news & views Unifying homology effects Carlo Cogoni University of Rome ‘La Sapienza’, Dipartimento di Biotecnologie Cellulari ed Ematologia, Viale Regina Elena, 324, 00161 Rome, Italy. e-mail:
[email protected] Meiotic silencing by unpaired DNA is a new mechanism, related to other homology-dependent gene silencing phenomena, with implications not only for genome protection against invasive nucleic acids but for genome maintenance and speciation as well. The ability of DNA sequences to pair with A single ancient origin silencing). Based on these characteristics, each other on the basis of sequence homol- Neurospora crassa is haploid during vege- which differ from transvection as it is ogy is well recognized, being the basis for tative growth, whereas in the sexual cycle, observed in Drosophila, meiotic transvec- homologous chromosome pairing during two haploid nuclei of opposite mating tion has been renamed ‘meiotic silencing meiotic prophase. In the past decade, it has type fuse to produce a diploid cell, the by unpaired DNA’ (MSUD). Shiu et al.2 become clear that homology-based interac- zygote. The zygote immediately under- also discovered that MSUD is not tions between nucleic acids, both DNA and goes the two meiotic divisions, followed restricted to a few genes as was previously RNA, can be influential not only in accom- by one mitotic division, to produce hap- believed, but that virtually the entire plishing pairing and recombination during loid ascospores. Thus, the brief period genome is subject to this process. A note- meiosis, but also in regulation of gene after karyogamy and before meiosis is the worthy finding is that a putative RNA- expression at other times in the cell cycle.