Evidence of Sexuality in the Cultivated Mushroom, Agaricus Bisporus

Mating-Type Locus Organization and Mating-Type Chromosome Differentiation in the Bipolar Edible Button Mushroom Agaricus bisporus

In heterothallic basidiomycete fungi, sexual compatibility is restricted by mating types, typically controlled by two loci: PR, encoding pheromone precursors and pheromone receptors, and HD, encoding two types of homeodomain transcription factors. We analysed the single mating-type locus of the commercial button mushroom variety, Agaricus bisporus var. bisporus, and of the related variety burnettii. We identified the location of the mating-type locus using genetic map and genome information, corresponding to the HD locus, the PR locus having lost its mating-type role. We found the mip1 and β-fg genes flanking the HD genes as in several Agaricomycetes, two copies of the β-fg gene, an additional HD2 copy in the reference genome of A. bisporus var. bisporus and an additional HD1 copy in the reference genome of A. bisporus var. burnettii. We detected a 140 kb-long inversion between mating types in an A. bisporus var. burnettii heterokaryon, trapping the HD genes, the mip1 gene and fragments of additional genes. The two varieties had islands of transposable elements at the mating-type locus, spanning 35 kb in the A. bisporus var. burnettii reference genome. Linkage analyses showed a region with low recombination in the mating-type locus region in the A. bisporus var. burnettii variety. We found high differentiation between β-fg alleles in both varieties, indicating an ancient event of recombination suppression, followed more recently by a suppression of recombination at the mip1 gene through the inversion in A. bisporus var. burnettii and a suppression of recombination across whole chromosomes in A. bisporus var. bisporus, constituting stepwise recombination suppression as in many other mating-type chromosomes and sex chromosomes.

Whole Genome Sequence of the Commercially Relevant Mushroom Strain Agaricus bisporus var. bisporus ARP23

Agaricus bisporus is an extensively cultivated edible mushroom. Demand for cultivation is continuously growing and difficulties associated with breeding programs now means strains are effectively considered monoculture. While commercial growing practices are highly efficient and tightly controlled, the over-use of a single strain has led to a variety of disease outbreaks from a range of pathogens including bacteria, fungi and viruses. To address this, the Agaricus Resource Program (ARP) was set up to collect wild isolates from diverse geographical locations through a bounty-driven scheme to create a repository of wild Agaricus germplasm. One of the strains collected, Agaricus bisporus var. bisporus ARP23, has been crossed extensively with white commercial varieties leading to the generation of a novel hybrid with a dark brown pileus commonly referred to as 'Heirloom'. Heirloom has been successfully implemented into commercial mushroom cultivation. In this study the whole genome of Agaricus bisporus var. bisporus ARP23 was sequenced and assembled with Illumina and PacBio sequencing technology. The final genome was found to be 33.49 Mb in length and have significant levels of synteny to other sequenced Agaricus bisporus strains. Overall, 13,030 putative protein coding genes were located and annotated. Relative to the other A. bisporus genomes that are currently available, Agaricus bisporus var. bisporus ARP23 is the largest A. bisporus strain in terms of gene number and genetic content sequenced to date. Comparative genomic analysis shows that the A. bisporus mating loci in unifactorial and unsurprisingly highly conserved between strains. The lignocellulolytic gene content of all A. bisporus strains compared is also very similar. Our results show that the pangenome structure of A. bisporus is quite diverse with between 60-70% of the total protein coding genes per strain considered as being orthologous and syntenically conserved. These analyses and the genome sequence described herein are the starting point for more detailed molecular analyses into the growth and phenotypical responses of Agaricus bisporus var. bisporus ARP23 when challenged with economically important mycoviruses.

Crosses among Homokaryons from Commercial and Wild-Collected Strains of the Mushroom Agaricus brunnescens (= A. bisporus )

A new technique for the production of hybrid strains of the cultivated mushroom Agaricus brunnescens is described. Homokaryons were recovered from regenerated protoplasts obtained from several heterokaryotic strains. A total of 16 novel hybrids were produced in 63 attempted crosses between paired homokaryons. Recovery of both homokaryons and hybrids was verified by analysis of restriction fragment length polymorphisms. Three of four hybrids fruited in small-scale tests, further confirming that the isolates were true hybrids. Colony morphology alone was found to be a poor indicator of hybrid status. In two instances, three homokaryons crossed successfully in all combinations, suggesting that there are at least three alleles at the putative mating-type locus. Crosses between homokaryons from commercial and wild-collected isolates indicated that these strains belong to the same biological species.

Restriction fragment length polymorphisms in the mushrooms Agaricus brunnescens and Agaricus bitorquis

Two Agaricus species, A. brunnescens (a commercial mushroom) and A. bitorquis (a wild, edible species), were examined for restriction fragment length polymorphisms. EcoRI-digested nuclear DNA from isolates of both species were cloned in plasmid vector pUC18. Ten random recombinant clones were used in Southern DNA-DNA hybridizations to probe EcoRI-digested DNA from 11 A. brunnescens isolates (7 commercial, 2 wild type, and 2 homokaryotic) and 7 A. bitorquis isolates. Most cloned fragments were polymorphic in both species. There were fewer different genotypes than expected, however, in the sample of commercial A. brunnescens strains. DNA from homokaryotic strains showed fewer bands in most hybridizations than DNA from heterokaryotic strains. All A. bitorquis isolates could be distinguished from each other as well as from every A. brunnescens strain. Putative homokaryons were detected by the loss of polymorphic bands among protoplast regenerates from one commercial strain and two strains collected in the wild.

Homokaryotization of Agaricus bitorquis (Quel.) Sacc. and Agaricus bisporus (Lange) Imb

After treatment of one strain of A. bitorquis and 12 strains of A. bisporus in modified monokaryotization solution, three types of mycelia were received: one is the original di- or heterokaryon, the other two were proven to be neohaplonts in A. bitorquis and two strains of A. bisporus. In neohaplonts of good fruiting strains, homokaryotic fruiting was observed.

Basidiospore numbers in Agaricus bisporus (Lange) Imbach

The number of spores per basidium in the cultivated mushroom Agaricus bisporus can be readily determined using the light microscope.

Study on development of Agaricus bisporus by fluorescent microscopy and scanning electron microscopy

Two strains of Agaricus bisporus have been investigated by fluorescent microscopy and scanning electron microscopy. Somatic nuclei, stained with auramin O and examined by fluorescent microscopy, appear to be randomly distributed, divide asynchronously, and assume a nonclassical or "two-track" configuration during mitotic metaphase. A similar configuration has been observed for nuclei during meiosis, but early meiosis in A. bisporus appears to be classical, usually with nine pairs of chromosomes evident during prophase I. Scanning electron microscopy has been used to document developmental stages in the formation and germination of basidiospores. Two-spored basidia were predominant, but occasionally one- or three-spored forms were observed. Four-spored basidia were absent, and uninucleated basidiospores were exceedingly rare to absent.

Fungal viruses

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