A-to-I mRNA editing controls spore death induced by a fungal meiotic drive gene in homologous and heterologous expression systems

Identification of novel genes responsible for a pollen killer present in local natural populations of Arabidopsis thaliana

Gamete killers are genetic loci that distort segregation in the progeny of hybrids because the killer allele promotes the elimination of the gametes that carry the sensitive allele. They are widely distributed in eukaryotes and are important for understanding genome evolution and speciation. We had previously identified a pollen killer in hybrids between two distant natural accessions of Arabidopsis thaliana. This pollen killer involves three genetically linked genes, and we previously reported the identification of the gene encoding the antidote that protects pollen grains from the killer activity. In this study, we identified the two other genes of the pollen killer by using CRISPR-Cas9 induced mutants. These two genes are necessary for the killer activity that we demonstrated to be specific to pollen. The cellular localization of the pollen killer encoded proteins suggests that the pollen killer activity involves the mitochondria. Sequence analyses reveal predicted domains from the same families in the killer proteins. In addition, the C-terminal half of one of the killer proteins is identical to the antidote, and one amino acid, crucial for the antidote activity, is also essential for the killer function. Investigating more than 700 worldwide accessions of A. thaliana, we confirmed that the locus is subject to important structural rearrangements and copy number variation. By exploiting available de novo genomic sequences, we propose a scenario for the emergence of this pollen killer in A. thaliana. Furthermore, we report the co-occurrence and behavior of killer and sensitive genotypes in several local populations, a prerequisite for studying gamete killer evolution in the wild. This highlights the potential of the Arabidopsis model not only for functional studies of gamete killers but also for investigating their evolutionary trajectories at complementary geographical scales.

A Fusarium verticillioides MAT1-2 Strain near Isogenic to the Sequenced FGSC7600 Strain for Producing Homozygous Multigene Mutants

Fungal genetic systems ideally combine molecular tools for genome manipulation and a sexual reproduction system to create an informative assortment of combinations of genomic modifications. When employing the sexual cycle to generate multi-mutants, the background genotype variations in the parents may result in progeny phenotypic variation obscuring the effects of combined mutations. Here, to mitigate this variation in Fusarium verticillioides, we generated a MAT1-2 strain that was near isogenic to the sequenced wild-type MAT1-1 strain, FGSC7600. This was accomplished by crossing FGSC7600 with the divergent wild-type MAT1-2 strain FGSC7603 followed by six sequential backcrosses (e.g., six generations) of MAT1-2 progeny to FGSC7600. We sequenced each generation and mapped recombination events. The parental cross involved twenty-six crossovers on nine of the eleven chromosomes. The dispensable chromosome 12, found in FGSC7603 but lacking in FGSC7600, was not present in the progeny post generation five. Inheritance of complete chromosomes without crossover was frequently observed. A deletion of approximately 140 kilobases, containing 54 predicted genes on chromosome 4, occurred in generation 4 and was retained in generation 5 indicating that these genes are dispensable for growth and both asexual and sexual reproduction. The final MAT1-2 strain TMRU10/35 is about 93% identical to FGSC7600. TMRU10/35 is available from the Fungal Genetics Stock Center as FGSC27326 and from the ARS Culture Collection as NRRL64809.

The micromammals

In this editorial, Senior Editor Susan Forsburg examines the reasons to keep studying eukaryotic microbes like S. pombe and S. cerevisiae—and other yeasts, algae, amoeba, and fungi—even as genetic and genomic technologies now allow manipulation and study of practically any organism. She explores the challenges and opportunities of working in these tiny organisms, pointing to the substantial biology their study has uncovered.

Proliferation and dissemination of killer meiotic drive loci

Killer meiotic drive elements are selfish genetic entities that manipulate the sexual cycle to promote their own inheritance via destructive means. Two broad classes are sperm killers, typical of animals and plants, and spore killers, which are present in ascomycete fungi. Killer meiotic drive systems operate via toxins that destroy or disable meiotic products bearing the alternative allele. To avoid suicidal autotargeting, cells that bear these selfish elements must either lack the toxin target, or express an antidote. Historically, these systems were presumed to require large nonrecombining haplotypes to link multiple functional interacting loci. However, recent advances on fungal spore killers reveal that numerous systems are enacted by single genes, and similar molecular genetic studies in Drosophila pinpoint individual loci that distort gamete sex. Notably, many meiotic drivers duplicate readily, forming gene families that can have complex interactions within and between species, and providing substrates for their rapid functional diversification. Here, we summarize the known families of meiotic drivers in fungi and fruit flies, and highlight shared principles about their evolution and proliferation that promote the spread of these noxious genes.

A Natural Fungal Gene Drive Enacts Killing via DNA Disruption

Fungal spore killers are a class of selfish genetic elements that positively bias their own inheritance by killing non-inheriting gametes following meiosis. As killing takes place specifically within the developing fungal ascus, a tissue which is experimentally difficult to isolate, our understanding of the mechanisms underlying spore killers are limited. In particular, how these loci kill other spores within the fungal ascus is largely unknown. Here, we overcome these experimental barriers by developing model systems in 2 evolutionary distant organisms, Escherichia coli (bacterium) and Saccharomyces cerevisiae (yeast), similar to previous approaches taken to examine the wtf spore killers. Using these systems, we show that the Podospora anserina spore killer protein SPOK1 enacts killing through targeting DNA. IMPORTANCE Natural gene drives have shaped the genomes of many eukaryotes and recently have been considered for applications to control undesirable species. In fungi, these loci are called spore killers. Despite their importance in evolutionary processes and possible applications, our understanding of how they enact killing is limited. We show that the spore killer protein Spok1, which has homologues throughout the fungal tree of life, acts via DNA disruption. Spok1 is only the second spore killer locus in which the cellular target of killing has been identified and is the first known to target DNA. We also show that the DNA disrupting activity of Spok1 is functional in both bacteria and yeast suggesting a highly conserved mode of action.

Gene drive by Fusarium SKC1 is dependent on its competing allele

The Fusarium verticillioides SKC1 gene driver is transmitted to offspring in a biased manner through spore killing. The mechanism that allows SKC1 to kill non-SKC1 offspring while sparing others is poorly understood. Here we report that gene drive by SKC1 is dependent on SKC1's competing allele. We propose that SKC1's competing allele influences the ability of a genome defense process to detect SKC1, and we provide evidence that this genome defense process is meiotic silencing by unpaired DNA (MSUD). Our findings suggest that the successful deployment of gene drivers to control pathogenic fungi will require researchers to consider how competing alleles influence the ability of gene drivers to be detected by genome defense processes.

On the Mechanistic Basis of Killer Meiotic Drive in Fungi

Spore killers are specific genetic elements in fungi that kill sexual spores that do not contain them. A range of studies in the last few years have provided the long-awaited first insights into the molecular mechanistic aspects of spore killing in different fungal models, including both yeast-forming and filamentous Ascomycota. Here we describe these recent advances, focusing on the wtf system in the fission yeast Schizosaccharomyces pombe; the Sk spore killers of Neurospora species; and two spore-killer systems in Podospora anserina, Spok and [Het-s]. The spore killers appear thus far mechanistically unrelated. They can involve large genomic rearrangements but most often rely on the action of just a single gene. Data gathered so far show that the protein domains involved in the killing and resistance processes differ among the systems and are not homologous. The emerging picture sketched by these studies is thus one of great mechanistic and evolutionary diversity of elements that cheat during meiosis and are thereby preferentially inherited over sexual generations.