SWITCH1 (SWI1): a novel protein required for the establishment of sister chromatid cohesion and for bivalent formation at meiosis

We have characterized a new gene, SWI1, involved in sister chromatid cohesi on during both male and female meiosis in Arabidopsis thaliana. A first all ele, swi1.1, was obtained as a T-DNA tagged mutant and was described previo usly as abnormal exclusively in female meiosis. We have isolated a new alle le, swi1.2, which is defective for both male and female meiosis. In swi1.2 male meiosis, the classical steps of prophase were not observed, especially because homologs do not synapse. Chromatid arms and centromeres lost their cohesion in a stepwise manner before metaphase I, and 20 chromatids instea d of five bivalents were seen at the metaphase plate, which was followed by an aberrant segregation. In contrast, swi1.2 female meiocytes performed a mitotic-like division instead of meiosis, indicating a distinct role for SW I1 or a different effect of the loss of SWI1 function in both processes. Th e SWI1 gene was cloned; the putative SWI1 protein did not show strong simil arity to any known protein. Plants transformed with a SWI1-GFP fusion indic ated that SWI1 protein is present in meiocyte nuclei, before meiosis and at a very early stage of prophase. Thus, SWI1 appears to be a novel protein i nvolved in chromatid cohesion establishment and in chromosome structure dur ing meiosis, but with clear differences between male and female meiosis.