Meiotic silencing and the epigenetics of sex

Abstract

The sensing of accurate homologous recognition and pairing between discreet chromosomal regions and/or entire chromosomes entering meiosis is an essential step in ensuring correct alignment for recombination. A component of this is the recognition of heterology, which is required to prevent recombination at ectopic sites and between non-homologous chromosomes. It has been observed that a number of diverged organisms add an additional layer to this process: regions or chromosomes without a homologous counterpart are targeted for silencing during meiotic prophase I. This phenomenon was originally described in filamentous fungi, but has since been observed in nematodes and mammals. In this review we will generally group these phenomena under the title of meiotic silencing, and describe what is known about the process in the organisms in which it is observed. We will additionally propose that the functions of meiotic silencing originate in genome defense, and discuss its potential contributions to genome evolution and speciation.

Neurospora crassa, a Model System for Epigenetics Research

“…Consistent with this possibility, the vast majority of relics of RIPare recognizably similar to transposons known from other organisms, and most strains of Neurospora lack active transposons (Galagan et al 2003;Selker et al 2003;Galagan and Selker 2004 (Nolan et al 2005). Information about the action of meiotic silencing suggests that this process will silence any transposed sequence in meiotic cells, even if it is only present as a single copy in the genome (Shiu et al 2001;Kelly and Aramayo 2007). In addition to dealing with errant transposons in meiosis, some of the genes involved in meiotic silencing also appear to play an important role in the process of speciation, as shown by the observation that mutants defective in meiotic silencing relieve the sterility of strains bearing large duplications of chromosome segments and allow closely related species to mate with N. crassa (Shiu et al 2001).…”

“…The most recent addition to the list of known silencing mechanisms is meiotic silencing, which was originally called "meiotic transvection" and later referred to as "meiotic silencing by unpaired DNA" (MSUD; Shiu et al 2001;Kelly and Aramayo 2007). As implied by its name, meiotic silencing operates only in meiosis, in which it evaluates the identity of the homologous chromosomes in two stages.…”

Molecular dissection of Neurospora Spore killer meiotic drive elements

“…MSUD, a process that silences expression from unpaired genes during meiosis (13)(14)(15), requires common RNAi proteins, such as an RNA-directed RNA polymerase, a Dicer, and an Argonaute slicer (16)(17)(18). The MSUD suppressors found in Sk-2 and Sk-3 are not as strong as the classic sad-1 Δ and sad-2 Δ suppressors, and they do not suppress the silencing of all unpaired loci (12).…”

Studying gene function in Caenorhabditis elegans using RNA-mediated interference

“…MiRNAs are encoded by cellular genes, production of mature miRNA relies on Dicer, and the interaction of an miRNA with its target mRNA involves an Argonaute protein. RNAi-related mechanisms that rely on endogenous siRNAs have been implicated in transposon silencing [10,11] and defence against infection by RNA viruses in a variety of organisms [1,6], and in meiotic silencing of unpaired DNA in Neurospora crassa [12].…”