Dual effect of the wheat Ph1 locus on chromosome synapsis and crossover

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Allopolyploids must possess a mechanism for fa- cilitating synapsis and crossover (CO) between homologues, in preference to homoeologues (related chromosomes), to en- sure successful meiosis. In hexaploid wheat, the Ph1 locus has a major effect on the control of these processes. Studying a wheat mutant lacking Ph1 provides an opportunity to explore the underlying mechanisms. Recently, it was proposed that Ph1 stabilises wheat during meiosis, both by promoting ho- mologue synapsis during early meiosis and preventing MLH1 sites on synapsed homoeologues from becoming COs later in meiosis. Here, we explore these two effects and demonstrate firstly that whether or not Ph1 is present, synapsis between homoeologues does not take place during the telomere bou- quet stage, with only homologous synapsis taking place dur- ing this stage. Furthermore, in wheat lacking Ph1, overall synapsis is delayed with respect to the telomere bouquet, with more synapsis occurring after the bouquet stage, when homoeologous synapsis is also possible. Secondly, we show that in the absence of Ph1, we can increase the number of MLH1 sites progressing to COs by altering environmental growing conditions; we show that higher nutrient levels in the soil or lower temperatures increase the level of both ho- mologue and homoeologue COs. These observations suggest opportunities to improve the exploitation of the Ph1 wheat mutant in breeding programmes.