Live observation of fission yeast meiosis in recombination-deficient mutants: a study on achiasmate chromosome segregation

Regular segregation of homologous chromosomes during meiotic divisions is e ssential for the generation of viable progeny. In recombination-proficient organisms, chromosome disjunction at meiosis I generally occurs by chiasma formation between the homologs (chiasmate meiosis). We have studied meiotic stages in living rec8 and rec7 mutant cells of fission yeast, with special attention to prophase and the first meiotic division. Both rec8 and rec7 a re early recombination mutants, and in rec7 mutants, chromosome segregation at meiosis I occurs without any recombination (achiasmate meiosis). Both m utants showed distinct irregularities in nuclear prophase movements. Additi onally, rec7 showed an extended first division of variable length and with single chromosomes changing back and forth between the cell poles. Two othe r early recombination deficient mutants (rec14 and rec15) showed very simil ar phenotypes to rec7 during the first meiotic division, and the fidelity o f achiasmate chromosome segregation slightly exceeded the expected random l evel. We discuss possible regulatory mechanisms of fission yeast to deal wi th achiasmate chromosome segregation.