Non-Mendelian determinants of morphology in fungi

Phenotypic instability in fungi

Fungi are prone to phenotypic instability, that is, the vegetative phase of these organisms, be they yeasts or molds, undergoes frequent switching between two or more behaviors, often with different morphologies, but also sometime having different physiologies without any obvious morphological outcome. In the context of industrial utilization of fungi, this can have a negative impact on the maintenance of strains and/or on their productivity. Instabilities have been shown to result from various mechanisms, either genetic or epigenetic. This chapter will review different types of instabilities and discuss some lesser-known ones, mostly in filamentous fungi, while it will direct readers to additional literature in the case of well-known phenomena such as the amyloid prions or fungal senescence. It will present in depth the "white/opaque" switch of Candida albicans and the "crippled growth" degeneration of the model fungus Podospora anserina. These are two of the most thoroughly studied epigenetic phenotypic switches. I will also discuss the "sectors" presented by many filamentous ascomycetes, for which a prion-based model exists but is not demonstrated. Finally, I will also describe intriguing examples of phenotypic instability for which an explanation has yet to be provided.

Using the residue of spirit production and bio-ethanol for protein production by yeasts

The residue (vinasse) formed during the distillation of bio-ethanol and cachaça, a traditional rum-type spirit produced from sugar-cane in Brazil, is highly harmful if discharged into the environment due to high values of COD and BOD. One possibility for minimizing the impact of vinasse in soils and waters is to use the residue in the production of microbial biomass for use as an animal feed supplement that will provide high levels on nitrogen (>9% d.m.) and low content of nucleic (≤ 10% d.m.) This paper reports the production and quality of biomass produced from fermentation of Saccharomyces cerevisiae and Candida parapsilosis in culture media under 12 different culture conditions and the respective effects of each variable (glucose, yeast extract, peptone, potassium phosphate, vinasse, pH and temperature). Of the S. cerevisiae isolates tested, two (VR1 and PE2) originating from fuel alcohol-producing plants were identified as offering the best potential for the industrial production of single cell protein from vinasse due to highest biomass productivity. Our results showed a potential viable and economic use of vinasse.

Transgenerational epigenetic inheritance: prevalence, mechanisms, and implications for the study of heredity and evolution

This review describes new developments in the study of transgenerational epigenetic inheritance, a component of epigenetics. We start by examining the basic concepts of the field and the mechanisms that underlie epigenetic inheritance. We present a comprehensive review of transgenerational cellular epigenetic inheritance among different taxa in the form of a table, and discuss the data contained therein. The analysis of these data shows that epigenetic inheritance is ubiquitous and suggests lines of research that go beyond present approaches to the subject. We conclude by exploring some of the consequences of epigenetic inheritance for the study of evolution, while also pointing to the importance of recognizing and understanding epigenetic inheritance for practical and theoretical issues in biology.

Prion proteins as genetic material in fungi

Prions are infectious proteins. Several prions have been identified in fungi where they behave as non-Mendelian cytoplasmic genetic elements. Most of these prions propagate as self-perpetuating amyloid aggregates thus providing an example of structural heredity. In yeast, prion propagation requires the Hsp104 disaggregase presumably to sheer amyloid assemblies and generate more fiber ends. Recent work in yeast shows that amyloid structure polymorphism underlies the prion strain phenomenon and influences species barriers. Structural models for the amyloid form of several fungal prion proteins are now available. All propose a cross beta-organization with parallel beta-sheets. Whether or not some of the fungal prions might be beneficial to their host is still a debated issue.

Genetic control of an epigenetic cell degeneration syndrome in Podospora anserina

Filamentous fungi frequently present degenerative processes, whose molecular basis is very often unknown. Here, we present three mutant screens that result in the identification of 29 genes that directly or indirectly control Crippled Growth (CG), an epigenetic cell degeneration of the filamentous ascomycete Podospora anserina. Two of these genes were previously shown to encode a MAP kinase kinase kinase and an NADPH oxidase involved in a signal transduction cascade that participates in stationary phase differentiations, fruiting body development and defence against fungal competitors. The numerous genes identified can be incorporated in a model in which CG results from the sustained activation of the MAP kinase cascade. Our data also emphasize the complex regulatory network underlying three interconnected processes in P. anserina: sexual reproduction, defence against competitors, and cell degeneration.

Mutation and epimutation load in haploid and diploid life forms

Epigenetic differentiation is the potentially heritable changes in levels of gene expression not caused by DNA sequence changes. Here, a classification scheme of mutations and epimutations is introduced, enabling a simple analysis of mutation and epimutation load in haploid and diploid organisms. It is found that the deleterious effect of epimutations is mainly determined by epimutation rate and degree of reversibility. Inherited epimutations have the same fitness consequences as inherited mutations. With complete reversibility and no inheritance, then epimutations have the same fitness consequences as somatic mutations. It is argued that organisms with somatic inheritance may experience more genetic load than organisms without somatic inheritance due to inherited epimutations in the former. This may partly explain the maintenance of soma/germ differentiation in many life forms. It is also argued that masking of deleterious somatic mutations may not necessarily explain the evolution of diploidy in life forms with inherited epimutations.

Bistability and hysteresis of the 'Secteur' differentiation are controlled by a two-gene locus in Nectria haematococca

Background

Bistability and hysteresis are increasingly recognized as major properties of regulatory networks governing numerous biological phenomena, such as differentiation and cell cycle progression. The full scope of the underlying molecular mechanisms leading to bistability and hysteresis remains elusive. Nectria haemaotcocca, a saprophytic or pathogenic fungus with sexual reproduction, exhibits a bistable morphological modification characterized by a reduced growth rate and an intense pigmentation. Bistability is triggered by the presence or absence of σ, a cytoplasmic determinant. This determinant spreads in an infectious manner in the hyphae of the growing margin, insuring hysteresis of the differentiation.

Results

Seven mutants specifically affected in the generation of σ were selected through two different screening strategies. The s₁ and s₂ mutations completely abolish the generation of σ and of its morphological expression, the Secteur. The remaining five mutations promote its constitutive generation, which determines an intense pigmentation but not growth alteration. The seven mutations map at the same locus, Ses (for 'Secteur-specific'). The s₂ mutant was obtained by an insertional mutagenesis strategy, which permitted the cloning of the Ses locus. Sequence and transcription analysis reveals that Ses is composed of two closely linked genes, SesA, mutated in the s₁ and s₂ mutant strains, and SesB, mutated in the s* mutant strains. SesB shares sequence similarity with animal and fungal putative proteins, with potential esterase/lipase/thioesterase activity, whereas SesA is similar to proteins of unknown function present only in the filamentous fungi Fusarium graminearum and Podospora anserina.

Conclusions

The cloning of Ses provides evidence that a system encoded by two linked genes directs a bistable and hysteretic switch in a eukaryote. Atypical regulatory relations between the two proteins may account for the hysteresis of Secteur differentiation.