Cosuppression is a silencing phenomenon triggered by the introduction of homologous DNA sequences into the genomes of organisms as diverse as plants, fungi, flies, and nematodes. Here we report sex-induced silencing (SIS), which is triggered by tandem integration of a transgene array in the human fungal pathogen Cryptococcus neoformans. A SXI2a-URA5 transgene array was found to be post-transcriptionally silenced during sexual reproduction. More than half of the progeny that inherited the SXI2a-URA5 transgene became uracil-auxotrophic due to silencing of the URA5 gene. In vegetative mitotic growth, silencing of this transgene array occurred at añ 250-fold lower frequency, indicating that silencing is induced during the sexual cycle. Central components of the RNAi pathway-including genes encoding Argonaute, Dicer, and an RNA-dependent RNA polymerase-are all required for both meiotic and mitotic transgene silencing. URA5-derived~22-nucleotide (nt) small RNAs accumulated in the silenced isolates, suggesting that SIS is mediated by RNAi via sequence-specific small RNAs. Through deep sequencing of the small RNA population in C. neoformans, we also identified abundant small RNAs mapping to repetitive transposable elements, and these small RNAs were absent in rdp1 mutant strains. Furthermore, a group of retrotransposons was highly expressed during mating of rdp1 mutant strains, and an increased transposition/mutation rate was detected in their progeny, indicating that the RNAi pathway squelches transposon activity during the sexual cycle. Interestingly, Ago1, Dcr1, Dcr2, and Rdp1 are translationally induced in mating cells, and Ago1, Dcr1, and Dcr2 localize to processing bodies (P bodies), whereas Rdp1 appears to be nuclear, providing mechanistic insights into the elevated silencing efficiency during sexual reproduction. We hypothesize that the SIS RNAi pathway operates to defend the genome during sexual development. Mobile genetic elements populate the genomes of virtually all eukaryotic organisms and challenge the genetic integrity of their hosts, necessitating mechanisms that limit their expansion. Early studies have pointed to links between RNAi and transposon control. For example, in C. elegans, several genes essential to RNAi are also involved in a transposon control pathway (Ketting et al. 1999). Additionally, a surprisingly large family of small RNAs mapping to specific transposable elements have been defined in Arabidopsis, Drosophila, C. elegans, and, recently, the budding yeast Saccharomyces castellii (Llave et al. 2002;Aravin et al. 2003;Drinnenberg et al. 2009). Among these, the piwi-interacting small RNAs (piRNAs) have been shown to bind the piwi proteins of the Argonaute superfamily and are required for silencing transposons in the animal germline (Brennecke et al. 2007). Thus, one conserved function of the RNAi pathway is to defend the genome from invasion by mobile elements.Both PTGS and TGS mechanisms have been implicated in transposon control. piRNA has the capacity to silence transposons solely at a ...

Sexual reproduction enables genetic exchange in eukaryotic organisms as diverse as fungi, animals, plants, and ciliates. Given its ubiquity, sex is thought to have evolved once, possibly concomitant with or shortly after the origin of eukaryotic organisms themselves. The basic principles of sex are conserved, including ploidy changes, the formation of gametes via meiosis, mate recognition, and cell-cell fusion leading to the production of a zygote. Although the basic tenants are shared, sex determination and sexual reproduction occur in myriad forms throughout nature, including outbreeding systems with more than two mating types or sexes, unisexual selfing, and even examples in which organisms switch mating type. As robust and diverse genetic models, fungi provide insights into the molecular nature of sex, sexual specification, and evolution to advance our understanding of sexual reproduction and its impact throughout the eukaryotic tree of life.