Methylation is not the main force repressing the retrotransposon MAGGY in Magnaporthe grisea

We: have introduced the LTR-retrotransposon MAGGY into a naive genome of Ma gnaporthe grisea and estimated the copy number of MAGGY in a cell by serial isolation of fungal protoplasts at certain time intervals. The number of M AGGY elements rapidly-increased for a short period following introduction. However, it did not increase geometrically and reached equilibrium at 20-30 copies per genome, indicating that MAGGY was repressed or silenced during proliferation, De novo methylation of MAGGY occurred immediately following invasion into the genome but the degree of methylation was constant and did not correlate with the repression of MABGGY. 5-Azacytidine treatment demet hylated and transcriptionally activated the MAGGY element in regenerants bu t did not affect transpositional frequency, suggesting that post-transcript ional suppression, not methylation, is the main force that represses MAGGY proliferation in M.grisea, Support for this conclusion was also obtained by examining the methylation status of MAGGY sequences in field isolates of M .grisea with active or inactive MAGGY elements. Methylation of the MAGGY se quences was detected in some isolates but not in others, However, the methy lation status did not correlate with the copy numbers and activity of the e lements.