A novel family of linear plasmids with homology to plasmid pAL2-1 of Podospora anserina

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.

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.

Control of mitochondrial integrity in ageing and disease

Various molecular and cellular pathways are active in eukaryotes to control the quality and integrity of mitochondria. These pathways are involved in keeping a 'healthy' population of this essential organelle during the lifetime of the organism. Quality control (QC) systems counteract processes that lead to organellar dysfunction manifesting as degenerative diseases and ageing. We discuss disease- and ageing-related pathways involved in mitochondrial QC: mtDNA repair and reorganization, regeneration of oxidized amino acids, refolding and degradation of severely damaged proteins, degradation of whole mitochondria by mitophagy and finally programmed cell death. The control of the integrity of mtDNA and regulation of its expression is essential to remodel single proteins as well as mitochondrial complexes that determine mitochondrial functions. The redundancy of components, such as proteases, and the hierarchies of the QC raise questions about crosstalk between systems and their precise regulation. The understanding of the underlying mechanisms on the genomic, proteomic, organellar and cellular levels holds the key for the development of interventions for mitochondrial dysfunctions, degenerative processes, ageing and age-related diseases resulting from impairments of mitochondria.

Calorie restriction in the filamentous fungus Podospora anserina

Calorie restriction (CR) is a regimen of reduced food intake that, although the underlying mechanism is unknown, in many organisms leads to life span extension. Podospora anserina is one of the few known ageing filamentous fungi and the ageing process and concomitant degeneration of mitochondria have been well-studied. CR in P. anserina increases not only life span but also forestalls the ageing-related decline in fertility. Here we review what is known about CR in P. anserina and about possibly involved mechanisms like enhanced mitochondrial stability, reduced production of reactive oxygen species and changes in the OXPHOS machinery. Additionally, we present new microscopic data on mitochondrial dynamics under rich nutritional and CR conditions at different points in life. Lines that have grown under severe CR for more than 50x the normal life span, show no accumulation of age-related damage, though fecundity is reduced in some of these lines. Finally, we discuss the possible role of CR in P. anserina in nature and the effect of CR at different points in life.

Induction of longevity by cytoplasmic transfer of a linear plasmid in Podospora anserina

In Podospora anserina the longevity inducing linear plasmid pAL2-1 was transferred from the extrachromosomal long-lived mutant AL2 to the short-lived wild-type strain A. The resulting strain, AL2-IV, exhibited the long-lived phenotype. In the short-lived progeny of crosses between this strain and wild-type strain A, the plasmid was absent. In contrast, all long-lived progeny contained both the autonomous plasmid as well as copies of it integrated in the mitochondrial DNA (mtDNA). Molecular analysis revealed that the integrated plasmid copies most likely resulted from a de novo integration of the autonomous element and the generation of AT-linker sequences at the integration site. We conclude that once the plasmid is present in mitochondria of a particular genetic background, it is able to integrate into the mtDNA and to induce longevity.