SWI1 is required for meiotic chromosome remodeling events

The Arabidopsis dsy10 mutant was previously identified as being defective in the synapsis of meiotic chromosomes resulting in male and female sterility. We report here the molecular analysis of the mutation and show that it represents a T-DNA insertion in the third exon of the SWI1 gene. Four mutations have now been identified in SWI1, several of which exhibit different phenotypes. For example, the swi1-1 and dyad mutations only affect meiosis in megasporocytes, while the swi1-2 and dsy10 mutations block both male and female meiosis. Furthermore, as part of a detailed cytological characterization of dsy10 meiocytes, we identified several differences during male meiosis between the swi1-2 and dys10 mutants, including variations in the formation of axial elements, the distribution of cohesin proteins and the timing of the premature loss of sister chromatid cohesion. We demonstrate that dsy10 represents a complete loss-of-function mutation, while a truncated form of SWI1 is expressed during meiosis in swi1-2 plants. We further show that dys10 meiocytes exhibit alterations in modified histone patterns, including acetylated histone H3 and dimethylated histone H3-Lysine 4.