| RNA silencing-based processes direct biological phenomena in a diverse range of eukaryotes. However, the importance of RNA silencing to fungal biology is unclear. While some fungal species completely lack core RNA silencing proteins, such as Dicer, Argonaute and RNA dependent RNA polymerase (RDRP), others carry one to several versions of these proteins. The evolutionary forces driving this variability are unknown.; This dissertation describes the discovery and characterization of RNA silencing in three Aspergillus fungi and a Fusarium species. When integrated into the genome, inverted repeat transgenes (IRT) correlated with suppression of endogenous target genes in all species examined. The IRT-based silencing phenotype was stable during growth on artificial media and during infection of host plants. Additionally, a single class of 25 nucleotide long RNAs consisting of IRT sequences was detected in A. nidulans IRT-carrying strains.; A. nidulans was found to encode a single Dicer, Argonaute and two RNA dependent RNA polymerases. Deletion of either Dicer or Argonaute eliminated IRT-based RNA silencing, but deletion of the RDRPs had no apparent effect on this process. Additionally, deletion of all four genes had no effect on physiological processes such as radial growth, conidiation, ascosporogenesis and crossing under the conditions tested.; Investigation of the A. nidulans genome also identified a truncated Dicer and Argonaute gene, suggesting that A. nidulans has recently lost some RNA silencing capabilities. The truncated genes are transcribed, spliced and poly-A tailed, but it is not known if they are translated into truncated proteins. Their deletion had no apparent effect on IRT-based RNA silencing, growth, conidiation and ascosporogenesis. Population and polymorphism studies suggest that the truncated genes are fixed within the species and that Dicer truncation preceded Argonaute truncation in evolutionary order.; Although a role for A. nidulans RNA silencing was not detected after experimental deletion of the full-length and truncated RNA silencing genes, the presence of an Aspergillus mycovirus correlated with RNA silencing suppression. This suggests that a major role of Aspergillus RNA silencing is virus defense. The possibility that host-virus interactions are a driving force of fungal RNA silencing gene evolution is discussed. |
