Maintenance of a steamlined genome is probably important to a free-living fungus. The period between fertilization and karyogamy in the life cycle of Neurospora and related fungi provides an ideal time for "genome-cleaning". Premeiotic intrachromosomal recombination deletes tandem repeats at high frequency in both homothallic and heterothallic filamentous ascomycetes. This eliminates excess copies of tandemly repeated genes and at the same time favors their homogenization. Heterothallic fungi such as Neurospora also take the bolder steps of mutating and modifying both copies of duplicated sequences, linked or unlinked, by RIP. Because these organisms are outbreeders, and because RIP operates immediately prior to meiosis in cells having nuclei from both parents, the process does not cause much lethality or loss of genetic information. RIP should effectively counter selfish and redundant DNA, and at the same time generate raw material for evolution. In addition, RIP should both prevent chromosomal rearrangements by causing divergence of dispersed repeated sequences and rid a population of duplication-generating rearrangements. Thus, this form of genetic instability potentially stabilizes the gross organization of the genome.