Biology subjects

Kronholm, I.

Publications and source records attributed to Kronholm, I..

14 recordsLinked to original sources

bioRxiv2026

SAD-6/ATRX enables broad genome surveillance and defense in fungi

The chromatin remodeler ATRX and its orthologs maintain genome function by regulating repetitive DNA and dynamic chromatin, and their activities have been canonically associated with replication-independent deposition of the histone H3.3 variant. This model is difficult to reconcile with fungi, which encode ATRX orthologs but lack H3 variants that may separately support replication-coupled and replication-independent deposition. Here we show that the fungal ATRX ortholog SAD-6 instead relies on a highly divergent histone H4 variant (H4v) to mediate broad genome surveillance and defense. Deposition of H4v is strictly SAD-6-dependent and thus provides a sensitive genome-wide readout of SAD-6 activity, revealing its functions at telomeres, tRNA and rDNA loci, AT-rich DNA, artificial transgenes, decaying mobile elements, and many genic regions. We further show that SAD-6 is required for a pathway of repeat-induced point mutation (RIP) that also requires DIM-5, a conserved SUV39 methyltransferase that mediates trimethylation of histone H3 lysine-9 in heterochromatin. Together, these findings establish ATRX-like remodelers as broad regulators of genome surveillance and defense in fungi that act through a highly divergent histone H4 variant rather than H3.3. Given that RIP is proposed to recognize repetitive DNA via recombination-independent homologous pairing, the requirement for SAD-6 in RIP suggests that ATRX-like remodelers may couple DNA pairing to heterochromatin nucleation on repeats.

molecular biology↗

bioRxiv2026

Centromeres are hotspots of cytosine methylation epimutations in a filamentous fungus

Epimutations are changes in chromatin modifications, such as DNA methylation or histone modifications. Some of these epigenetic changes can be inherited for several generations, and they potentially contribute to evolutionary processes. Estimates of epimutation rates now exists in a few species, but the presence and function of epigenetic marks are not conserved across different species. To understand the properties of epimutations in fungi, we performed a mutation accumulation experiment with the filamentous fungus Neurospora crassa and investigated spontaneous changes in DNA methylation and trimethylation of lysine 9 on histone H3 (H3K9me3) in the mutation accumulation lines. We observed that centromeric regions are hotspots of spontaneous DNA methylation changes in N. crassa. In these hotspot regions, DNA methylation changes were transmitted across mitoses, but changes occurring in euchromatin were not maintained. The rate of DNA methylation changes was around 30 000 fold faster than the genetic mutation rate. We did not observe spontaneous changes in H3K9me3 that were transmitted across mitoses. Our results show that while spontaneous epimutations occur in this species, they occur predominantly in gene poor heterochromatic regions, so their impact for evolutionary adaptation may be limited.

genetics↗

bioRxiv2024

The effect of population size on adaptation to fluctuating temperatures

Climate change exposes populations to more frequent periods of extreme temperatures and faster temperature fluctuations. Theoretical models suggest that different types of adaptations should occur in constant versus fluctuating environments of varying frequency. Furthermore, evolutionary adaptation to one environment may weaken the adaptations to alternative environments due to antagonistically pleiotropic alleles. However, fitness trade-offs are rarely observed in experiments and it has been hypothesized that the number and severity of trade-offs evolving in fluctuating environments might depend on population size. To evaluate whether specific types of adaptations evolve at fluctuating temperatures and how population size affects the evolution of trade-offs, we performed an evolution experiment with fission yeast (Schizosaccharomyces pombe). The small and large populations evolved for 500 generations at constant and fluctuating temperatures, after which the evolved strains competed against ancestral strains in respective selection environments and in alternative environments to detect trade-offs. We observed significant adaptation and maladaptation only to constant heat, but not to fluctuating temperatures. Overall, the population size did not have significant effects on adaptation capacity or trade-offs in alternative environments. Our results suggest that constant extreme temperatures may act as stronger selective pressures than temperature variation and that trade-offs are unlikely to constrain adaptation to fluctuating temperatures.

evolutionary biology↗

bioRxiv2024

Genome-wide association mapping for growth rate at fluctuating and extreme temperatures

Constant temperatures and fluctuations of varying frequency affect fitness differently, which has led to suggestions of distinct genetic architectures and adaptation strategies between constant and fluctuating thermal environments. However, very little is known about the possible fitness trade-offs and genetic constraints underlying thermal adaptation and how they affect species ability to confront climatic changes. We addressed this gap in knowledge by integrating quantitative genetics and genome-wide association mapping in the filamentous fungus Neurospora crassa. Growth rates were measured for 434 strains under fast and slow frequency fluctuations, at high and low temperature range with respect to the species tolerance, and at constant mean and extreme temperatures of these ranges. We found strong genetic correlations between fast and slow frequency fluctuations, and between fluctuations and their mean temperatures, but not with the highest extreme temperature. Positive correlations were supported by high heritability values, pointing that in N. crassa there are no significant trade-offs or genetic constraints in adaptation when variance in temperature increases. Altogether, our results indicated clearly polygenic basis of thermal tolerance, with most of the variation in overall performance (83 %), and clearly less in hot-cold trade-off (8 %), or heat stress tolerance (4 %). Interestingly, GWAS discovered many SNPs associated with growth rate only at constant temperatures or at fast and slow fluctuations at high and low thermal range. However, the cellular functions of the associated genes were overlapping, and no opposite allelic effects were found between treatments. Hence, large-effect loci indicated no trade-offs, but a shared physiology across temperatures, probably owing to the general stress response or individuals overall fitness.

evolutionary biology↗

bioRxiv2024

Genome-wide association for loci influencing thermal performance curves in Neurospora crassa

Temperature poses a unique challenge to ectothermic species, as it affects all biochemical reactions in the cell and causes physiological stress. The effect of temperature on an organism can be described by a thermal performance curve (TPC), which displays organismal performance, such as growth rate, as a function of temperature. Previous studies on thermal performance have revealed different amounts of genetic variation and trade-offs in TPC shape and position within species and populations. However, very little is known about the genetic architecture of TPCs on the level of individual loci and alleles. We asked what is the identity of loci contributing to genetic variation in TPCs, and do the alleles exhibit trade-offs or thermodynamic scaling across the temperature range? We used genome-wide association mapping to find loci influencing growth rate at different temperatures and TPC traits in the filamentous fungus Neurospora crassa. We also evaluated the directions and magnitudes of allelic effects to investigate possible trade-offs. We observed both unique associations at specific temperatures, as some loci affected growth rate only at low, intermediate, or high temperatures, and associations that were shared across multiple temperatures. However, only weak evidence of trade-offs was detected, indicating that the evolution of TPCs in N. crassa is not constrained by allelic effects in opposite directions at hot and cold temperatures. Our findings indicate that trade-offs contribute little to variation in TPCs.

genetics↗

bioRxiv2022

Parental effects in a filamentous fungus: phenotype, fitness, and mechanism

Adaptation to changing environments often requires meaningful phenotypic modifications to match the current conditions. However, obtaining information about the surroundings during an organisms own lifetime may only permit accommodating relatively late developmental modifications. Therefore, it may be advantageous to rely on inter-generational or trans-generational cues that provide information about the environment as early as possible to allow development along an optimal trajectory. Transfer of information or resources across generations, known as parental effects, is well documented in animals and plants but not in other eukaryotes, such as fungi. Understanding parental effects and their evolutionary consequences in fungi is of vital importance as they perform crucial ecosystem functions. In this study, we investigated whether parental effects are present in the filamentous fungus Neurospora crassa, how long do they last, are the effects adaptive, and what is their mechanism. We performed a fully factorial match / mismatch experiment for a good and poor quality environment, in which we measured mycelium size of strains that experienced either a matched or mismatched environment in their previous generation. We found a strong silver spoon effect in initial mycelium growth, which lasted for one generation, and increased fitness during competition experiments. By using deletion mutants that lacked key genes in epigenetic processes, we show that epigenetic mechanisms are not involved in this effect. Instead, we show that spore glycogen content, glucose availability and a radical transcription shift in spores are the main mechanisms behind this parental effect.

microbiology↗

bioRxiv2022

Variation in spontaneous mutation rate and spectrum across the genome of Neurospora crassa

While mutation rates have been extensively studied, variation in mutation rates throughout the genome is poorly understood. To understand patterns of genetic variation, it is important to understand how mutation rates vary. Chromatin modifications may be an important factor in determining variation in mutation rates in eukaryotic genomes. To study variation in mutation rates, we performed a mutation accumulation experiment in the filamentous fungus Neurospora crassa, and sequenced the genomes of the 40 MA lines that had been propagated asexually for approximately 1015 [1003, 1026] mitoses. We detected 1322 mutations in total, and observed that the mutation rate was higher in regions of low GC, in domains of H3K9 trimethylation, in centromeric regions, and in domains of H3K27 trimethylation. The rate of single nucleotide mutations in euchromatin was 2.46 [2.19, 2.77] x 10-10. In contrast, the mutation rate in H3K9me3 domains was tenfold higher: 2.43 [2.25, 2.62] x 10-9. We also observed that the spectrum of single nucleotide mutations was different between H3K9me3 and euchromatic domains. Our statistical model of mutation rate variation predicted a moderate amount of extant genetic variation, suggesting that the mutation rate is an important factor in determining levels of natural genetic variation. Furthermore, we characterized mutation rates of structural variants, complex mutations, and the effect of local sequence context on the mutation rate. Our study highlights that chromatin modifications are associated with mutation rates, and accurate evolutionary inferences should take variation in mutation rates across the genome into account.

genetics↗

bioRxiv2020

Evolution of anticipatory effects mediated by epigenetic changes

Anticipatory effects mediated by epigenetic changes occur when parents modify the phenotype of their offspring by making epigenetic changes in their gametes guided by information from an environmental cue. To investigate when do anticipatory effects mediated by epigenetic changes evolve in a fluctuating environment, I use an individual based simulation model with explicit genetic architecture. The model allows for the population to respond to environmental changes by evolving plasticity, bet-hedging, or by tracking the environment with genetic adaptation, in addition to the evolution of anticipatory effects. The results show that anticipatory effects evolve when the environmental cue provides reliable information about the environment and the environment changes at intermediate rates, provided that fitness costs of anticipatory effects are rather low. Moreover, evolution of anticipatory effects is quite robust to different genetic architectures when reliability of the environmental cue is high. Anticipatory effects always give smaller fitness benefits than within generation plasticity, suggesting a possible reason for generally small observed anticipatory effects in empirical studies.

evolutionary biology↗

bioRxiv2020

Quantitative genetics of temperature performance curves of Neurospora crassa

Earths temperature is increasing due to anthropogenic CO2 emissions; and organisms need either to adapt to higher temperatures, migrate into colder areas, or face extinction. Temperature affects nearly all aspects of an organisms physiology via its influence on metabolic rate and protein structure, therefore genetic adaptation to increased temperature may be much harder to achieve compared to other abiotic stresses. There is still much to be learned about the evolutionary potential for adaptation to higher temperatures, therefore we studied the quantitative genetics of growth rates in different temperatures that make up the thermal performance curve of the fungal model system Neurospora crassa. We studied the amount of genetic variation for thermal performance curves and examined possible genetic constraints by estimating the G-matrix. We observed a substantial amount of genetic variation for growth in different temperatures, and most genetic variation was for performance curve elevation. Contrary to common theoretical assumptions, we did not find strong evidence for genetic trade-offs for growth between hotter and colder temperatures. We also simulated short term evolution of thermal performance curves of N. crassa, and suggest that they can have versatile responses to selection.

evolutionary biology↗

bioRxiv2019

Environmentally triggered evolutionary cascade across trophic levels in an experimental phage-bacteria-insect system

Viruses are key actors of ecosystems and have major impacts on global biogeochemical cycles. Prophages deserve particular attention as they are ubiquitous in bacterial genomes and can enter a lytic cycle when triggered by environmental conditions. We explored how temperature affects the interactions between prophages and other biological levels by using an opportunistic pathogen, the bacterium Serratia marcescens, that harbours several prophages and that had undergone an evolution experiment under several temperature regimes. We found that the release of one of the prophages was temperature-sensitive and malleable to evolutionary changes. We further discovered that the virulence of the bacterium in an insect model also evolved and was positively correlated with phage release rates. We determined through analysis of genetic and epigenetic data that changes in the outer cell wall structure possibly explain this phenomenon. We hypothezise that the temperature-dependent phage release rate acted as a selection pressure on S. marcescens and that it resulted in modified bacterial virulence in the insect host. Our study system illustrates how viruses can mediate the influence of abiotic environmental changes to other biological levels and thus be involved in ecosystem feedback loops.

evolutionary biology↗

bioRxiv2019

Roles of adenosine and cytosine methylation changes and genetic mutations in adaptation to different temperatures

Epigenetic modifications can contribute to adaptation, but the relative contributions of genetic and epigenetic variation are unknown. Previous studies on the role of epigenetic changes in adaptation in eukaryotes have nearly exclusively focused on cytosine methylation (m5C), while prokaryotes exhibit a richer system of methyltransferases targetting adenines (m6A) or cytosines (m4C, m5C). DNA methylation in prokaryotes has many roles, but its potential role in adaptation still needs further investigation. We collected phenotypic, genetic, and epigenetic data using single molecule real-time sequencing of clones of the bacterium Serratia marcescens that had undergone experimental evolution in contrasting temperatures to investigate the relationship between environment and genetic, epigenetic, and phenotypic changes. This data provided a detailed description of the methylation landscape of S. marcescens and allowed us to examine the potential contributions of genetic and epigenetic changes to phenotypic adaptation. The genomic distribution of GATC motifs, which are the main target for m6A methylation, and of partially methylated epiloci pointed to a link between m6A methylation and regulation of gene expression in S. marcescens. Evolved strains, while genetically homogeneous, exhibited many polymorphic m6A epiloci. There was no strong support for a genetic control of methylation changes in our experiment, and no clear evidence of parallel environmentally-induced or environmentally-selected methylation changes at specific epiloci was found. Both some genetic and epigenetic variants were associated with some phenotypic traits. Overall, our results suggest that both genetic and adenine methylation changes have potential to contribute to phenotypic adaptation in S. marcescens, but that any environmentally-induced epigenetic change occurring in our experiment would probably have been quite labile.

evolutionary biology↗

bioRxiv2019

Marked Neurospora crassa strains for competition experiments and Bayesian methods for fitness estimates

The filamentous fungus Neurospora crassa, a model microbial eukaryote, has a life cycle with many features that make it suitable for studying experimental evolution. However, it has lacked a general tool for estimating relative fitness of different strains in competition experiments. To remedy this need, we constructed N. crassa strains that contain a modified csr-1 locus and developed an assay for detecting the proportion of the marked strain using a post PCR high resolution melting assay. DNA extraction from spore samples can be performed on 96-well plates, followed by a PCR step, which allows many samples to be processed with ease. Furthermore, we suggest a Bayesian approach for estimating relative fitness from competition experiments that takes into account the uncertainty in measured strain proportions. We show that there is a fitness effect of the mating type locus, as mating type mat a has a higher competitive fitness than mat A. The csr-1* marker also has a small fitness effect, but is still a suitable marker for competition experiments. As a proof of concept, we estimate the fitness effect of the qde-2 mutation, a gene in the RNA interference pathway, and show that its competitive fitness is lower than what would be expected from its mycelial growth rate alone.

genetics↗

bioRxiv2017

Effects of acclimation time and epigenetic mechanisms on growth of Neurospora in fluctuating environments

Reaction norms or tolerance curves have often been used to predict how organisms deal with fluctuating environments. A potential drawback is that reaction norms measured in different constant environments may not capture all aspects of organismal responses to fluctuating environments. We examined growth of the filamentous fungus Neurospora crassa in fluctuating temperatures and tested if growth in fluctuating temperatures can be explained simply by growth in different constant temperatures or if more complex models are needed. In addition, as previous studies on fluctuating environments have revealed that past temperatures that organisms have experienced can affect their response to current temperature, we tested the roles of different epigenetic mechanisms in response to fluctuating environments using different mutants. We found that growth of Neurospora can be predicted in fluctuating temperatures to some extent if acclimation times are taken into account in the model. Interestingly, while fluctuating environments have been linked with epigenetic responses we found only some evidence of involvement of epigenetic mechanisms on tolerating fluctuating temperatures. Mutants which lacked H3K4 or H3K36 methylation had slightly impaired response to temperature fluctuations, in addition the H3K4 methylation mutant and a mutant in the RNA interference pathway had altered acclimation times.

genetics↗

bioRxiv2017

Ecological plant epigenetics: Evidence from model and non-model species, and the way forward

Growing evidence makes a strong case that epigenetic mechanisms contribute to complex traits, with implications across many fields of biology from dissecting developmental processes to understanding aspects of human health and disease. In ecology, recent studies have merged ecological experimental design with epigenetic analyses to elucidate the contribution of epigenetics to plant phenotypes, stress response, adaptation to habitat, or species range distributions. While there has been some progress in revealing the role of epigenetics in ecological processes, many studies with non-model species have so far been limited to describing broad patterns based on anonymous markers of DNA methylation. In contrast, studies with model species have benefited from powerful genomic resources, which allow for a more mechanistic understanding but have limited ecological realism. To understand the true significance of epigenetics for plant ecology and evolution, we must combine both approaches transferring knowledge and methods from model-species research to genomes of evolutionarily divergent species, and examining responses to complex natural environments at a more mechanistic level. This requires transforming genomics tools specifically for studying non-model species, which is challenging given the large and often polyploid genomes of plants. Collaboration between molecular epigeneticists, ecologists and bioinformaticians promises to enhance our understanding of the mutual links between genome function and ecological processes.

ecology↗