Evolution of Silencing at the Mating-Type Loci in Hemiascomycetes

This chapter explores how a unique silencing mechanism evolved in the hemiascomycete class of fungi, which includes the model yeast Saccharomyces cerevisiae. It reviews what is known about silencing in species related to S. cerevisiae and explores the origins and development of the unique Sirmediated silencing mechanism. The phylogenetic relationships of the species discussed in this chapter are illustrated. The whole-genome duplication played an important role in the development of Sirmediated silencing. For clarity, in the remainder of this chapter, gene and protein names are preceded by the initials of the genus and species of the host organism. The chapter also explores the evolution of the Sir proteins and then examines the changing nature of the genomic regions most likely to be silenced: silent mating-type cassettes, telomeres, and centromeres. Beyond the hemiascomycetes, SpSir2p contributes to classical silencing at centromeres, telomeres, and mating-type loci in S. pombe, where it deacetylates H3-K9, thereby promoting methylation of this lysine and the association of SpSwi6. An interesting example of the evolution of Sir2p is the emergence of a new paralog, Hst1p, which arose in the whole-genome duplication. An examination of the distribution of silencing proteins in the hemiascomycete class of fungi, coupled with experimental studies of silencing in a few species, suggests the following speculative model for the evolution of Sirmediated silencing.