Neurospora Spore KillersSk-2andSk-3Suppress Meiotic Silencing by Unpaired DNA

Abstract

In Neurospora crassa, pairing of homologous DNA segments is monitored during meiotic prophase I. Any genes not paired with a homolog, as well as any paired homologs of that gene, are silenced during the sexual phase by a mechanism known as meiotic silencing by unpaired DNA (MSUD). Two genes required for MSUD have been described previously: sad-1 (suppressor of ascus dominance), encoding an RNAdirected RNA polymerase, and sad-2, encoding a protein that controls the perinuclear localization of SAD-1. Inactivation of either sad-1 or sad-2 suppresses MSUD. We have now shown that MSUD is also suppressed by either of two Spore killer strains, Sk-2 and Sk-3. These were both known to contain a haplotype segment that behaves as a meiotic drive element in heterozygous crosses of killer 3 sensitive. Progeny ascospores not carrying the killer element fail to mature and are inviable. Crosses homozygous for either of the killer haplotypes suppress MSUD even though ascospores are not killed. The killer activity maps to the same 30-unit-long region within which recombination is suppressed in killer 3 sensitive crosses. We suggest that the region contains a suppressor of MSUD. S EXUAL reproduction in Neurospora crassa is mediated by the fusion of two haploid nuclei that carry compatible mating-type genes (mat A and mat a). The haploid nuclei proliferate in specialized premeiotic ascogenous tissue (precroziers and three-celled croziers) within the fertilized fruiting body (perithecium). Two nuclei of opposite mating type that are sequestered in the subapical cell of the crozier then migrate into a tubular sac (ascus) and fuse to create the zygote, which immediately undergoes two meiotic divisions and a postmeiotic mitosis. The resulting eight haploid nuclei reside in the same cytoplasm until they are sequestered into eight linearly ordered ascospores. The mating-type locus and other segregating markers show 1:1 segregation (Raju 1992;Davis 2000).Meiotic drive is a phenomenon in which certain chromosomal loci show a progeny ratio different from 1:1 and one allele is favored over the other. Examples of meiotic drive in animals include the segregation distorter (SD) system of Drosophila melanogaster and the t haplotype in the mouse (Lyttle 1991a,b;Schimenti 2000;Kusano et al. 2003). In heterozygous males, the meiotic products that do not carry the drive element (SD chromosome or t haplotype) fail to differentiate into functional sperm. Although meiotic drive is widespread in insects, mammals, plants, and fungi, very few cases have been dissected at the molecular level (Pennisi 2003;Burt and Trivers 2006).In Neurospora, Spore killers are chromosomal elements that cause the death of ascospores that do not contain the killer element (Turner and Perkins 1979, 1991). For example, in a cross of Spore killer 3 wild type (Sk sensitive), every mature ascus contains four normalsized, black, viable ascospores and four tiny, undeveloped ascospores that are inviable (see Figure 2A). All survivors carry the killer element. When both kill...

Perithecium morphogenesis in Sordaria macrospora

“…In N. crassa, between 200 and 400 asci develop from the ascogenous hyphae in each perithecium (Raju, 1986(Raju, , 2008Raju et al, 2007). The formation of each ascus involves fusion of two dikaryotic nuclei within an ascus mother cell (Raju, 1987), followed by meiosis of the resulting diploid nucleus inside the young ascus.…”

Relationship between transmission ratio distortion and genetic divergence in intraspecific rice crosses

“…In both systems, the "drivers" (i.e., preferentially transmitted alleles) are transmitted predominantly to the progeny of heterozygous males by inducing dysfunction of the non-carrier sperm cells and are maintained owing to this "selWsh" behavior, despite the fact that there is reduced male fertility (»50% fertility), and in the t complex system, even Wtness reduction in homozygous genotypes (t/t) (Hartl and Clark 1997). A particular knobbed chromosome in maize referred to as abnormal chromosome 10 (Ab10) which is preferentially transmitted through female meiosis and represents a case of true meiotic drive (Rhoades 1942;Buckler IV et al 1999), and Spore killer in Neurospora (Turner and Perkins 1979;Raju et al 2007) have been extensively studied as parallel examples of meiotic drive.…”

Carrefour Mme. Gras: A wild-isolated Neurospora crassa strain that suppresses meiotic silencing by unpaired DNA and uncovers a novel ascospore stability defect

“…Both Sk-2 and Sk-3 map to a 30 map unit region that spans the centromere of linkage group (LG) III in which recombination is blocked in the Sk-heterozygous crosses. Although we do not know how Sk-2 and Sk-3 suppress meiotic silencing, both elements were shown to increase the productivity of Dp-heterozygous crosses (Raju et al, 2007).…”

“…The other, called meiotic silencing by unpaired DNA (or, simply, meiotic silencing), occurs subsequently, in the stage following karyogamy and lasts until the eight haploid nuclei generated by meiosis and a post-meiotic mitosis become sequestered into the eight ascospores of the developing ascus. RIP induces G:C to A:T hypermutation and methylation of cytosines in DNA sequences that are duplicated in an otherwise haploid nucleus (Cambareri et al, 1989;Selker, 1990;Galagan and Selker, 2004), whereas, meiotic silencing is presumed to be an RNAi-based elimination of transcripts of any gene not properly paired with a homolog in meiosis, thereby silencing it, and genes homologous to it, regardless of whether the latter were themselves paired (Aramayo and Metzenberg, 1996;Shiu et al, 2001Shiu et al, , 2006Raju et al, 2007). RIP and meiotic silencing are believed to have contributed to maintaining the Neurospora genome free of transposons and other repeated DNA (Galagan et al, 2003).…”

“…A decrease in the level of any of these proteins might cause an overall lowering of meiotic silencing efficiency, thereby alleviating the silencing of duplication-borne genes, and thus increase the productivity of the duplication-heterozygous cross. The Spore killer meiotic drive elements Sk-2 and Sk-3 also are dominant suppressors of meiotic silencing (Raju et al, 2007). In a heterozygous Sk  wild type cross, the Sk element causes the death of ascospores that do not inherit it; all mature asci contain four normal sized, viable black ascospores and four tiny unpigmented and undeveloped inviable ascospores; the viable ascospores being the ones that inherited the killer element.…”