Sex and the single gene: Meiotic silencing by unpaired DNA

In the fungus Neurospora crassa, crosses made between normal sequence strains and some translocation strains can yield three kinds of progeny: normal sequence (like one parent), translocation sequence (like the other parent) and a novel class that is duplicated for the translocated segment. The duplication progeny arise when one component of the translocation segregates with a normal sequence chromosome; deletion progeny resulting from segregation of the complementary component are inviable (see figure 1). Regardless of the particular genomic segment duplicated (i.e. regardless of the translocation strain used in the parental cross), duplication strains in general show a characteristic barren phenotype when they, in turn, are crossed with non-duplication strains. Barren crosses produce plenty of perithecia (fruiting bodies) but yield only a few exceptional ascospores (haploid products of meiosis). A series of remarkable studies by Robert Metzenberg and colleagues (in particular, Rodolfo Aramayo and Patrick Shiu) has now revealed that the barren phenotype is caused by a newly discovered gene-silencing process called meiotic silencing by unpaired DNA (MSUD). MSUD occurs during meiosis and silences any gene that is unpaired and all genes homologous to it, including those that may themselves be paired. Large duplications can span many genes, including some that code for products required for meiosis and ascospore development. In crosses heterozygous for such duplications one copy of each such gene is unpaired so the silencing of all three copies by MSUD renders the