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 pattes.including acetylated histone H3 and dimethylated histone H3-Lysine 4.