A Meiotic Drive Element in the Maize Pathogen Fusarium verticillioides Is Located Within a 102 kb Region of Chromosome V.
ABSTRACT: Fusarium verticillioides is an agriculturally important fungus because of its association with maize and its propensity to contaminate grain with toxic compounds. Some isolates of the fungus harbor a meiotic drive element known as Spore killer (Sk(K)) that causes nearly all surviving meiotic progeny from an Sk(K) × Spore killer-susceptible (Sk(S)) cross to inherit the Sk(K) allele. Sk(K) has been mapped to chromosome V but the genetic element responsible for meiotic drive has yet to be identified. In this study, we used cleaved amplified polymorphic sequence markers to genotype individual progeny from an Sk(K) × Sk(S) mapping population. We also sequenced the genomes of three progeny from the mapping population to determine their single nucleotide polymorphisms. These techniques allowed us to refine the location of Sk(K) to a contiguous 102 kb interval of chromosome V, herein referred to as the Sk region. Relative to Sk(S) genotypes, Sk(K) genotypes have one extra gene within this region for a total of 42 genes. The additional gene in Sk(K) genotypes, herein named SKC1 for Spore Killer Candidate 1, is the most highly expressed gene from the Sk region during early stages of sexual development. The Sk region also has three hyper-variable regions, the longest of which includes SKC1 The possibility that SKC1, or another gene from the Sk region, is an essential component of meiotic drive and spore killing is discussed.
Project description:Meiotic drive is a non-Mendelian inheritance phenomenon in which certain selfish genetic elements skew sexual transmission in their own favor. In some cases, progeny or gametes carrying a meiotic drive element can survive preferentially because it causes the death or malfunctioning of those that do not carry it. In Neurospora, meiotic drive can be observed in fungal spore killing. In a cross of Spore killer (Sk) × WT (Sk-sensitive), the ascospores containing the Spore killer allele survive, whereas the ones with the sensitive allele degenerate. Sk-2 and Sk-3 are the most studied meiotic drive elements in Neurospora, and they each theoretically contain two essential components: a killer element and a resistance gene. Here we report the identification and characterization of the Sk resistance gene, rsk (resistant to Spore killer). rsk seems to be a fungal-specific gene, and its deletion in a killer strain leads to self-killing. Sk-2, Sk-3, and naturally resistant isolates all use rsk for resistance. In each killer system, rsk sequences from an Sk strain and a resistant isolate are highly similar, suggesting that they share the same origin. Sk-2, Sk-3, and sensitive rsk alleles differ from each other by their unique indel patterns. Contrary to long-held belief, the killer targets not only late but also early ascospore development. The WT RSK protein is dispensable for ascospore production and is not a target of the spore-killing mechanism. Rather, a resistant version of RSK likely neutralizes the killer element and prevents it from interfering with ascospore development.
Project description: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.
Project description:Sk-2 is a meiotic drive element that was discovered in wild populations of Neurospora fungi over 40 years ago. While early studies quickly determined that Sk-2 transmits itself through sexual reproduction in a biased manner via spore killing, the genetic factors responsible for this phenomenon have remained mostly unknown. Here, we identify and characterize rfk-1, a gene required for Sk-2-based spore killing. The rfk-1 gene contains four exons, three introns, and two stop codons, the first of which undergoes RNA editing to a tryptophan codon during sexual development. Translation of an unedited rfk-1 transcript in vegetative tissue is expected to produce a 102-amino acid protein, whereas translation of an edited rfk-1 transcript in sexual tissue is expected to produce a protein with 130 amino acids. These findings indicate that unedited and edited rfk-1 transcripts exist and that these transcripts could have different roles with respect to the mechanism of meiotic drive by spore killing. Regardless of RNA editing, spore killing only succeeds if rfk-1 transcripts avoid silencing caused by a genome defense process called meiotic silencing by unpaired DNA (MSUD). We show that rfk-1's MSUD avoidance mechanism is linked to the genomic landscape surrounding the rfk-1 gene, which is located near the Sk-2 border on the right arm of chromosome III. In addition to demonstrating that the location of rfk-1 is critical to spore-killing success, our results add to accumulating evidence that MSUD helps protect Neurospora genomes from complex meiotic drive elements.
Project description:Mendel laws of inheritance can be cheated by Meiotic Drive Elements (MDs), complex nuclear genetic loci found in various eukaryotic genomes and distorting segregation in their favor. Here, we identify and characterize in the model fungus Podospora anserina Spok1 and Spok2, two MDs known as Spore Killers. We show that they are related genes with both spore-killing distorter and spore-protecting responder activities carried out by the same allele. These alleles act as autonomous elements, exert their effects independently of their location in the genome and can act as MDs in other fungi. Additionally, Spok1 acts as a resistance factor to Spok2 killing. Genetical data and cytological analysis of Spok1 and Spok2 localization during the killing process suggest a complex mode of action for Spok proteins. Spok1 and Spok2 belong to a multigene family prevalent in the genomes of many ascomycetes. As they have no obvious cellular role, Spok1 and Spok2 Spore Killer genes represent a novel kind of selfish genetic elements prevalent in fungal genome that proliferate through meiotic distortion.
Project description:Meiotic drivers are selfish alleles that subvert gametogenesis to increase their transmission into progeny. Drivers impose a fitness cost, putting pressure on the genome to evolve suppressors. Here we investigate the wtf gene family from Schizosaccharomyces pombe, previously shown to contain meiotic drivers in wild isolates. We discovered that wtf13 found in lab stocks is a meiotic driver. wtf13 kills spores that do not inherit it by generating both a diffusible poison and a spore-specific antidote. Additionally, we demonstrate that wtf13 is suppressed by another wtf gene, wtf18-2, that arose spontaneously in the lab and makes only an antidote. Wtf18-2 does not act indiscriminately to prevent spore destruction. Instead, it rescues only the spores that inherit wtf18-2. In this way, wtf18-2 selfishly gains a transmission advantage of its own while dampening the drive of wtf13. This establishes a novel paradigm for meiotic drive suppressors and provides insight into the mechanisms and evolution of drive systems.
Project description:Meiotic drive is the preferential transmission of a particular allele during sexual reproduction. The phenomenon is observed as spore killing in multiple fungi. In natural populations of Podospora anserina, seven spore killer types (Psks) have been identified through classical genetic analyses. Here we show that the Spok gene family underlies the Psks. The combination of Spok genes at different chromosomal locations defines the spore killer types and creates a killing hierarchy within a population. We identify two novel Spok homologs located within a large (74-167 kbp) region (the Spok block) that resides in different chromosomal locations in different strains. We confirm that the SPOK protein performs both killing and resistance functions and show that these activities are dependent on distinct domains, a predicted nuclease and kinase domain. Genomic and phylogenetic analyses across ascomycetes suggest that the Spok genes disperse through cross-species transfer, and evolve by duplication and diversification within lineages.
Project description:Meiotic drive is widespread in nature. The conflict it generates is expected to be an important motor for evolutionary change and innovation. In this study, we investigated the genomic consequences of two large multi-gene meiotic drive elements, Sk-2 and Sk-3, found in the filamentous ascomycete Neurospora intermedia. Using long-read sequencing, we generated the first complete and well-annotated genome assemblies of large, highly diverged, non-recombining regions associated with meiotic drive elements. Phylogenetic analysis shows that, even though Sk-2 and Sk-3 are located in the same chromosomal region, they do not form sister clades, suggesting independent origins or at least a long evolutionary separation. We conclude that they have in a convergent manner accumulated similar patterns of tandem inversions and dense repeat clusters, presumably in response to similar needs to create linkage between genes causing drive and resistance.
Project description:Background:Streptokinase (SK), a heterogeneous plasminogen activator (PA) protein from groups A, C, and G streptococci (GAS, GCS, GGS, respectively) contains three structural domains (SK?, SK?, and SK?). Based on the variable region of SK?, GAS-SK (ska) are clustered as SK1 and SK2 (including cluster2-streptokinase (SK2a)/SK2b), which show low and high fibrinogen (FG)-dependent plasminogen (Plg) activation properties, respectively. Despite being co-clustered as SK2a, GCS/GGS-SK (skcg) variants display properties similar to SK1. Herein, by SK? exchange between GGS (G88) and GAS-SK2a (STAB902) variants, the potential roles of SK domains in amidolytic/proteolytic activity and FG-bound-Plg activation are represented. Methods:Two parental SKG88 and SKSTAB902 genes were cloned into the NdeI/XhoI site of pET26b expression vector. The two chimeric SK?-exchanged constructs (SKC1: ?G88-?STAB-?G88 and SKC2; ?STAB-?G88-?STAB) were constructed by BstEII/BsiWI digestion/cross-ligation in parental plasmids. SK were expressed in E. coli and purified by nickel-nitriloacetic acid chromatography. PA potencies of SKs were measured by colorimetric assay. Results:SDS-PAGE and Western-blot analyses confirmed the proper expression of 47-kDa SK. Analyses indicated that the catalytic efficiency (Kcat/Km) for amidolytic and proteolytic activity were less and moderately dependent on SK?, respectively. The increase of FG-bound-Plg activation for SKSTAB902/SKC1 containing SK2a? was around six times, whereas for SKG88/SKC2 containing skcg?, it was four times. Conclusion:Although SK? has noticeable contribution in FG-bound-Plg activation activity, it had minor contribution in fibrin-independent, amidolytic activity. These data might be of interest for engineering fibrin-specific versions of SK.
Project description:To date, research on the evolution of sex chromosomes has focused on sexually antagonistic selection among diploids, which has been shown to be a potent driver of the strata and reduced recombination that characterize many sex chromosomes. However, significant selection can also occur on haploid genotypes during less conspicuous life cycle stages, e.g., competition among sperm/pollen or meiotic drive during gamete/spore production. These haploid selective processes are typically sex-specific, e.g., gametic/gametophytic competition typically occurs among sperm/pollen, and meiotic drive typically occurs during either spermatogenesis or oogenesis. We use models to investigate whether sex-specific selection on haploids could drive the evolution of recombination suppression on the sex chromosomes, as has been demonstrated for sex-specific selection among diploids. A potential complication is that zygotic sex-ratios become biased when haploid selected loci become linked to the sex-determining region because the zygotic sex ratio is determined by the relative number and fitness of X- vs. Y-bearing sperm. Despite causing biased zygotic sex-ratios, we find that a period of sex-specific haploid selection generally favors recombination suppression on the sex chromosomes. Suppressed recombination is favored because it allows associations to build up between haploid-beneficial alleles and the sex that experiences haploid selection most often (e.g., pollen beneficial alleles become strongly associated with the male determining region, Y or Z). Haploid selected loci can favor recombination suppression even in the absence of selective differences between male and female diploids. Overall, we expand our view of the sex-specific life cycle stages that can drive sex chromosome evolution to include gametic competition and meiotic drive. Based on our models, sex chromosomes should become enriched for genes that experience haploid selection, as is expected for genes that experience sexually antagonistic selection. Thus, we generate a number of predictions that can be evaluated in emerging sex chromosome systems.