Evidence of genetic divergence in two populations of Agaricus bisporus

Experimental Outcrossing in Agaricus bisporus Revealed a Major and Unexpected Involvement of Airborne Mycelium Fragments

Agaricus bisporus var. bisporus, the button mushroom, has a predominantly pseudohomothallic life cycle. Most of its spores are heterokaryotic and give rise to fertile heterokaryons. However, previous studies have suggested that outcrossing should not be rare in wild populations. In order to discover how outcrossing occurs, we experimentally favored it between aerial propagules of a fruiting donor mycelium and a delayed receiver mycelium that only invaded culture trays. Two donor/receiver pairs were studied, and potentially hybrid basidiomata collected on the receiver trays were analyzed with a mitochondrial marker, two unlinked nuclear CAPS markers, then haplotype markers based on DNA sequences obtained after PCR cloning of the rDNA ITS region and the fruk gene. For one of the two pairs, most basidiomata were hybrids between the donor and the receiver. Genotyping of the hybrids revealed only two genotypes consistent with outcrossing involving airborne mycelium fragments rather than basidiospores. The resident receiver heterokaryon that provided its mitochondria to the hybrid basidiomata is suspected to have had a trophic contribution to their growth and successful fruiting. The high level of heterozygosity and the cultivar introgression previously revealed in wild populations of this pseudohomothallic species may result from outcrossing involving airborne pieces of mycelium.

Identification of Resistance to Wet Bubble Disease and Genetic Diversity in Wild and Cultivated Strains of Agaricus bisporus

Outbreaks of wet bubble disease (WBD) caused by Mycogone perniciosa are increasing across the world and seriously affecting the yield of Agaricus bisporus. However, highly WBD-resistant strains are rare. Here, we tested 28 A. bisporus strains for WBD resistance by inoculating M. perniciosa spore suspension on casing soil, and assessed genetic diversity of these strains using 17 new simple sequence repeat (SSR) markers developed in this study. We found that 10 wild strains originating from the Tibetan Plateau in China were highly WBD-resistant strains, and 13 cultivated strains from six countries were highly susceptible strains. A total of 88 alleles were detected in these 28 strains, and the observed number of alleles per locus ranged from 2 to 8. Cluster and genetic structure analysis results revealed the wild resources from China have a relatively high level of genetic diversity and occur at low level of gene flow and introgression with cultivated strains. Moreover, the wild strains from China potentially have the consensus ancestral genotypes different from the cultivated strains and evolved independently. Therefore, the highly WBD-resistant wild strains from China and newly developed SSR markers could be used as novel sources for WBD-resistant breeding and quantitative trait locus (QTL) mapping of WBD-resistant gene of A. bisporus.

Molecular diversity assessment of arctic and boreal Agaricus taxa

We provide a phylogenetic diversity assessment study in genus Agaricus as part of our ongoing work to saturate ITS and LSU rDNA sequence diversity of soil-dwelling fungi in Alaska. Pairwise sequence similarity-based groupings and statistical parsimony analyses were applied to delimit operational taxonomic unit (OTU) and were compared to results of full phylogenetic analyses. Our results show that the proportion of section Arvenses taxa is particularly high in the boreal forest and hypo-arctic (low arctic) regions, whereas the genus is represented by section Agaricus in high arctic habitats. Furthermore our findings suggest that the commercially important A. bisporus occurs naturally in the boreal region of interior Alaska, substantially expanding the known northern limit of the species. Delimitations of OTU varied greatly with different methods. In general 95% similarity-based grouping proved to be the least sensitive method, often resulting in section- and subsection-level groups. The 95% connection-limit statistical parsimony separated far more groups. The 98% similarity-based groups and the 98% connection limit networks recognized respectively 11 and 13 OTU containing our specimens. The 98% connection limit statistical parsimony was the only method in which all recognized OTU consisted of members grouped by branches with significant (> .95) posterior probabilities, providing an independent support for the groups. Our results also point out that considerable additional efforts will be needed to elucidate the evolution of this diverse genus and to assess its phylogenetic diversity, given that most taxa in our analyses could not be placed convincingly within well characterized species using ITS/LSU data.

Comparison of random amplified polymorphic DNA markers and morphological characters in identification of homokaryon isolates of white button mushroom (Agaricus bisporus)

The secondarily homothallic life cycle of the white button mushroom that results in scarcity of uninucleate basidiospores (homokaryons) in its progeny, is the most important impediment for genetic improvement of the commercial strains. Identification of homokaryons for breeding programs of Agaricus bisporus (button mushroom) is, therefore, crucial. Verifying homokaryons through fruiting trial is time consuming and unreliable. In this study, ability of RAPD markers, compared to morphological characters for identification of homokaryon isolates, was investigated. Based on morphological characters, 42 isolates were screened and exposed to RAPD markers. The results showed that RAPD markers could discriminate homokaryons from heterokaryons, based on number of bands generated. The numbers of band in homokaryons were significantly less than those of heterokaryons. Results also showed that cluster analysis, based on average of band number generated, could separate homokaryon from heterokaryon isolates. It is suggested that RAPDs could be used to identify hyomokaryons from heterokaryons for breeding program of A. bisporus.

Evidence for outcrossing via the Buller phenomenon in a substrate simultaneously inoculated with spores and mycelium of Agaricus bisporus

In Agaricus bisporus, traditional cultivars and most of the wild populations belong to A. bisporus var. bisporus, which has a predominantly pseudohomothallic life cycle in which most meiospores are heterokaryons (n + n). A lower proportion of homokaryotic (n) meiospores, which typify the heterothallic life cycle, also are produced. In wild populations, pseudohomothallism was thought previously to play a major role, but recent analyses have found that significant outcrossing also may occur. We inoculated a standard substrate for A. bisporus cultivation simultaneously with homokaryotic mycelium from one parent and spores from a second parent. Culture trays produced numerous sporocarps that could theoretically have resulted from five different reproductive modes (pseudohomothallism, selfing or outcrossing via heterothallism, and selfing or outcrossing via the Buller phenomenon [i.e., between a homokaryon and a heterokaryon]). Most or all of the sporocarps resulted from outcrossing between the inoculated homokaryon and the inoculated heterokaryotic spores (or mycelia that grew from them). These data broaden our understanding of population dynamics under field conditions and provide an outcrossing method that could be used in commercial breeding programs.

Intratetrad mating, heterozygosity, and the maintenance of deleterious alleles in Microbotryum violaceum (=Ustilago violacea)

The mating system of Microbotryum violaceum was investigated in populations that are polymorphic for mating-type bias, where individuals produce viable haploids of only one of the two required mating types. The cause of mating-type bias was identified as deleterious recessive alleles linked to mating type. Maintenance of the deleterious alleles was promoted by early conjugation among products of single meioses, such that the duration of the free-living haploid stage is minimized. This development was also observed in nonbiased isolates. As a consequence, the mating system tends toward mating within the tetrad, which might be expected to reduce heterozygosity. However, complete centromere linkage of mating type ensures conjugation between first division meiotic products, such that mating in M. violaceum is analogous to forms of meiotic parthenogenesis with first division restitution (i.e. automixis with central fusion). This fungus was used to test the prediction that this mating system would maintain heterozygosity in regions of the genome linked to centromeres. Therefore, populations were screened for additional heterozygous lethal recessive alleles linked to centromeres, and several examples were found. Furthermore, the occurrence of intratetrad mating in M. violaceum provides an explanation for low variation among individuals within populations, inconsistent estimates of outcrossing rates, low levels of mating between tetrads of one diploid individual, and high frequencies of haplo-lethal alleles in natural populations.

Global genetic resources forAgaricusbreeding and cultivation

Until recently the natural history and resource status of the button mushroom Agaricus bisporus has been but poorly known. At present, five and perhaps six genetically distinctive, reproductively isolated populations of this species from western North America, Europe, and western Asia have been located, sampled, and partially characterized. Morphology, diversity, reproductive syndrome, and other economically important traits vary among these populations. Other populations may exist in northern and central Africa and in Australia. Available evidence suggests that the studied populations are ancient. However, probably through cultivation, European germ plasm has invaded the three known North American populations and has already displaced a large portion of the indigenous coastal Californian population; the trend is less advanced in Alberta and the California desert. The recovery of diverse wild germ plasm of this mushroom, still in its infancy, is likely to become increasingly difficult. The value of these genetic resources, the threats that confront them, and appropriate responses by the mushroom industry are discussed. Hypotheses about the outcomes of heterokaryon somatogamy and the concept of the individual in this species are presented. The term metagenotype is coined to describe the ancestral or consensus genotype of a multigenerational intramictic lineage group. Key words: Agaricus bisporus, genetic resources, biodiversity, breeding, population structure, resource management.

Breeding of edible fungi with emphasis on the variability among French genetic resources ofAgaricus bisporus

Our laboratory (INRA) has developed breeding programs for several species of edible fungi. For example, "sporeless" strains were obtained by mutagenesis for Pleurotus ostreatus and Pleurotus pulmonarius, a hybrid strain of Lepista nuda has been marketed, and genetic variability has been studied in Tuber melanosporum. At present, the largest program concerns Agaricus bisporus, for which a collection of about 200 French isolates from 44 sites has been gathered. High genetic polymorphism appeared even within each site. Nine genetically different isolates from the same site were studied in experimental culture. Variability was observed for color of the cap, susceptibility to bacterial blotch, and the elevated basidial spore number trait. A single isolate, Bs 261, gave sporocarps having a majority of tetrasporic basidia, and spores which, for the most part, were homokaryotic. This strain was interfertile with typical bisporic strains. The resulting hybrids were either bisporic or tetrasporic, according to which homokaryon of Bs 261 was used. More investigations will be necessary to understand the presence of a rare tetrasporic strain in a bisporic population and also to study the relationship between Bs 261 and the tetrasporic strains of the previously described A. bisporus var. burnettii. Key words: edible fungi, Agaricus bisporus, genetic resources, breeding, basidial spore number, tetrasporic trait.

Indigenous and introduced populations ofAgaricus bisporus, the cultivated button mushroom, in eastern and western Canada: implications for population biology, resource management, and conservation of genetic diversity

Agaricus bisporus is known from field collections in several parts of southern Canada and the border states of Washington and Idaho. In Ontario, the species is associated with urban horticultural sites, agricultural areas, and thoroughfares. In British Columbia, Washington, and Idaho, the species is less well documented but occurs in habitats similar to those in Ontario. All studied isolates from these two regions were genotypically similar to those collected in Europe and also to a representative sample of cultivar isolates believed to be of European origin. In contrast, a population from forests of Picea in the Rocky Mountains of Alberta had genotypes that are very different from European and cultivar groups, including the putatively introduced populations in other parts of Canada. To a lesser extent, the Albertan individuals also differed from three other isolated natural populations, two in California and one in Israel. Two of 35 isolates from Alberta had nuclear genotypes that were partially or entirely consistent with European ancestry, indicating that foreign (probably cultivar) germ plasm has become established in the native habitat. Thirty of 35 isolates from the Alberta field sample had one of six mitochondrial DNA (mtDNA) haplotypes known only from Alberta; the remaining five had an mtDNA type known from cultivar strains. However, nuclear genotypic similarity to other Albertan or European-cultivar isolates was not significantly correlated with presence or absence of this latter mtDNA type. The genetic diversity represented by the previously unknown Albertan population, and to a lesser extent by the heirloom varietal lineages and their derivatives that have become established in Ontario, British Columbia, Washington, and Idaho, is an important resource for breeders of this valuable fungal crop species. Threats to resource populations of this species, and possible responses, are discussed. Key words: Agaricus bisporus, mushroom population biology, resource management, germ plasm conservation, genetic diversity, microphylogeny.

Investigation of Mitochondrial Transmission in Selected Matings between Homokaryons from Commercial and Wild-Collected Isolates of Agaricus bisporus (= Agaricus brunnescens )

Ten heterokaryons of Agaricus bisporus (= Agaricus brunnescens ) were shown to carry four different mitochondrial (mt) genotypes by analysis of mt restriction fragment length polymorphisms (RFLPs). Fifteen homokaryons derived from these strains were used to investigate mt inheritance in A. bisporus. One hundred eighty-nine pairings were performed in 25 different combinations. Pairings in 15 different combinations produced heterokaryons on the basis of nuclear RFLP analyses and/or fruiting trials. The mt genotype of each new intraspecies hybrid was examined by using mt RFLPs as genetic markers. Our results suggest the following. (i) Recombination between the mt genomes was not a common event. (ii) From most individual pairings, all heterokaryons carried the same mt genotype. (iii) Heterokaryons carrying either of the two possible mt genotypes were observed in certain crosses after modification of the pairing procedure. A biparental transmission pattern was demonstrated for some crosses, but there appears to be a preference for one of the mt genotypes to predominate in any specific pairing.

DNA amplification polymorphisms of the cultivated mushroom Agaricus bisporus

Single 10-bp primers were used to generate random amplified polymorphic DNA (RAPD) markers from commercial and wild strains of the cultivated mushroom Agaricus bisporus via the polymerase chain reaction. Of 20 primers tested, 19 amplified A. bisporus DNA, each producing 5 to 15 scorable markers ranging from 0.5 to 3.0 kbp. RAPD markers identified seven distinct genotypes among eight heterokaryotic strains; two of the commercial strains were shown to be related to each other through single-spore descent. Homokaryons recovered from protoplast regenerants of heterokaryotic strains carried a subset of the RAPD markers found in the heterokaryon, and both of the haploid nuclei from two heterokaryons were distinguishable. RAPD markers also served to verify the creation of a hybrid heterokaryon and to analyze meiotic progeny from this new strain: most of the basidiospores displayed RAPD fingerprints identical to that of the parental heterokaryon, although a few selected slow growers were homoallelic at a number of loci that were heteroallelic in the parent, suggesting that they represented rare homokaryotic basidiospores; crossover events between a RAPD marker locus and its respective centromere appeared to be infrequent. These results demonstrate that RAPD markers provide an efficient alternative for strain fingerprinting and a versatile tool for genetic studies and manipulations of A. bisporus.