Two distinct modes of meiotic chromosome synapsis

The pairwise alignment of homologous chromosomes within the synaptonemal complex (SC) is important for meiotic crossover recombination and fertility ^1–3 . However, chromosomes do not need sequence homology to synapse, with meiotic recombination defects often leading to synapsis of non-homologous chromosome segments ^2,4,5 . Here we show that such heterologous synapsis reflects a distinctly regulated mode of meiotic chromosome synapsis that also happens during the early stages of wild-type yeast meiosis and occurs in parallel to the well-known synapsis initiation at crossover-designated sites ^3,5,6 . Heterologous synapsis initiates along chromosome arms after double-strand break resection and is accompanied by canonical markers of crossover repair, but does not need recombinase-dependent strand invasion. Instead, it requires the DNA-damage sensor kinase ATR/Mec1, which phosphorylates of a specific amino acid in Zip1, the major transverse filament protein of the SC. Phospho-mimetic mutants in ZIP1 rescue the synapsis defect of mec1 mutants and also partially restore gamete viability, indicating that this particular MEC1 function is important for the faithful completion of meiosis. Importantly, crossover repair quickly rectifies heterologous synapsis, allowing successful completion of meiosis even when most of the genome initially synapses independently of homology. Our data identify meiotic chromosome synapsis as a dynamic and reversible process that becomes optimized as result of recombination-dependent chromosome pairing.