Spore-killing meiotic drive factors in a natural population of the fungus Podospora anserina

Longevity of Fungal Mycelia and Nuclear Quality Checks: a New Hypothesis for the Role of Clamp Connections in Dikaryons

This paper addresses the stability of mycelial growth in fungi and differences between ascomycetes and basidiomycetes. Starting with general evolutionary theories of multicellularity and the role of sex, we then discuss individuality in fungi. Recent research has demonstrated the deleterious consequences of nucleus-level selection in fungal mycelia, favoring cheaters with a nucleus-level benefit during spore formation but a negative effect on mycelium-level fitness. Cheaters appear to generally be loss-of-fusion (LOF) mutants, with a higher propensity to form aerial hyphae developing into asexual spores. Since LOF mutants rely on heterokaryosis with wild-type nuclei, we argue that regular single-spore bottlenecks can efficiently select against such cheater mutants. We then zoom in on ecological differences between ascomycetes being typically fast-growing but short-lived with frequent asexual-spore bottlenecks and basidiomycetes being generally slow-growing but long-lived and usually without asexual-spore bottlenecks. We argue that these life history differences have coevolved with stricter nuclear quality checks in basidiomycetes. Specifically, we propose a new function for clamp connections, structures formed during the sexual stage in ascomycetes and basidiomycetes but during somatic growth only in basidiomycete dikaryons. During dikaryon cell division, the two haploid nuclei temporarily enter a monokaryotic phase, by alternatingly entering a retrograde-growing clamp cell, which subsequently fuses with the subapical cell to recover the dikaryotic cell. We hypothesize that clamp connections act as screening devices for nuclear quality, with both nuclei continuously testing each other for fusion ability, a test that LOF mutants will fail. By linking differences in longevity of the mycelial phase to ecology and stringency of nuclear quality checks, we propose that mycelia have a constant and low lifetime cheating risk, irrespective of their size and longevity.

Individual Genetic Heterogeneity

Genetic variation has been widely covered in literature, however, not from the perspective of an individual in any species. Here, a synthesis of genetic concepts and variations relevant for individual genetic constitution is provided. All the different levels of genetic information and variation are covered, ranging from whether an organism is unmixed or hybrid, has variations in genome, chromosomes, and more locally in DNA regions, to epigenetic variants or alterations in selfish genetic elements. Genetic constitution and heterogeneity of microbiota are highly relevant for health and wellbeing of an individual. Mutation rates vary widely for variation types, e.g., due to the sequence context. Genetic information guides numerous aspects in organisms. Types of inheritance, whether Mendelian or non-Mendelian, zygosity, sexual reproduction, and sex determination are covered. Functions of DNA and functional effects of variations are introduced, along with mechanism that reduce and modulate functional effects, including TARAR countermeasures and intraindividual genetic conflict. TARAR countermeasures for tolerance, avoidance, repair, attenuation, and resistance are essential for life, integrity of genetic information, and gene expression. The genetic composition, effects of variations, and their expression are considered also in diseases and personalized medicine. The text synthesizes knowledge and insight on individual genetic heterogeneity and organizes and systematizes the central concepts.

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.

Reflection on the Challenges, Accomplishments, and New Frontiers of Gene Drives

Ongoing pest and disease outbreaks pose a serious threat to human, crop, and animal lives, emphasizing the need for constant genetic discoveries that could serve as mitigation strategies. Gene drives are genetic engineering approaches discovered decades ago that may allow quick, super-Mendelian dissemination of genetic modifications in wild populations, offering hopes for medicine, agriculture, and ecology in combating diseases. Following its first discovery, several naturally occurring selfish genetic elements were identified and several gene drive mechanisms that could attain relatively high threshold population replacement have been proposed. This review provides a comprehensive overview of the recent advances in gene drive research with a particular emphasis on CRISPR-Cas gene drives, the technology that has revolutionized the process of genome engineering. Herein, we discuss the benefits and caveats of this technology and place it within the context of natural gene drives discovered to date and various synthetic drives engineered. Later, we elaborate on the strategies for designing synthetic drive systems to address resistance issues and prevent them from altering the entire wild populations. Lastly, we highlight the major applications of synthetic CRISPR-based gene drives in different living organisms, including plants, animals, and microorganisms.

Allorecognition genes drive reproductive isolation in Podospora anserina

Allorecognition, the capacity to discriminate self from conspecific non-self, is a ubiquitous organismal feature typically governed by genes evolving under balancing selection. Here, we show that in the fungus Podospora anserina, allorecognition loci controlling vegetative incompatibility (het genes), define two reproductively isolated groups through pleiotropic effects on sexual compatibility. These two groups emerge from the antagonistic interactions of the unlinked loci het-r (encoding a NOD-like receptor) and het-v (encoding a methyltransferase and an MLKL/HeLo domain protein). Using a combination of genetic and ecological data, supported by simulations, we provide a concrete and molecularly defined example whereby the origin and coexistence of reproductively isolated groups in sympatry is driven by pleiotropic genes under balancing selection.

Fungal Cell Death: The Beginning of the End

Death is an important part of an organism's existence and also marks the end of life. On a cellular level, death involves the execution of complex processes, which can be classified into different types depending on their characteristics. Despite their "simple" lifestyle, fungi carry out highly specialized and sophisticated mechanisms to regulate the way their cells die, and the pathways underlying these mechanisms are comparable with those of plants and metazoans. This review focuses on regulated cell death in fungi and discusses the evidence for the occurrence of apoptotic-like, necroptosis-like, pyroptosis-like death, and the role of the NLR proteins in fungal cell death. We also describe recent data on meiotic drive elements involved in "spore killing" and the molecular basis of allorecognition-related cell death during cell fusion of genetically dissimilar cells. Finally, we discuss how fungal regulated cell death can be relevant in developing strategies to avoid resistance and tolerance to antifungal agents.

The spore killers, fungal meiotic driver elements

During meiosis, both alleles of any given gene should have equal chances of being inherited by the progeny. There are a number of reasons why, however, this is not the case, with one of the most intriguing instances presenting itself as the phenomenon of meiotic drive. Genes that are capable of driving can manipulate the ratio of alleles among viable meiotic products so that they are inherited in more than half of them. In many cases, this effect is achieved by direct antagonistic interactions, where the driving allele inhibits or otherwise eliminates the alternative allele. In ascomycete fungi, meiotic products are packaged directly into ascospores; thus, the effect of meiotic drive has been given the nefarious moniker, "spore killing." In recent years, many of the known spore killers have been elevated from mysterious phenotypes to well-described systems at genetic, genomic, and molecular levels. In this review, we describe the known diversity of spore killers and synthesize the varied pieces of data from each system into broader trends regarding genome architecture, mechanisms of resistance, the role of transposable elements, their effect on population dynamics, speciation and gene flow, and finally how they may be developed as synthetic drivers. We propose that spore killing is common, but that it is under-observed because of a lack of studies on natural populations. We encourage researchers to seek new spore killers to build on the knowledge that these remarkable genetic elements can teach us about meiotic drive, genomic conflict, and evolution more broadly.

Molecular Mechanisms and Evolutionary Consequences of Spore Killers in Ascomycetes

In this review, we examine the fungal spore killers. These are meiotic drive elements that cheat during sexual reproduction to increase their transmission into the next generation. Spore killing has been detected in a number of ascomycete genera, including Podospora, Neurospora, Schizosaccharomyces, Bipolaris, and Fusarium. There have been major recent advances in spore killer research that have increased our understanding of the molecular identity, function, and evolutionary history of the known killers. The spore killers vary in the mechanism by which they kill and are divided into killer-target and poison-antidote drivers. In killer-target systems, the drive locus encodes an element that can be described as a killer, while the target is an allele found tightly linked to the drive locus but on the nondriving haplotype. The poison-antidote drive systems encode both a poison and an antidote element within the drive locus. The key to drive in this system is the restricted distribution of the antidote: only the spores that inherit the drive locus receive the antidote and are rescued from the toxicity of the poison. Spore killers also vary in their genome architecture and can consist of a single gene or multiple linked genes. Due to their ability to distort meiosis, spore killers gain a selective advantage at the gene level that allows them to increase in frequency in a population over time, even if they reduce host fitness, and they may have significant impact on genome architecture and macroevolutionary processes such as speciation.

Diverse mating phenotypes impact the spread of wtf meiotic drivers in Schizosaccharomyces pombe

Meiotic drivers are genetic elements that break Mendel's law of segregation to be transmitted into more than half of the offspring produced by a heterozygote. The success of a driver relies on outcrossing (mating between individuals from distinct lineages) because drivers gain their advantage in heterozygotes. It is, therefore, curious that Schizosaccharomyces pombe, a species reported to rarely outcross, harbors many meiotic drivers. To address this paradox, we measured mating phenotypes in S. pombe natural isolates. We found that the propensity for cells from distinct clonal lineages to mate varies between natural isolates and can be affected both by cell density and by the available sexual partners. Additionally, we found that the observed levels of preferential mating between cells from the same clonal lineage can slow, but not prevent, the spread of a wtf meiotic driver in the absence of additional fitness costs linked to the driver. These analyses reveal parameters critical to understanding the evolution of S. pombe and help explain the success of meiotic drivers in this species.

Invasion and maintenance of meiotic drivers in populations of ascomycete fungi

Meiotic drivers (MDs) are selfish genetic elements that are able to become overrepresented among the products of meiosis. This transmission advantage makes it possible for them to spread in a population even when they impose fitness costs on their host organisms. Whether an MD can invade a population, and subsequently reach fixation or coexist in a stable polymorphism, depends on the one hand on the biology of the host organism, including its life cycle, mating system, and population structure, and on the other hand on the specific fitness effects of the driving allele on the host. Here, we present a population genetic model for spore killing, a type of drive specific to fungi. We show how ploidy level, rate of selfing, and efficiency of spore killing affect the invasion probability of a driving allele and the conditions for its stable coexistence with a nondriving allele. Our model can be adapted to different fungal life cycles, and is applied here to two well-studied genera of filamentous ascomycetes known to harbor spore-killing elements, Neurospora and Podospora. We discuss our results in the light of recent empirical findings for these two systems.

The Enterprise, a massive transposon carrying Spok meiotic drive genes

The genomes of eukaryotes are full of parasitic sequences known as transposable elements (TEs). Here, we report the discovery of a putative giant tyrosine-recombinase-mobilized DNA transposon, Enterprise, from the model fungus Podospora anserina Previously, we described a large genomic feature called the Spok block which is notable due to the presence of meiotic drive genes of the Spok gene family. The Spok block ranges from 110 kb to 247 kb and can be present in at least four different genomic locations within P. anserina, despite what is an otherwise highly conserved genome structure. We propose that the reason for its varying positions is that the Spok block is not only capable of meiotic drive but is also capable of transposition. More precisely, the Spok block represents a unique case where the Enterprise has captured the Spoks, thereby parasitizing a resident genomic parasite to become a genomic hyperparasite. Furthermore, we demonstrate that Enterprise (without the Spoks) is found in other fungal lineages, where it can be as large as 70 kb. Lastly, we provide experimental evidence that the Spok block is deleterious, with detrimental effects on spore production in strains which carry it. This union of meiotic drivers and a transposon has created a selfish element of impressive size in Podospora, challenging our perception of how TEs influence genome evolution and broadening the horizons in terms of what the upper limit of transposition may be.

The taxonomy of the model filamentous fungus Podospora anserina

The filamentous fungus Podospora anserina has been used as a model organism for more than 100 years and has proved to be an invaluable resource in numerous areas of research. Throughout this period, P. anserina has been embroiled in a number of taxonomic controversies regarding the proper name under which it should be called. The most recent taxonomic treatment proposed to change the name of this important species to Triangularia anserina. The results of past name changes of this species indicate that the broader research community is unlikely to accept this change, which will lead to nomenclatural instability and confusion in literature. Here, we review the phylogeny of the species closely related to P. anserina and provide evidence that currently available marker information is insufficient to resolve the relationships amongst many of the lineages. We argue that it is not only premature to propose a new name for P. anserina based on current data, but also that every effort should be made to retain P. anserina as the current name to ensure stability and to minimise confusion in scientific literature. Therefore, we synonymise Triangularia with Podospora and suggest that either the type species of Podospora be moved to P. anserina from P. fimiseda or that all species within the Podosporaceae be placed in the genus Podospora.

Combinations of Spok genes create multiple meiotic drivers in Podospora

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.

In many organisms, most cells carry two versions of a given gene, one coming from the mother and the other from the father. An exception is sexual cells such as eggs, sperm, pollen or spores, which should only contain one variant of a gene. During their formation, these cells usually have an equal chance of inheriting one of the two gene versions.

However, a certain class of gene variants called meiotic drivers can cheat this process and end up in more than half of the sexual cells; often, the cells that contain the drivers can kill sibling cells that do not carry these variants. This results in the selfish genetic elements spreading through populations at a higher rate, sometimes with severe consequences such as shifting the ratio of males to females.

Meiotic drivers have been discovered in a wide range of organisms, from corn to mice to fruit flies and bread mold. They also exist in the fungus Podospora anserina, where they are called ‘spore killers’. Fungi are often used to study complex genetic processes, yet the identity and mode of action of spore killers in P. anserina were still unknown.

Vogan, Ament-Velásquez et al. used a combination of genetic methods to identify three genes from the Spok family which are responsible for certain spores being able to kill their siblings. Two of these were previously unknown, and they could be found in different locations throughout the genome as part of a larger genetic region. Depending on the combination of Spok genes it carries, a spore can kill or be protected against other spores that contain different permutations of the genes. Copies of these genes were also shown to be present in other fungi, including species that are a threat to crops.

Scientists have already started to create synthetic meiotic drivers to manipulate how certain traits are inherited within a population. This could be useful to control or eradicate pests and insects that transmit dangerous diseases. The results by Vogan, Ament-Velásquez et al. shine a light on the complex ways that natural meiotic drivers work, including how they can be shared between species; this knowledge could inform how to safely deploy synthetic drivers in the wild.

A family of killers

Spok genes are meiotic drivers that increase their own chances of transmission by killing gametes that do not inherit them.

A century of bias in genetics and evolution

Mendel proposed that the heritable material is particulate and that transmission of alleles is unbiased. An assumption of unbiased transmission was necessary to show how variation can be preserved in the absence of selection, so overturning an early objection to Darwinism. In the second half of the twentieth century, it was widely recognised that even strongly deleterious alleles can invade if they have strongly biased transmission (i.e. strong segregation distortion). The spread of alleles with distorted segregation can explain many curiosities. More recently, the selectionist-neutralist duopoly was broken by the realisation that biased gene conversion can explain phenomena such as mammalian isochore structures. An initial focus on unbiased transmission in 1919, has thus given way to an interest in biased transmission in 2019. A focus on very weak bias is now possible owing to technological advances, although technical biases may put a limit on resolving power. To understand the relevance of weak bias we could profit from having the concept of the effectively Mendelian allele, a companion to the effectively neutral allele. Understanding the implications of unbiased and biased transmission may, I suggest, be a good way to teach evolution so as to avoid psychological biases.

Supergenes and complex phenotypes

Understanding the molecular underpinnings of evolutionary adaptations is a central focus of modern evolutionary biology. Recent studies have uncovered a panoply of complex phenotypes, including locally adapted ecotypes and cryptic morphs, divergent social behaviours in birds and insects, as well as alternative metabolic pathways in plants and fungi, that are regulated by clusters of tightly linked loci. These 'supergenes' segregate as stable polymorphisms within or between natural populations and influence ecologically relevant traits. Some supergenes may span entire chromosomes, because selection for reduced recombination between a supergene and a nearby locus providing additional benefits can lead to locus expansions with dynamics similar to those known for sex chromosomes. In addition to allowing for the co-segregation of adaptive variation within species, supergenes may facilitate the spread of complex phenotypes across species boundaries. Application of new genomic methods is likely to lead to the discovery of many additional supergenes in a broad range of organisms and reveal similar genetic architectures for convergently evolved phenotypes.

Genes that bias Mendelian segregation

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.

Chromosome segregation during meiosis ensures that paternal and maternal chromosomes are equally transmitted to the progeny. Meiotic Drive Elements (MDs) are known to distort this 1∶1 ratio in many animal, plant, and fungal species by killing the gametes not carrying them. Most of the known MDs are complex genetic loci with separate genes for the killing activity and the resistance to said killing. Here, we report in a model fungus on two genes endowed with MD properties previously unreported. Both genes produce a single polypeptide and confer both killing and resistance. They exert their effect irrespective of their position in the genome. They can cross species barriers and promote bias in segregation in other species. As related genes are frequently observed in fungal genomes, we propose that they are representative of a novel kind of selfish genes that propagate by distorting the Mendel laws of segregation.

High natural prevalence of a fungal prion

Prions are infectious proteins that cause fatal diseases in mammals. Prions have also been found in fungi, but studies on their role in nature are scarce. The proposed biological function of fungal prions is debated and varies from detrimental to benign or even beneficial. [Het-s] is a prion of the fungus Podospora anserina. The het-s locus exists as two antagonistic alleles that constitute an allorecognition system: the het-s allele encoding the protein variant capable of prion formation and the het-S allele encoding a protein variant that cannot form a prion. We document here that het-s alleles, capable of prion formation, are nearly twice as frequent as het-S alleles in a natural population of 112 individuals. Then, we report a 92% prevalence of [Het-s] prion infection among the het-s isolates and find evidence of the role of the [Het-s]/het-S allorecognition system on the incidence of infection by a deleterious senescence plasmid. We explain the het-s/het-S allele ratios by the existence of two selective forces operating at different levels. We propose that during the somatic stage, the role of [Het-s]/HET-S in allorecognition leads to frequency-dependent selection for which an equilibrated frequency would be optimal. However, in the sexual cycle, the [Het-s] prion causes meiotic drive favoring the het-s allele. Our findings indicate that [Het-s] is a selected and, therefore, widespread prion whose activity as selfish genetic element is counteracted by balancing selection for allorecognition polymorphism.

Identification of the het-r vegetative incompatibility gene of Podospora anserina as a member of the fast evolving HNWD gene family

In fungi, vegetative incompatibility is a conspecific non-self recognition mechanism that restricts formation of viable heterokaryons when incompatible alleles of specific het loci interact. In Podospora anserina, three non-allelic incompatibility systems have been genetically defined involving interactions between het-c and het-d, het-c and het-e, het-r and het-v. het-d and het-e are paralogues belonging to the HNWD gene family that encode proteins of the STAND class. HET-D and HET-E proteins comprise an N-terminal HET effector domain, a central GTP binding site and a C-terminal WD repeat domain constituted of tandem repeats of highly conserved WD40 repeat units that define the specificity of alleles during incompatibility. The WD40 repeat units of the members of this HNWD family are undergoing concerted evolution. By combining genetic analysis and gain of function experiments, we demonstrate that an additional member of this family, HNWD2, corresponds to the het-r non-allelic incompatibility gene. As for het-d and het-e, allele specificity at the het-r locus is determined by the WD repeat domain. Natural isolates show allelic variation for het-r.

Neurospora spore killers Sk-2 and Sk-3 suppress meiotic silencing by unpaired DNA

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 RNA-directed 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 x 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 x sensitive crosses. We suggest that the region contains a suppressor of MSUD.

Non-mendelian inheritance of the HET-s prion or HET-s prion domains determines the het-S spore killing system in Podospora anserina

Two alleles of the het-s/S locus occur naturally in the filamentous fungus Podospora anserina, het-s and het-S. The het-s encoded protein can form a prion that propagates a self-perpetuating amyloid aggregate, resulting in two phenotypes for the het-s strains. The prion-infected [Het-s] shows an antagonistic interaction to het-S whereas the prion-free [Het-s*] is neutral in interaction to het-S. The antagonism between [Het-s] and het-S is seen as heterokaryon incompatibility at the somatic level and as het-S spore killing in the sexual cycle. Two different domains of the HET-s and HET-S proteins have been identified, and a structure-function relationship has been established for interactions at the somatic level. In this study, we correlate accumulation of the HET-s and HET-S proteins (visualized using GFP) during the sexual cycle with timing of het-S spore abortion. Also, we present the structure-function relationship of the HET-s domains for interactions in the sexual cycle. We show that the constructs that ensure het-s incompatibility function in somatic mycelium are also active in het-S spore killing in the sexual cycle. In addition, paternal prion transmission and het-S spore killing has been found with the HET-s(157-289) truncated protein. The consequences of the unique transition from a coenocytic to a cellular state in the sexual phase and the timing, and localization of paternal and maternal HET-s and HET-S expression that are pertinent to prion transmission, and het-S spore killing are elaborated. These data further support our previously proposed model for het-S spore killing.

Sexual transmission of the [Het-S] prion leads to meiotic drive in Podospora anserina

In the filamentous fungus Podospora anserina, two phenomena are associated with polymorphism at the het-s locus, vegetative incompatibility and ascospore abortion. Two het-s alleles occur naturally, het-s and het-S. The het-s encoded protein is a prion propagating as a self-perpetuating amyloid aggregate. When prion-infected [Het-s] hyphae fuse with [Het-S] hyphae, the resulting heterokaryotic cells necrotize. [Het-s] and [Het-S] strains are sexually compatible. When, however, a female [Het-s] crosses with [Het-S], a significant percentage of het-S spores abort, in a way similar to spore killing in Neurospora and Podospora. We report here that sexual transmission of the [Het-s] prion after nonisogamous mating in the reproductive cycle of Podospora is responsible for the killing of het-S spores. Progeny of crosses between isogenic strains with distinct wild-type or introduced, ectopic het-s/S alleles were cytologically and genetically analyzed. The effect of het-s/S overexpression, ectopic het-s/S expression, absence of het-s expression, loss of [Het-s] prion infection, and the distribution patterns of HET-s/S-GFP proteins were categorized during meiosis and ascospore formation. This study unveiled a het-S spore-killing system that is governed by dosage of and interaction between the [Het-s] prion and the HET-S protein. Due to this property of the [Het-s] prion, the het-s allele acts as a meiotic drive element favoring maintenance of the prion-forming allele in natural populations.

The fitness of filamentous fungi

Fitness is a common currency in comparative biology. Without data on fitness, hypotheses about the adaptive significance of phenotypes or basic mechanisms of evolution, for example natural selection, remain speculative. Experiments with fungi can address questions specific to fungi or questions with a broader significance. Fungi can challenge the generality of fundamental evolutionary principles, yet there are no standard measures of fungal fitness. We argue that focusing on a single aspect of a complex life cycle, or a single measure of fitness (e.g. the number of asexual spores) is appropriate. Choosing which aspect of fitness to measure can be facilitated by an understanding of how fitness measures are correlated. Choices can also be based on the ecology of a species, for example whether a fungus is semelparous and reproduces once, or iteroparous and reproduces multiple times.

Geographic Distribution of Neurospora Spore Killer Strains and Strains Resistant to Killing

Spore killer strains, found in Neurospora, provided the first recognized example of meiotic drive in fungi. In the present study, natural populations throughout the world were examined for the presence of killer strains and strains that are resistant to killing. In N. intermedia, Sk-2 and Sk-3 are present but are rare. Killer strains were found at only five sites, in Borneo, Java, and Papua New Guinea. Nonkiller strains that are resistant to killing by Sk-2 or Sk-3 are frequent in that part of the world where the killer strains are present, but resistant stains were not found in regions where killers are absent. In N. sitophila, Sk-1 killer strains are common in nature, but only 1 of 392 nonkiller strains was resistant. In N. crassa, no killer strain was found among >500, but widely scattered Sk-2-resistant strains were present, suggesting the past or present existence of killers.