Sex and deleterious mutations

The evolutionary advantage of sexual reproduction has been considered as one of the most pressing questions in evolutionary biology. While a pluralistic view of the evolution of sex and recombination has been suggested by some, here we take a simpler view and try to quantify the conditions under which sex can evolve given a set of minimal assumptions. Since real populations are finite and also subject to recurrent deleterious mutations, this minimal model should apply generally to all populations. We show that the maximum advantage of recombination occurs for an intermediate value of the deleterious effect of mutations. Furthermore we show that the conditions under which the biggest advantage of sex is achieved are those that produce the fastest fitness decline in the corresponding asexual population and are therefore the conditions for which Muller's ratchet has the strongest effect. We also show that the selective advantage of a modifier of the recombination rate depends on its strength. The quantification of the range of selective effects that favors recombination then leads us to suggest that, if in stressful environments the effect of deleterious mutations is enhanced, a connection between sex and stress could be expected, as it is found in several species.     

Betaine-homocysteine methyltransferase in the fungus Aspergillus nidulans.

A betaine:homocysteine methyltransferase activity was demonstrated in the cell-free extracts from the fungus $\text{Aspergillus}\text{nidulans}$. Among methionine-requiring mutants which do not grow on homocysteine one class responds to betaine indicating that this compound can serve as a methyl donor in methionine synthesis $\text{in vivo}$. Mutants of the second class which grow only on methionine were shown to have betaine: homocysteine — and methyltetrahydrofolate: homocysteine methyltransferases simultaneously impaired.     

Repair of alkylation damage in the fungus Aspergillus nidulans

The repair of alkylation damage in Aspergillus nidulans was investigated. We have assayed soluble protein fractions for enzymes known to be involved in the repair of this type of damage in DNA. The presence of a glycosylase activity that can remove 3-methyladenine from DNA was demonstrated, as well as a DNA methyltransferase activity that appears to act against O6-methylguanine. In addition to this approach, a series of mutants were isolated which display increased sensitivity to alkylating agents (sag mutants). 5 such mutants were further characterized, and at least 4 are shown to map to genes which have not previously been characterized. The behaviour of double mutant combinations demonstrates the existence of at least 2 pathways for the repair of alkylation damage. The majority of the sag mutants (sagA1, sagB2, sag4 and sagE5) exhibit an increased sensitivity to a range of alkylating agents, but not to UV light, while sagC3, when irradiated at the germling stage, also shows sensitivity to UV. None of the mutants isolated are defective in either the 3-methyladenine DNA glycosylase activity, or the DNA methyltransferase activity, and the nature of the defects in these strains remains to be determined.     

Mitotic recombination accelerates adaptation in the fungus Aspergillus nidulans

Understanding the prevalence of sexual reproduction in eukaryotes is a hard problem. At least two aspects still defy a fully satisfactory explanation, the functional significance of genetic recombination and the great variation among taxa in the relative lengths of the haploid and diploid phases in the sexual cycle. We have performed an experimental study to explore the specific advantages of haploidy or diploidy in the fungus Aspergillus nidulans. Comparing the rate of adaptation to a novel environment between haploid and isogenic diploid strains over 3,000 mitotic generations, we demonstrate that diploid strains, which during the experiment have reverted to haploidy following parasexual recombination, reach the highest fitness. This is due to the accumulation of recessive deleterious mutations in diploid nuclei, some of which show their combined beneficial effect in haploid recombinants. Our findings show the adaptive significance of mitotic recombination combined with flexibility in the timing of ploidy level transition if sign epistasis is an important determinant of fitness.     

New indole-diterpenoids from the algal-associated fungus Aspergillus nidulans

Two new compounds, including a chlorinated indole-diterpenoid 19-hydroxypenitrem A (1) and its dechlorinated derivative 19-hydroxypenitrem E (2), along with two known congeners (3 and 4), were isolated and identified from the cultures of Aspergillus nidulans EN-330, an endophytic fungus obtained from the marine red alga Polysiphonia scopulorum var. villum. Their structures and absolute configurations were assigned on the basis of 1D and 2D NMR and CD experiments. Compounds 1–4 exhibited cytotoxic activity against brine shrimp with LD₅₀ values of 3.2, 4.6, 1.7, and 8.7 μM, respectively. Besides, the chlorinated 19-hydroxypenitrem A (1) showed antimicrobial activity against four human- and aqua-pathogens. Preliminary SAR study revealed that the Cl-substitution at C-6 enhanced the cytotoxic activity against brine shrimp and antimicrobial activity, while the 19-OH substitution suppressed the activity.     

Osmotic adjustment in the filamentous fungus Aspergillus nidulans.

Aspergillus nidulans was shown to be xerotolerant, with optimal radial growth on basal medium amended with 0.5 M NaCl (osmotic potential [psi s] of medium, -3 MPa), 50% optimal growth on medium amended with 1.6 M NaCl (psi s of medium, -8.7 MPa), and little growth on medium amended with 3.4 M NaCl (psi s of medium, -21 MPa). The intracellular content of soluble carbohydrates and of selected cations was measured after growth on basal medium, on this medium osmotically amended with NaCl, KCl, glucose, or glycerol, and also after hyperosmotic and hypoosmotic transfer. The results implicate glycerol and erythritol as the major osmoregulatory solutes. They both accumulated during growth on osmotically amended media, as well as after hyperosmotic transfer, except on glycerol-amended media, in which erythritol did not accumulate. Furthermore, they both decreased in amount after hypoosmotic transfer. With the exception of glycerol, the extracellular osmotic solute did not accumulate intracellularly when mycelium was grown in osmotically amended media, but it accumulated after hyperosmotic transfer. It was concluded that the extracellular solute usually plays only a transient role in osmotic adaptation. The intracellular content of soluble carbohydrates and cations measured could reasonably account for the intracellular osmotic potential of mycelium growing on osmotically amended media.     

Farnesol-induced cell death in the filamentous fungus Aspergillus nidulans

FOH (farnesol), a non-sterol isoprenoid produced by dephosphorylation of farnesyl pyrophosphate, has been shown to inhibit proliferation and induce apoptosis. We have been using Aspergillus nidulans and FOH as a model system and cell death stimulus, respectively, aiming to understand by which means filamentous fungi are driven towards cell death. Here, we review some of our findings about FOH-induced cell death in A. nidulans.     

Glycerol uptake mutants of the hyphal fungus Aspergillus nidulans

A new class of glycerol non-utilizing mutants, designated glcC, has been isolated. The glcC gene was mapped in linkage group VI and mutants were found to complement the reference strains glcA1 (linkage group V) and glcB33 (linkage group I) in diploids. The new mutants were unable to grow on glycerol. However, in contrast to the glcA and glcB phenotype these mutants did grow well on dihydroxyacetone and D-galacturonate. By in vivo 13C NMR spectroscopy it was shown that the glcC mutant did not take up glycerol but did take up dihydroxyacetone. The latter substrate was converted intracellularly into glycerol which was then catabolized as normal.     

Lactose and D-galactose catabolism in the filamentous fungus Aspergillus nidulans

The disaccharide lactose is a byproduct of cheese production accumulating to amounts of 800,000 tons per year worldwide, of which 15% is used as a carbon source for various microbial fermentations. Nevertheless, little is known about the regulation of its metabolism in filamentous fungi. Lactose is metabolized slowly, and some important fungi such as A. niger cannot use it at all. A more detailed knowledge on the rate-limiting steps would be helpful to improve its industrial application. We have chosen A. nidulans as an object for investigating how lactose and galactose metabolism are regulated because it has long become a model system for biochemical and genetic research on fungi, and mutants in the lactose-metabolizing pathway of A. nidulans are available. In this paper, we will review the contributions of our research group achieved on this field.     

Conditionally lethal tubA alpha-tubulin mutations in Aspergillus nidulans

Summary We have mapped 17 extragenic suppressors of benA 33, a heat-sensitive -tubulin mutation of Aspergillus nidulans, to the tubA tubulin locus. Fifteen of these tubA mutations cause cold sensitivity in a genetic background with benA 33 and appear to cause lethality in a background with the wild-type benA allele. We examined the microtubule-mediated processes, nuclear division and nuclear migration, in seven different cold-sensitive double mutants, each carrying benA 33 and a different cold-sensitive tubA allele. Nuclear division and migration were inhibited at a restrictive temperature in each case, suggesting that cold sensitivity is due to the inhibition of microtubule function at low temperatures. A single allele, tubA4, suppressed the heat sensitivity conferred by benA33 but did not confer cold sensitivity in a benA33 background, however in a wildtype benA background, tubA4 conferred supersensitivity to antimicrotubule agents and weak cold sensitivity. TubA4 did not suppress the heat sensitivity conferred by two other benA alleles. The cold sensitivity conferred by tubA4 was suppressed by the microtubule stabilizing agent deuterium oxide, and the suppression of heat sensitivity conferred by four other tubA mutations was reversed by deuterium oxide. These results suggest that these mutations may affect hydrophobic interactions between -and -tubulin.     

On the accumulation of deleterious mutations during range expansions

We investigate the effect of spatial range expansions on the evolution of fitness when beneficial and deleterious mutations cosegregate. We perform individual‐based simulations of 1D and 2D range expansions and complement them with analytical approximations for the evolution of mean fitness at the edge of the expansion. We find that deleterious mutations accumulate steadily on the wave front during range expansions, thus creating an expansion load. Reduced fitness due to the expansion load is not restricted to the wave front, but occurs over a large proportion of newly colonized habitats. The expansion load can persist and represent a major fraction of the total mutation load for thousands of generations after the expansion. The phenomenon of expansion load may explain growing evidence that populations that have recently expanded, including humans, show an excess of deleterious mutations. To test the predictions of our model, we analyse functional genetic diversity in humans and find patterns that are consistent with our model.     

Chromosome-specific recombinant DNA libraries from the fungus Aspergillus nidulans.

Development of physical genomic maps is facilitated by identification of overlapping recombinant DNA clones containing long chromosomal DNA inserts. To simplify the analysis required to determine which clones in a genomic library overlap one another, we partitioned Aspergillus nidulans cosmid libraries into chromosome-specific subcollections. The eight A. nidulans chromosomes were resolved by pulsed field gel electrophoresis and hybridized to filter replicas of cosmid libraries. The subcollections obtained appeared to be representative of the chromosomes based on the correspondence between subcollection size and chromosome length. A sufficient number of clones was obtained in each chromosome-specific subcollection to predict the overlap and assembly of individual clones into a limited number of contiguous regions. This approach should be applicable to many organisms whose genomes can be resolved by pulsed field gel electrophoresis.