Pre-meiotic S phase is linked to reductional chromosome segregation and recombination

Meiosis is initiated from G1 of the cell cycle and is characterized by a pr e-meiotic S phase followed by two successive nuclear divisions. The first o f these, meiosis I, differs from mitosis in having a reductional pattern of chromosome segregation(1,2). Here we show that meiosis can be initiated fr om G2 in fission yeast cells by ectopically activating the meiosis-inducing network. The subsequent meiosis I occurs without a pre-meiotic S phase and with decreased recombination, and exhibits a mitotic pattern of equational chromosome segregation. The subsequent meiosis II results in random chromo some segregation. This behaviour is similar to that observed in cells lacki ng the meiotic cohesin Rec8 (refs 3, 4), which becomes associated with chro mosomes at G1/S phase, including the inner centromere, a region that is pro bably critical for sister-centromere orientation(5). If the expression of R ec8 is delayed to S phase/G2, then the centromeres behave equationally. We propose that the presence of Rec8 in chromatin is required at the pre-meiot ic S phase to construct centromeres that behave reductionally and chromosom e arms capable of a high level of recombination, and that this explains why meiosis is initiated from G1 of the cell cycle.