Mus81 nuclease and Sgs1 helicase are essential for meiotic recombination in a protist lacking a synaptonemal complex.

Mus81 resolvase and Sgs1 helicase have well-established roles in mitotic DNA repair. Moreover, Mus81 is part of a minor crossover (CO) pathway in the meiosis of budding yeast, plants and vertebrates. The major pathway depends on meiosis-specific synaptonemal complex (SC) formation, ZMM proteins and the MutLγ complex for CO-directed resolution of joint molecule (JM)-recombination intermediates. Sgs1 has also been implicated in this pathway, although it may mainly promote the non-CO outcome of meiotic repair. We show in Tetrahymena, that homologous chromosomes fail to separate and JMs accumulate in the absence of Mus81 or Sgs1, whereas deletion of the MutLγ-component Mlh1 does not affect meiotic divisions. Thus, our results are consistent with Mus81 being part of an essential, if not the predominant, CO pathway in Tetrahymena. Sgs1 may exert functions similar to those in other eukaryotes. However, we propose an additional role in supporting homologous CO formation by promoting homologous over intersister interactions. Tetrahymena shares the predominance of the Mus81 CO pathway with the fission yeast. We propose that in these two organisms, which independently lost the SC during evolution, the basal set of mitotic repair proteins is sufficient for executing meiotic recombination.