De novo methylation of repeated sequences in Coprinus cinereus.

We have examined the stability of duplicated DNA sequences in the sexual phase of the life cycle of the basidiomycete fungus, Coprinus cinereus. We observed premeiotic de novo methylation in haploid nuclei containing either a triplication, a tandem duplication, or an ectopic duplication. Methylation changes were not observed in unique sequences. Repeated sequences underwent methylation changes during the dikaryotic stage. In one cross, 27% of the segregants exhibited methylation-directed gene inactivation. However, all auxotrophs eventually reverted to prototrophy. C to T transition mutation were not observed in this study. Our studies also revealed one inversion that occurred in 50% of the segregants in a single triplication cross, and a single pop-out event that occurred during vegetative growth. These alterations were similar to changes reported in experiments with duplicated sequences in Neurospora crassa and Ascobolus immersus. However, significant differences were also noted. First, the extent of methylation was much less in C. cinereus than in the other two fungi. Second, CpG sequences appeared to be the preferred targets of methylation.