which degenerate. The eighth haploid gametic nucleus copies itself by mitosis and the two identical haploid nuclei fuse to from a completely homozygous diploid zygotic nucleus. Two post-zygotic mitotic divisions yield four diploid germline nuclei, which migrate to positions at the anterior and posterior of the cell. The two nuclei at the cell posterior differentiate into new MACs, the two nuclei at the cell anterior are the new MICs. During the first, caryonidal, cell division the macronuclear anlage do not divide. One is distributed to each daughter cell as they continue to endoreplicate DNA to attain the final MAC copy number of ∼800 n. After a second cell division in which both the MICs and the MAC divide, the sexual progeny enter the vegetative phase. Note that the progressively fragmented maternal MAC is present throughout meiosis, fertilization, and MAC differentiation and remains transcriptionally active. The fragments are lost by dilution in the course of the first cell divisions. The same events occur during conjugation; however, there is reciprocal exchange of haploid gametic MICs, so that the diploid zygotic nucleus produced by fertilization in each conjugating cell is heterozygous at all loci. Both sexual processes can be induced by standard laboratory protocols (Sonneborn 1974). (B) Genome reorganization. During MAC differentiation, DNA is amplified to a final copy number of ∼800 n. Different classes of repeated sequences, such as transposable elements and minisatellite repeats, schematized here, are eliminated by an imprecise mechanism that leads to chromosome fragmentation and de novo telomere addition, but which in some cases can be resolved by variable internal deletions. Internal Eliminated Sequences (IES) are short (<1 kb) unique-copy elements that interrupt both coding and noncoding sequences. IESs are precisely excised between 5′-TA-3′ dinucleotides at each end of the IES. A single TA remains in the MAC DNA. For a detailed review, see Bétermier (2004).