Methylation of repeated DNA sequences and genome stability inAscobolus immersus

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Abstract
In the ascomycete Ascobolus immersus, artificially repeated DNA fragments are subject to a process of methylation induced premeiotically (MIP). Artificially repeated genes are inactivated as a consequence of this methylation. Once established, both methylation and inactivation are stably maintained (although they can be reversed) through vegetative as well as sexual reproduction, even after the different copies of the repeat have segregated from each other. Therefore, MIP constitutes a process of epimutation. The biological significance of MIP remains unknown. Two likely hypotheses, which are not mutually exclusive, are that MIP acts to limit the spread of transposable elements throughout the genome or that it acts to reduce ectopic recombination between dispersed sequences. In this second hypothesis, targets for MIP are also likely to be mainly transposable elements. For these reasons, we have recently started a search for such elements in Ascobolus. Results obtained so far indicate that several types of transposable elements or remnants of them are present in Ascobolus. Analysis of their methylation status suggests that they are indeed likely targets of MIP and in one case points to a possible strategy that transposons might use to escape MIP, simply by reducing their size. Key words: DNA repeats, methylation, genome stability, Ascobolus immersus.
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