**3.** *Schizosaccharomyces pombe*

*S. pombe***,** also referred to as "fission yeast," is a unicellular rod-shaped eukaryotic microorganism in the Division *Ascomycota*. It grows vegetatively primarily as a haploid organism. *S. pombe* is facultatively sexual, so that when nutrients are limiting cells of opposite mating type tend to undergo syngamy (union of gametes) to form diploid zygotes [24]. The zygote can then enter meiosis leading to the production of four haploid products (spores) initially enclosed in a sac called an ascus.

Several different types of experiments have shown that DNA damages induce the sexual cycle and meiotic recombination in *S. pombe*. First, exposure of *S. pombe* cells to hydrogen peroxide, a reactive chemical that causes oxidative DNA damage, was observed to lead to an increase in sexual reproduction associated with a 4- to 18-fold increase in the formation of meiotic spores [25]. Second, DNA damages, in which the base cytosine is deaminated to uracil, forming the inappropriate base pair dU:dG, stimulate meiotic recombination [26]. Third, faulty processing of DNA replication intermediates (referred to as Okazaki fragments) produces DNA damages, including single-strand breaks or gaps, that stimulate meiotic recombination [27].

The fission yeast *S. pombe*, like the budding yeast *S. cerevisiae* (see above)*,* switches mating type during vegetative growth, though they each use different mechanisms [28]. This provides *S. pombe* with increased mating opportunities with close relatives. The decreased opportunity for outcrossing in *S. pombe* indicates that the production of genetic variation is unlikely to be the principal selective force maintaining meiotic sex in these organisms. Overall, the findings with *S. pombe*, like those with the other yeasts, *S. cerevisiae* and *S. paradoxus,* suggest that meiotic recombination is primarily an adaptation for repairing DNA damage.
