METHYLATION OF REPEATED DNA-SEQUENCES AND GENOME STABILITY IN ASCOBOLUS-IMMERSUS

In the ascomycete Ascobolus immersus, artificially repeated DNA fragme nts are subject to a process of methylation induced premeiotically (MI P). Artificially repeated genes are inactivated as a consequence of th is methylation. Once established, both methylation and inactivation ar e stably maintained (although they can be reversed) through vegetative as well as sexual reproduction, even after the different copies of th e repeat have segregated from each other. Therefore, MIP constitutes a process of epimutation. The biological significance of MIP remains un known. Two likely hypotheses, which are not mutually exclusive, are th at MIP acts to limit the spread of transposable elements throughout th e genome or that it acts to reduce ectopic recombination between dispe rsed sequences. In this second hypothesis, targets for MIP are also li kely to be mainly transposable elements. For these reasons, we have re cently started a search for such elements in Ascobolus. Results obtain ed so far indicate that several types of transposable elements or remn ants of them are present in Ascobolus. Analysis of their methylation s tatus suggests that they are indeed likely targets of MIP and in one c ase points to a possible strategy that transposons might use to escape MIP, simply by reducing their size.