Evidence for Outcrossing via the Buller Phenomenon in a Substrate Simultaneously Inoculated with Spores and Mycelium of Agaricus bisporus

Abstract

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.Agaricus bisporus (Lange) Imbach, the button mushroom, has a unifactorial mating system (18) with multiple alleles (11) at the MAT locus located on chromosome 1 (26). Most wild populations and all of the traditional cultivars of A. bisporus belong to A. bisporus var. bisporus. In this variety, the life cycle is amphithallic; i.e., both heterothallic genetic reproduction and (in this case, predominantly) pseudohomothallic (ϭsec-ondary homothallic) genetic reproduction occur (22). Heterothallic basidiospores are homokaryotic (n) and generally give rise to self-sterile homokaryotic mycelia. Plasmogamy between two sexually compatible homokaryons restores a fertile heterokaryon. Pseudohomothallic basidiospores are heterokaryons (n ϩ n) and contain two nonsister postmeiotic nuclei with different mating type alleles. Mycelia growing from these spores are sexually fertile and usually maintain the parental genotype in the offspring (15,21,23). Amphithallism is not rare, and 9% of ϳ500 species of holobasidiomycetes with lamellae are considered amphithallic (16). In wild strains of A. bisporus var. bisporus only 1.3% of the basidia, on average, are tetrasporic (5) and produce homokaryotic basidiospores. In wild populations, successive generations of pseudohomothallism should generate pseudoclonal lineages (12). Little evidence of pseudohomothallic reproduction was found in a recent spatial-temporal analysis of two local French populations (25), suggesting that outcrossing and recombination played significant roles in the history of both populations. Determining the relative roles of heterothallism and pseudohomothallism in wild populations by genetic analysis of collected sporocarps is difficult because of migration between sites and because such samples do not include individu...

Spore behaviors reveal a category of mating-competent infertile heterokaryons in the offspring of the medicinal fungus Agaricus subrufescens

“…Such positive mating reactions between homokaryons and heterokaryons (the Buller phenomenon) are not rare in Basidiomycota (Buller 1931;Quintanilha 1937;Raper et al 1972;Callac et al 2006). Considering that all these heterokaryons should theoretically mate with all tester homokaryons regardless their mating type alleles (Fig.…”

“…However, they differ from the homokaryons on a major point: they can transmit not only recessive deleterious or lethal alleles but also advantageous alleles that could be on linked loci and which could not be found in viable homokaryotic single spore isolates. Such a linkage has been shown through the Buller phenomenon in A. bisporus, for a locus involved in disease resistance (Callac et al 2008).…”

Fungal Sex: The Basidiomycota

“…We call these species homothallic, and several proximal mechanisms have been proposed to account for such mating behavior, namely: (i) the presence of compatible sets of MAT genes, either fused or unlinked, in one haploid genome (primary homothallism); (ii) mating-type switching (either bidirectional or unidirectional); and (iii) the lack of any gene-based mating-type discrimination (also known as ability for unisexual reproduction or same-sex mating) [reviewed in (157, 158)]. Some other fungal species, referred to as pseudohomothallic, also have the ability to complete the sexual cycle without the apparent need for syngamy (159–161). However, pseudohomothallism is achieved by packaging two nuclei of compatible mating types derived from the same meiosis into a single spore.…”

Sex, outcrossing and mating types: unsolved questions in fungi and beyond

“…Neurospora tetrasperma , Merino et al. , 1996; Raju & Perkins, 1994; Agaricus bisporus , Callac et al. , 2006; Saccharomycodes ludwigii, Zakharov, 2005).…”

“…Interestingly, recent studies have suggested that pseudo‐homothallic fungi may exhibit a mechanism favouring outcrossing over automixis. In A. bisporus , a choice experiment in seminatural conditions has shown that outcrossing was indeed more frequent than automixis in this pseudo‐homothallic mushroom (Callac et al. , 2006).…”