S. pombe wtf drivers use dual transcriptional regulation and selective protein exclusion from spores to cause meiotic drive

Meiotic drivers bias gametogenesis to ensure their transmission into more than half the offspring of a heterozygote. In Schizosaccharomyces pombe, wtf meiotic drivers destroy the meiotic products (spores) that do not inherit the driver from a heterozygote, thereby reducing fertility. wtf drivers encode both a Wtfpoison protein and a Wtfantidote protein using alternative transcriptional start sites. Here, we analyze how the expression and localization of the Wtf proteins are regulated to achieve drive. We show that transcriptional timing and selective protein exclusion from developing spores ensure that all spores are exposed to Wtf4poison, but only the spores that inherit wtf4 receive a dose of Wtf4antidote sufficient for survival. In addition, we show that the Mei4 transcription factor, a master regulator of meiosis, controls the expression of the wtf4poison transcript. This transcriptional regulation, which includes the use of a critical meiotic transcription factor, likely complicates the universal suppression of wtf genes without concomitantly disrupting spore viability. We propose that these features contribute to the evolutionary success of the wtf drivers.

The wtf meiotic driver gene family has unexpectedly persisted for over 100 million years

Meiotic drivers are selfish elements that bias their own transmission into more than half of the viable progeny produced by a driver+/driver- heterozygote. Meiotic drivers are thought to exist for relatively short evolutionary timespans because a driver gene or gene family is often found in a single species or in a group of very closely related species. Additionally, drivers are generally considered doomed to extinction when they spread to fixation or when suppressors arise. In this study, we examine the evolutionary history of the wtf meiotic drivers first discovered in the fission yeast Schizosaccharomyces pombe. We identify homologous genes in three other fission yeast species, S. octosporus, S. osmophilus, and S. cryophilus, which are estimated to have diverged over 100 million years ago from the S. pombe lineage. Synteny evidence supports that wtf genes were present in the common ancestor of these four species. Moreover, the ancestral genes were likely drivers as wtf genes in S. octosporus cause meiotic drive. Our findings indicate that meiotic drive systems can be maintained for long evolutionary timespans.

What can we learn from selfish loci that break Mendel’s law?

Exceptions to Mendel’s law of segregation were important for demonstrating that chromosomes carry genetic material. Scrutiny of other exceptional inheritance patterns has the potential to unravel unsolved mysteries of genetics.

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.

The wtf4 meiotic driver utilizes controlled protein aggregation to generate selective cell death

Meiotic drivers are parasitic loci that force their own transmission into greater than half of the offspring of a heterozygote. Many drivers have been identified, but their molecular mechanisms are largely unknown. The wtf4 gene is a meiotic driver in Schizosaccharomyces pombe that uses a poison-antidote mechanism to selectively kill meiotic products (spores) that do not inherit wtf4. Here, we show that the Wtf4 proteins can function outside of gametogenesis and in a distantly related species, Saccharomyces cerevisiae. The Wtf4poison protein forms dispersed, toxic aggregates. The Wtf4antidote can co-assemble with the Wtf4poison and promote its trafficking to vacuoles. We show that neutralization of the Wtf4poison requires both co-assembly with the Wtf4antidote and aggregate trafficking, as mutations that disrupt either of these processes result in cell death in the presence of the Wtf4 proteins. This work reveals that wtf parasites can exploit protein aggregate management pathways to selectively destroy spores.

Atypical meiosis can be adaptive in outcrossed Schizosaccharomyces pombe due to wtf meiotic drivers

Killer meiotic drivers are genetic parasites that destroy ‘sibling’ gametes lacking the driver allele. The fitness costs of drive can lead to selection of unlinked suppressors. This suppression could involve evolutionary tradeoffs that compromise gametogenesis and contribute to infertility. Schizosaccharomyces pombe, an organism containing numerous gamete (spore)-killing wtf drivers, offers a tractable system to test this hypothesis. Here, we demonstrate that in scenarios analogous to outcrossing, wtf drivers generate a fitness landscape in which atypical spores, such as aneuploids and diploids, are advantageous. In this context, wtf drivers can decrease the fitness costs of mutations that disrupt meiotic fidelity and, in some circumstances, can even make such mutations beneficial. Moreover, we find that S. pombe isolates vary greatly in their ability to make haploid spores, with some isolates generating up to 46% aneuploid or diploid spores. This work empirically demonstrates the potential for meiotic drivers to shape the evolution of gametogenesis.

Meiotic drive

In this Quick Guide, Srinivasa and Zanders provide an overview of meiotic drivers and the diverse mechanisms these genetic elements use to bias their transmission to the next generation.

Gene conversion generates evolutionary novelty that fuels genetic conflicts

Genetic conflicts arise when the evolutionary interests of two genetic elements are not aligned. Conflicts between genomes (e.g. pathogen versus host) or within the same genome (e.g. internal parasitic DNA sequences versus the rest of the host genome) can both foster 'molecular arms races', in which genes on both sides of the conflict rapidly evolve due to bouts of adaptation and counter-adaptation. Importantly, a source of genetic novelty is needed to fuel these arms races. In this review, we highlight gene conversion as a major force in generating the novel alleles on which selection can act. Using examples from both intergenomic and intragenomic conflicts, we feature the mechanisms by which gene conversion facilitates the rapid evolution of genes in conflict.

A family of killers

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

Fertility Costs of Meiotic Drivers

In sexual reproduction, opportunities are limited and the stakes are high. This inevitably leads to conflict. One pervasive conflict occurs within genomes between alternative alleles at heterozygous loci. Each gamete and thus each offspring will inherit only one of the two alleles from a heterozygous parent. Most alleles 'play fair' and have a 50% chance of being included in any given gamete. However, alleles can gain an enormous advantage if they act selfishly to force their own transmission into more than half, sometimes even all, of the functional gametes. These selfish alleles are known as 'meiotic drivers', and their cheating often incurs a high cost on the fertility of eukaryotes ranging from plants to mammals. Here, we review how several types of meiotic drivers directly and indirectly contribute to infertility, and argue that a complete picture of the genetics of infertility will require focusing on both the standard alleles - those that play fair - as well as selfish alleles involved in genetic conflict.

Killer Meiotic Drive and Dynamic Evolution of the wtf Gene Family

Natural selection works best when the two alleles in a diploid organism are transmitted to offspring at equal frequencies. Despite this, selfish loci known as meiotic drivers that bias their own transmission into gametes are found throughout eukaryotes. Drive is thought to be a powerful evolutionary force, but empirical evolutionary analyses of drive systems are limited by low numbers of identified meiotic drive genes. Here, we analyze the evolution of the wtf gene family of Schizosaccharomyces pombe that contains both killer meiotic drive genes and suppressors of drive. We completed assemblies of all wtf genes for two S. pombe isolates, as well as a subset of wtf genes from over 50 isolates. We find that wtf copy number can vary greatly between isolates and that amino acid substitutions, expansions and contractions of DNA sequence repeats, and nonallelic gene conversion between family members all contribute to dynamic wtf gene evolution. This work demonstrates the power of meiotic drive to foster rapid evolution and identifies a recombination mechanism through which transposons can indirectly mobilize meiotic drivers.

A suppressor of a wtf poison-antidote meiotic driver acts via mimicry of the driver's antidote

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.

Genetic Villains: Killer Meiotic Drivers

Unbiased allele transmission into progeny is a fundamental genetic concept canonized as Mendel's Law of Segregation. Not all alleles, however, abide by the law. Killer meiotic drivers are ultra-selfish DNA sequences that are transmitted into more than half (sometimes all) of the meiotic products generated by a heterozygote. As their name implies, these loci gain a transmission advantage in heterozygotes by destroying otherwise viable meiotic products that do not inherit the driver. We review and classify killer meiotic drive genes across a wide spectrum of eukaryotes. We discuss how analyses of these ultra-selfish genes can lead to greater insight into the mechanisms of gametogenesis and the causes of infertility.

Veni, vidi, vici: the success of wtf meiotic drivers in fission yeast

Meiotic drivers are selfish DNA loci that can bias their own transmission into gametes. Owing to their transmission advantages, meiotic drivers can spread in populations even if the drivers or linked variants decrease organismal fitness. Meiotic drive was first formally described in the 1950s and is thought to be a powerful force shaping eukaryotic genomes. Classic genetic analyses have detected the action of meiotic drivers in plants, filamentous fungi, insects and vertebrates. Several of these drive systems have limited experimental tractability and relatively little is known about the molecular mechanisms of meiotic drive. Recently, however, meiotic drivers were discovered in a yeast species. The Schizosaccharomyces pombe wtf gene family contains several active meiotic drive genes. This review summarizes what is known about the wtf family and highlights its potential as a highly tractable experimental model for molecular and evolutionary characterization of meiotic drive.

wtf genes are prolific dual poison-antidote meiotic drivers

Meiotic drivers are selfish genes that bias their transmission into gametes, defying Mendelian inheritance. Despite the significant impact of these genomic parasites on evolution and infertility, few meiotic drive loci have been identified or mechanistically characterized. Here, we demonstrate a complex landscape of meiotic drive genes on chromosome 3 of the fission yeasts Schizosaccharomyces kambucha and S. pombe. We identify S. kambucha wtf4 as one of these genes that acts to kill gametes (known as spores in yeast) that do not inherit the gene from heterozygotes. wtf4 utilizes dual, overlapping transcripts to encode both a gamete-killing poison and an antidote to the poison. To enact drive, all gametes are poisoned, whereas only those that inherit wtf4 are rescued by the antidote. Our work suggests that the wtf multigene family proliferated due to meiotic drive and highlights the power of selfish genes to shape genomes, even while imposing tremendous costs to fertility.

DOI:http://dx.doi.org/10.7554/eLife.26033.001

Animals, plants and fungi produce sex cells – known as gametes – when they are preparing to reproduce. These cells are made when cells containing two copies of every gene in the organism divide to produce new cells that each only have one copy of each gene. Therefore, a particular gene copy usually has a 50% chance of being carried by each gamete. There is a group of genes that selfishly increase their chances of being transmitted to the next generation by destroying the gametes that do not carry them. These “gamete killer” genes can lead to infertility and other health problems.

Fission yeast is a fungus that is widely used in research. Previous studies revealed that the yeast are likely to have several gamete killers, but the identities of these genes or how they work were not clear. Nuckolls, Bravo Núñez et al. sought to identify at least one gamete killer gene and understand how it works.

The experiments found that a gene called wtf4 acts as a gamete killer in fission yeast. This gene encodes two different proteins, one that acts as a poison and one that acts as an antidote. The antidote remains inside the gametes that contain the wtf4 gene, while the poison is released in the surrounding environment. The poison is capable of killing all of the gametes, but the antidote protects the gametes that contain the wtf4 gene. Further experiments show that wtf4 is just one member of a large family of genes that are also likely to play roles in selectively killing gametes.

A separate study by Hu et al. found that two other members of the wtf family also act as gamete killers in fission yeast. Together, these findings expand our understanding of the nature of gamete killers and how they can contribute to infertility. This may guide the search for gamete killers in humans and other organisms. In the future, gamete killers could potentially be used to eradicate populations of pests that damage crops or spread diseases in humans.

DOI:http://dx.doi.org/10.7554/eLife.26033.002

Chromosome segregation: human female meiosis breaks all the rules

A major hindrance in studying human meiosis has been the inability to assess all four products of female meiosis. Overcoming this hurdle, a new study discovers a high incidence of non-canonical 'reverse meiosis' and a new form of meiotic drive.

Genome rearrangements and pervasive meiotic drive cause hybrid infertility in fission yeast

Hybrid sterility is one of the earliest postzygotic isolating mechanisms to evolve between two recently diverged species. Here we identify causes underlying hybrid infertility of two recently diverged fission yeast species Schizosaccharomyces pombe and S. kambucha, which mate to form viable hybrid diploids that efficiently complete meiosis, but generate few viable gametes. We find that chromosomal rearrangements and related recombination defects are major but not sole causes of hybrid infertility. At least three distinct meiotic drive alleles, one on each S. kambucha chromosome, independently contribute to hybrid infertility by causing nonrandom spore death. Two of these driving loci are linked by a chromosomal translocation and thus constitute a novel type of paired meiotic drive complex. Our study reveals how quickly multiple barriers to fertility can arise. In addition, it provides further support for models in which genetic conflicts, such as those caused by meiotic drive alleles, can drive speciation.

DOI:http://dx.doi.org/10.7554/eLife.02630.001

It is widely thought that all of the billions of species on Earth are descended from a common ancestor. New species are created via a process called speciation, and nature employs various ‘barriers’ to keep closely related species distinct from one another. One of these barriers is called hybrid sterility. Horses and donkeys, for example, can mate to produce hybrids called mules, but mules cannot produce offspring of their own because they are infertile.

Hybrid sterility can occur for a number of reasons. Mules are infertile because they inherit 32 chromosomes from their horse parent, but only 31 chromosomes from their donkey parent—and so have an odd chromosome that they cannot pair-off when they make sperm or egg cells. However, even if a hybrid inherits the same number of chromosomes from each parent, if the chromosomes from the two parents have different structures, the hybrid may still be infertile.

Zanders et al. have now looked at two species of fission yeast—S. pombe and S. kambucha—that share 99.5% of their DNA sequence. Although hybrids of these two species inherit three chromosomes from each parent, the majority of spores (the yeast equivalent of sperm) that these hybrids produce fail to develop into new yeast cells. Zanders et al. identified two causes of this infertility: one of these was chromosomal rearrangement; the other was due to three different sites in the DNA of S. kambucha that interfere with the development of the spores that inherit S. pombe chromosomes.

Since these two yeast species are so closely related, the findings of Zanders et al. reveal how quickly multiple barriers to fertility can arise. In addition, these findings provide further support for models in which conflicts between different genes in genomes can drive the process of speciation.

DOI:http://dx.doi.org/10.7554/eLife.02630.002