CENH3 morphogenesis reveals dynamic centromere associations during synaptonemal complex formation and the progression through male meiosis in hexaploid wheat.

During meiosis, centromeres in some species undergo a series of associations, but the processes and progression to homologous pairing is still a matter of debate. Here, we aimed to correlate meiotic centromere dynamics and early telomere behaviour to the progression of synapotonemal complex (SC) construction in hexaploid wheat (2n=42) by triple immunolabelling of CENH3 protein marking functional centromeres, and SC proteins ASY1 (unpaired lateral elements) and ZYP1 (central elements in synapsed chromosomes). We show that single or multiple centromere associations formed in meiotic interphase undergo a progressive polarisation (clustering) at the nuclear periphery in early leptotene, leading to formation of the telomere bouquet. Critically, immunolabelling shows the dynamics of these presynaptic centromere associations and a structural reorganisation of the centromeric chromatin coinciding with key events of synapsis initiation from the subtelomeric regions. As short stretches of subtelomeric synapsis emerged at early zygotene, centromere clusters lost their strong polarization, gradually resolving as individual centromeres indicated by more than 21 CENH3 foci associated with unpaired lateral elements. Only following this centromere depolarisation were homologous chromosome arms connected, as observed by the alignment and fusion of interstitial ZYP1 loci elongating at zygotene so synapsis at centromeres is a continuation of the interstitial synapsis. Our results thus reveal that centromere associations are a component of the timing and progression of chromosome synapsis, and the gradual release of the individual centromeres from the clusters correlates with the elongation of interstitial synapsis between the corresponding homologues.