A Critical Component of Meiotic Drive in Neurospora Is Located Near a Chromosome Rearrangement

Abstract

Neurospora fungi harbor a group of meiotic drive elements known as Spore killers (Sk). Spore killer-2 (Sk-2) and Spore killer-3 (Sk-3) are two Sk elements that map to a region of suppressed recombination. Although this recombination block is limited to crosses between Sk and Sk-sensitive (Sk S ) strains, its existence has hindered Sk characterization. Here we report the circumvention of this obstacle by combining a classical genetic screen with next-generation sequencing technology and three-point crossing assays. This approach has allowed us to identify a novel locus called rfk-1, mutation of which disrupts spore killing by Sk-2. We have mapped rfk-1 to a 45-kb region near the right border of the Sk-2 element, a location that also harbors an 11-kb insertion (Sk-2 INS1 ) and part of a .220-kb inversion (Sk-2 INV1 ). These are the first two chromosome rearrangements to be formally identified in a Neurospora Sk element, providing evidence that they are at least partially responsible for Sk-based recombination suppression. Additionally, the proximity of these chromosome rearrangements to rfk-1 (a critical component of the spore-killing mechanism) suggests that they have played a key role in the evolution of meiotic drive in Neurospora.G ENOMES typically consist of large groups of cooperating genes that encode information needed to maintain and reproduce themselves as well as their cellular environments. However, individual genes within genomes do not always cooperate (Dawkins 2006;Burt and Trivers 2008). For example, ranGAP in fruit flies and tcd1 in mice are selfish genes that collude with others to bias meiotic transmission ratios to their advantage (Kusano et al. 2003;Lyon 2003). While ranGAP works within a gene complex known as Segregation Distorter, tcd1 works within another called the t haplotype. Because these selfish gene complexes "drive" through meiosis, they are referred to as meiotic drive elements. Such elements are found throughout sexual organisms, with other well-known examples being X d in stalkeyed flies, Ab10 in maize, and the Spore killers in various fungi (Rhoades 1942;Raju 1994;Presgraves et al. 1997;Burt and Trivers 2008).Neurospora fungi spend most of their life cycle in a vegetative haploid phase. However, specific environmental cues (e.g., nitrogen limitation) can trigger initiation of the sexual cycle, which includes a brief diploid phase that allows meiotic drive to function in this group of fungi. Sexual reproduction in Neurospora begins when an asexual spore (conidium) from a male parent donates a nucleus to an immature fruiting body (protoperithecium) of a female parent. Pairs of nuclei, one from each parent, fuse within the meiotic cells (asci) of the fertilized fruiting body (perithecium) to form a diploid nucleus that passes through the standard stages of meiosis (Raju 1980). The four haploid meiotic products then undergo a postmeiotic mitosis to produce eight nuclei, each of which is incorporated into an American football-shaped ascospore (Raju 1980). Ascospores darken as...

Identification of a genetic element required for spore killing in Neurospora

“…The shortest abortion-inducing interval identified by the above experiments is AH36 , located between positions 27,899 and 29,381 of the 45 kb rfk-1 region (Figure 2C and Table 2). Because the research path that led us to AH36 began with mapping the position of rfk-1 in strain ISU-3211 (Harvey et al 2014), AH36 in ISU-3211 (referred to as AH36 3211 ) should harbor at least one mutation that disrupts rfk-1 function. To test this hypothesis, we transferred AH36 3211 to an Sk S genetic background and crossed the resulting strain with an Sk S sad-2 Δ mating partner.…”

“…However, for a few isolates of N. intermedia , asci with atypical phenotypes were due to chromosomal factors called Spore killer-2 ( Sk-2 ) and Spore killer-3 ( Sk-3 ). Sk-2 and Sk-3 are not single genes; rather, they are complexes of genes that span approximately 30 cM of chromosome III, and they are transmitted through meiosis as single units due to a recombination suppression mechanism thought to be enforced by inversions (Turner and Perkins 1979; Campbell and Turner 1987; Hammond et al 2012; Harvey et al 2014). Unlike standard genetic elements, which display a Mendelian transmission rate of 50% through sexual reproduction, Sk-2 and Sk-3 are transmitted at levels approaching 100% (Turner and Perkins 1979).…”

“…As described above, Sk S × Sk-2 rsk Δ Sk-2 crosses produce abortive asci. We recently used this characteristic to screen for mutations that disrupt spore killing (Harvey et al 2014). Specifically, we fertilized an Sk S mating partner with mutagenized Sk-2 rsk Δ Sk-2 conidia (asexual spores that also function as fertilizing propagules).…”

A novel meiotic drive in Neurospora crosses heterozygous for hybrid translocation strains disfavors homokaryotic progeny derived from alternate, but not adjacent-1 segregation

“…A similar model was proposed to explain the production of more aneuploid or diploid progeny by hybrid S. kambucha (Sk) / Schizosaccharomyces pombe (Sp) diploids than even the pure haploid Sk and Sp parental genotypes (Zanders et al, 2014). Spore killer elements in Neurospora and Podospora (viz.,Spok1 and Spok2) trigger inviability in ascospores not inheriting them (Turner and Perkins, 1979;Hammond et al, 2012;Grognet et al, 2014;Harvey et al, 2014). This model requires that MDE be correctly positioned in T(IBj5) Nt and T(B362i) Nt .…”

A mus-51 RIP allele for transformation of Neurospora crassa

“…BDS medium with hygromycin (200 μg/ml) and/or cyclosporin A (5 μg/ml) was used to screen for hygromycin and cyclosporin A resistance. Top and Bottom Agar were used in transformation experiments as previously described (Harvey et al ., 2014). The key strains used in this study are listed in Table 1.…”