Standing guard: Perinuclear localization of an RNA-dependent RNA polymerase
- Department of Biology, Emory University, 1510 Clifton Road, Atlanta, GA 30322
Sexual reproduction has both potential benefits and potential hazards for the fitness of a genome. The genetic shuffling that accompanies meiosis is recognized as a way of testing genetic variation against environmental changes to select for evolutionary fitness. However, sexual reproduction can also assist the spread of infectious genetic parasites by intermingling two genomes. Meiosis-specific defense mechanisms have arisen to combat this hazard, and this type of defense is strikingly evident in the filamentous fungus Neurospora crassa. A recent study by Shiu et al. (1) in this issue of PNAS sheds new light on a meiotic process that targets transcripts from unpaired regions of chromosomes for destruction during meiosis.
The maintenance of a relatively stable genome is paramount to the survival of a species. Transposable genetic elements (TEs) can have immediate and drastic impacts on the structure of a genome through insertional mutagenesis and defective repair of double-stranded breaks. Given the potential danger of these elements, extensive invasion of genomes seems to have been tolerated surprisingly well in most species. To wit, 45% of the sequence of the human genome is comprised of TEs or their degenerated remnants. The situation in plants can be even more extreme, with TEs providing as much as 80% of the genomic content in some species (2, 3).
The presence of TEs in all eukaryotic genomes indicates an ancient origin and continuous maintenance during evolution. Their ubiquity and extensive contribution to genome structure have led many to propose that genome evolution and organism complexity have been driven by both the elements themselves and attempts by host genomes to tame or repress them (2, 4). Indeed, many of the mechanisms that regulate transcriptional programs in eukaryotic development also target and repress TE proliferation. These include DNA methylation, repressive chromatin assembly, and RNA-mediated …
*E-mail: bkelly{at}emory.edu
