Fungal fidelity: nuclear divorce from a dikaryon by mating or monokaryon regeneration

Sexual reproduction is the null hypothesis for life cycles of rust fungi

Sexual reproduction, mutation, and reassortment of nuclei increase genotypic diversity in rust fungi. Sexual reproduction is inherent to rust fungi, coupled with their coevolved plant hosts in native pathosystems. Rust fungi are hypothesised to exchange nuclei by somatic hybridisation with an outcome of increased genotypic diversity, independent of sexual reproduction. We provide criteria to demonstrate whether somatic exchange has occurred, including knowledge of parental haplotypes and rejection of fertilisation in normal rust life cycles.

Nuclear conditions of basidiospores and hyphal cells in the edible mushroom Oudemansiella aparlosarca

Oudemansiella aparlosarca is an edible mushroom possessing medicinal and health benefits. Although there are studies on the cultivation of O. aparlosarca, only a few studies have focused on its genetics and life cycle. Therefore, the main objective of this study was to identify the nuclear conditions of basidiospores and homokaryotic and heterokaryotic hyphal cells and to determine the influence of different nuclear conditions on basidiospore diameter in O. aparlosarca. Two parental strains: strain-55 and strain-81 were used. Staining of basidiospores and hyphal cells in the apical region was performed. We observed the following nuclear conditions: non-nucleate, mononucleate, binucleate, and multinucleate. In both parental strains, binucleate spores were predominant, while the number of non-nucleate spores was the lowest. The diameter of non-nucleate spores was the smallest, being 11.52 µm and 12.15 µm in parental strain-81 and strain-55, respectively, while multinucleate spores had the largest diameter, being 14.78 µm in both parental strains. Both homokaryotic and heterokaryotic strains were identified in isolated single spores from parental strains. Binucleate cells were majorly present in heterokaryotic hyphal cells, and multinucleate cells were predominant in homokaryotic hyphal cells. We conclude that O. aparlosarca contains homokaryotic and heterokaryotic basidiospores, which indicates an amphithallic life cycle. The observed binucleate spores might be the result of post-meiotic mitosis.

Variations in Nuclear Number and Size in Vegetative Hyphae of the Edible Mushroom Lentinula edodes

In basidiomycete fungi, the number of nuclei and their ploidy level per nucleus can vary tremendously among species; however, within species, nuclear number and ploidy levels are traditionally considered fixed in their vegetative hyphae. In the edible mushroom Lentinula edodes, the hyphae are classified as either monokaryotic or dikaryotic, with each monokaryotic hyphal cell containing one haploid nucleus, and each dikaryotic hyphal cell containing two haploid nuclei. The dikaryotic hyphae are the results of mating between two genetically distinct monokaryons with different mating types. In this study, we examined the nuclear number and size (a potential correlate to ploidy) of L. edodes mycelia throughout its vegetative growth. We found that the number of nuclei within individual hyphal cells varied widely from non-nucleated to uninucleated, dinucleated, and multinucleated. Additionally, different nuclei within the same cell appeared very different in size, with a maximum nucleus cross-sectional area of 4.94 μm2 and the minimum nucleus cross-sectional area at only 0.37 μm2. Moreover, as culture time increased, more cells appeared to be devoid of any nuclei, with transmission electron microscopy and terminal deoxynucleotidyl transferase dUTP nick-end labeling (TUNEL) assays of late-stage cultures showing autophagosomes fusing and dissolving the nuclei and resulting in a large number of TUNEL-positive DNA fragments in non-nucleated cells. These results indicated that non-nucleated cells were likely caused by autophagy and apoptosis-like activities within aging L. edodes hyphae.

Evolutionarily advanced ant farmers rear polyploid fungal crops

Innovative evolutionary developments are often related to gene or genome duplications. The crop fungi of attine fungus-growing ants are suspected to have enhanced genetic variation reminiscent of polyploidy, but this has never been quantified with cytological data and genetic markers. We estimated the number of nuclei per fungal cell for 42 symbionts reared by 14 species of Panamanian fungus-growing ants. This showed that domesticated symbionts of higher attine ants are polykaryotic with 7-17 nuclei per cell, whereas nonspecialized crops of lower attines are dikaryotic similar to most free-living basidiomycete fungi. We then investigated how putative higher genetic diversity is distributed across polykaryotic mycelia, using microsatellite loci and evaluating models assuming that all nuclei are either heterogeneously haploid or homogeneously polyploid. Genetic variation in the polykaryotic symbionts of the basal higher attine genera Trachymyrmex and Sericomyrmex was only slightly enhanced, but the evolutionarily derived crop fungi of Atta and Acromyrmex leaf-cutting ants had much higher genetic variation. Our opposite ploidy models indicated that the symbionts of Trachymyrmex and Sericomyrmex are likely to be lowly and facultatively polyploid (just over two haplotypes on average), whereas Atta and Acromyrmex symbionts are highly and obligatorily polyploid (ca. 5-7 haplotypes on average). This stepwise transition appears analogous to ploidy variation in plants and fungi domesticated by humans and in fungi domesticated by termites and plants, where gene or genome duplications were typically associated with selection for higher productivity, but allopolyploid chimerism was incompatible with sexual reproduction.

Transmission of the G143A QoI-resistance point mutation through anastomosis in Magnaporthe grisea

BACKGROUND

Soon after the introduction of Qo inhibitor fungicides in 1996, the point mutation leading to the amino acid exchange glycine to alanine at the 143 position of the mitochondrial cytochrome b gene was identified as the main cause of resistance. The present study describes the role of anastomosis in the transmission of the G143A mutation in Magnaporthe grisea.

RESULTS

Two M. grisea mutants were co-cultivated on oatmeal agar and also co-inoculated on barley leaves. The mutants differed by the presence of the G143A mutation in one isolate and a disrupted AOX gene by insertion of a hygromycin gene in the other (M-145). Specific resistant (r) or sensitive (s) phenotypes of 409 monosporic cultures were determined on media amended with either hygromycin (H) or azoxystrobin (S) plus SHAM. The phenotypes identified reflected not only the phenotypes of mutants M-145 and G143A but also the wild-type parent phenotype HsSs and a new HrSr isolate.

CONCLUSION

Identification of the M. grisea phenotypes HrSr and HsSs suggests that anastomosis occurred during co-cultivation and co-inoculation of the mutants M-145 and G143A, allowing the transfer of the G143A point mutation from the QoI-resistant isolate to the susceptible isolate.

On the asymmetry of mating in natural populations of the mushroom fungus Schizophyllum commune

Before a mycelium of a mushroom-forming basidiomycete develops mushrooms, the monokaryotic mycelium needs to become fertilized. Although the mechanistic details of mating in mushrooms have been studied thoroughly in laboratory research, very little is known on mating patterns in nature. In this study, we performed fine-scale analyses of three populations of Schizophyllum commune from their natural substrate (i.e. dead beech branches). From the three branches, 24, 12, and 24 fruiting bodies were isolated and for each mushroom, the origins of its nuclei and cytoplasm were reconstructed using DNA markers. Nuclear genotypes were determined using sequencing data and mating types, and mitochondrial haplotypes using SNP markers. From these combined data we reconstructed colonization and mating patterns of the mycelia. On each branch, we found multiple dikaryons (3, 3, and 8, respectively); in two instances one nuclear haplotype was shared between two dikaryons and in two other cases a nuclear haplotype was shared between three dikaryons. Each dikaryon always had a single mitochondrial haplotype. These findings indicate that mating usually is not symmetrical and that a monokaryon is most likely fertilized by a small monokaryon, a spore or a dikaryon. Sharing of nuclear haplotype between different dikaryons resulted either from multiple fertilizations of a single monokaryon, if the dikaryons had identical mitochondrial haplotypes, or, if the dikaryons had different mitochondrial haplotypes, most likely from secondary matings between a monokaryon and a dikaryon (Buller phenomenon). We conclude that mating in S. commune between same-sized monokaryons with reciprocal migration, as generally described in textbooks, is rare in nature. We discuss the implications of non-symmetric mating for basidiomycete evolution.