Transposable Elements and Repeat-Induced Point Mutation

This chapter concentrates on the impact of repeat-induced point mutation (RIP) on transposable elements (TEs). It is clear that RIP does not follow an identical pattern in all fungi: whereas RIP in Neurospora crassa is intense enough to reduce the C+G content of the most affected elements to below 30% and widespread enough for unmutated TEs to be absent from the sequenced genome, in Magnaporthe grisea and Podospora anserina RIP is described as "light" or "mild", and in P. anserina it was observed only in late-maturing ascospores. The genomes of Neurospora and other filamentous fungi show a large variety of sequences homologous to TEs found in other organisms. The distribution of TEs in fungal genomes suggests that there are a limited number of innocuous genomic locations. A striking feature of some of the TE sequences is illustrated. Like other genomic components, TEs provide a record of the evolutionary processes to which they were subjected, but unlike most genomic components, their turnover is rapid and the record they leave is largely one of decay, including defeat by the defense mechanisms of the host. RIP is the most potent genome defense system known in eukaryotes. It is perhaps surprising that RIP is apparently unique to filamentous ascomycetes, but genome defense mechanisms appear to be unusually labile in evolution perhaps because they have to be retailored to meet each new emergency encountered by the host.