Abstract
The presence of meiosis, which is a conserved component of sexual reproduction, across organisms from all eukaryotic kingdoms, strongly argues that sex is a primordial feature of eukaryotes. However, extant meiotic structures and processes can vary considerably between organisms. The ciliated protist Tetrahymena thermophila, which diverged from animals, plants, and fungi early in evolution, provides one example of a rather unconventional meiosis. Tetrahymena has a simpler meiosis compared with most other organisms: It lacks both a synaptonemal complex (SC) and specialized meiotic machinery for chromosome cohesion and has a reduced capacity to regulate meiotic recombination. Despite this, it also features several unique mechanisms, including elongation of the nucleus to twice the cell length to promote homologous pairing and prevent recombination between sister chromatids. Comparison of the meiotic programs of Tetrahymena and higher multicellular organisms may reveal how extant meiosis evolved from proto-meiosis.
Highlights
Meiosis is a special type of cell division through which eukaryotic germ progenitor cells halve the somatic diploid chromosome set to generate the gametic haploid set for sexual reproduction
DNA ends at double-strand breaks (DSBs) sites can interact with intact DNA molecules to probe homology, pair, and initiate homology-dependent repair, which leads to interhomolog crossovers (COs)
The choice of yeast was a good one because many features of yeast meiosis turn out to be typical of canonical meiosis, which prevails in vertebrates and plants, and whose understanding is critical for clinical human genetics and plant breeding
Summary
Meiosis is a special type of cell division through which eukaryotic germ progenitor cells halve the somatic diploid chromosome set to generate the gametic haploid set for sexual reproduction. The MIC functions as the germline: During vegetative growth, it divides mitotically, and during sexual reproduction, it undergoes meiosis and produces gametic nuclei. Studies of deletion mutants showed that of more than 100 genes with an expression peak at the early mating stage, only about one-quarter could be assigned a function in meiotic pairing and/or recombination (Table 1).
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