Roles of recombination in chromosome pairing and synapsis during meiotic prophase I in Arabidopsis.

<p>In meiotic interphase and leptotene, chromosomes are decondensed and centromeres are dispersed in the nucleus (a). Centromeres cluster in early zygotene (b), separate and couple non-homologously (c), until homologous centromere pairing is established (d), following which arms pair and full synapsis is established. The transition from centromere coupling to homologous centromere pairing requires initiation of recombination by SPO11 and DMC1. In absence of these proteins only coupling of centromeres of non-homologous chromosomes will occur leading to the formation of univalents at metaphase I. Once homologous centromere pairing has been established, pairing and synapsis along chromosome arms will proceed with the help of RAD51 and its paralogues RAD51C and XRCC3 (e). Arabidopsis telomeres have been shown to pair and cluster at the nucleolus during early meiosis (see <a href="http://www.plosgenetics.org/article/info:doi/10.1371/journal.pgen.1002636#pgen.1002636-Roberts1" target="_blank">[64]</a>). However, their behaviour in these recombination mutants is not known and telomere associations have not been included in the diagram for clarity.