Timing of meiosis: Microtubules on the move

Abstract

Meiosis represents a typical feature of the eukaryotic life style, which is tightly associated with sexual reproduction. Its fundamental role is the formation of haploid gametes from diploid germ-line cells. This process involves several crucial events orchestrated in the 2 rounds of meiotic divisions and requires the pairing and recombination of homologous chromosomes. The failure of proper chromosomal segregation is the major cause of chromosomal aneuploidy, resulting in spontaneous miscarriages and congenital birth defects in human.1 As meiosis emerged early in eukaryotic evolution, its mechanism and corresponding genes are conserved across the eukaryotes.2 In fission yeast, Schizosaccharomyces pombe, haploid cells with opposite mating types mate, their nuclei undergo karyogamy, and the diploid nucleus readily enters into meiotic program, resulting in the formation of four haploid spores. Two recent studies3,4 point to the role of microtubule-associated proteins in proper timing of karyogamy and the first meiotic division, in which the chromosome number is reduced. During meiosis I, the chromosomal termini (telomeres) cluster at the nuclear envelope, resulting in a polarized configuration of chromosomes called the “bouquet” that plays a key role in coordinating the microtubule-organizing center and the spindle during meiosis.5 The bouquet formation is followed by oscillatory movement of the elongated nucleus (termed “horsetail”).6 The telomere clustering, horsetail movement, and recombination are essential for homologous chromosome pairing in meiosis I.