Interchromosomal transfer of epigenetic states in Ascobolus: transfer of DNA methylation is mechanistically related to homologous recombination.

The transfer of methylation between alleles represents a plausible epigenetic mutational mechanism to explain loss of imprinting in mammals and paramutation in plants. Here, we have exploited advantages unique to the fungus Ascobolus immersus to obtain direct experimental evidence that methylation transfer can occur between homologous chromosomes. A methylated allele and an unmethylated allele of the Ascobolus b2 spore color gene were brought together in individual meiotic cells. Frequent transfer of methylation to the unmethylated allele was observed. This transfer was polarized 5' to 3' along the b2 gene, as is gene conversion, and always accompanied the latter process when tested in the same cross. These and other observations strongly suggest that methylation transfer and recombination are mechanistically related.