Licensing MLH1 sites for crossover during meiosis

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During meiosis, homologues synapse and recombine at sites marked by the binding of the mismatch repair protein MLH1, with each site leading to a recombination event or crossover. In hexaploid wheat, the Ph1 locus has a major effect on whether crossover occurs between homologues, or between related homoeologues. Here we report that, surprisingly, in wheat-rye hybrids where homologues are absent, neither the level of synapsis, nor the number of MLH1 sites is affected by Ph1.  Thus in the case of wheat-wild relative hybrids, Ph1 must affect whether MLH1 sites are able to progress to crossover. The level of synapsis also implies that Ph1 promotes homologue pairing, rather than suppressing homoeologue pairing in wheat itself. Therefore Ph1 stabilises wheat as a polyploid during meiosis, by both promoting homologue pairing and preventing MLH1 sites on paired homoeologues from becoming crossovers