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ABSTRACT ZMM proteins have been defined in budding yeast as factors that are collectively involved in the formation of interfering crossovers (COs) and synaptonemal complexes (SCs), and they are a hallmark of the predominant meiotic recombination pathway of most organisms. In addition to this so-called class I CO pathway, a minority of crossovers are formed by a class II pathway, which Involves the Mus81-Mms4 endonuclease complex. This Is the only CO pathway In the SC-less meiosis of the fission yeast. ZMM proteins (including SC components) were always found to be co-occurring and hence have been regarded as functionally linked. Like the fission yeast, the protist Tetrahymena thermophila does not possess a SC, and Its COs are dependent on Mus81-Mms4. Here we show that the ZMM proteins Msh4 and Msh5 are required for normal chiasma formation, and we propose that they have a pro-CO function outside a canonical class I pathway in Tetrahymena. Thus, the two-pathway model is not tenable as a general rule.
MEIOSIS is a specialized cell division by which the diploid somatic chromosome set is halved prior to the production of gametes. This reduction is achieved by the separation of homologous chromosomes. To ensure faithful segregation, homologous chromosomes must first identify each other and become stably linked. This is achieved by physical connections between homologs due to the invasion and exchange of homologous DNA strands (see Ehmsen and Heyer 2008). An invading single strand forms a heteroduplex with the complementary strand of its target double-stranded DNA (dsDNA). Subsequent strand ligation leads to a more stable intermediate, a Holliday junction (HJ). A possible outcome of HJ formation are crossovers (COs), which mature into chiasmata and hold homologs together and, at the same time, are the basis of genetic recombination. However, a subset of strand invasion events, ranging from less than half in budding yeast (Mancera et al. 2008; Qi et al. 2009) to a considerable excess in Drosophila (Comeron et al. 2012; Miller et al. 2012), Arabidopsis (Yang et al. 2012), and humans (Wang et al. 2012), result in noncrossovers (NCOs), which are tractable as genetic or sequence conversion events.
COs and NCOs are initiated by DNA double-strand breaks (DSBs) generated by the meiosis-specific nuclease Spoil (see Keeney 2001; de Massy 2013), and...