The germplasm of Agaricus bisporus:: Main results after ten years of collecting in France, in Greece, and in North America

Until recently, the genetic resources of the button mushroom (Agaricus bisporus) were limited to a. few cultivars having a pseudohomothallic life cycle unfavourable for outcrossing. Ten years ago, INRA associated with CTC (Centre Technique du Champignon) in France, collaborating the last four years with NAGREF in Greece, and simultaneously the ARP (Agaricus Resource Program) in USA, had gradually constituted their respective collections on which are based the observations and the analyses summarised here. More than 500 wild specimens, mostly collected either on cypress or spruce litter, or on manure, or in sandy semi-arid habitats, are now maintained in liquid nitrogen; their distribution mainly covers Europe, the Mediterranean region, and North America. Populations from France, Greece, Alberta, and coastal California are predominantly pseudohomothallic while the isolates of the population of the Sonoran desert of California have a predominantly heterothallic life cycle. The latter life cycle favours outcrossing; therefore the knowledge of its control by a dominant allele at the BSN locus located on chromosome I has dramatically modified our breeding strategies. Finally, a third type of life cycle, homothallism, is found in some rare European strains producing haploid fruitbodies and spores. If we except these rare haploid isolates, alloenzymatic or molecular codominant markers generally reveal a high level of heterozygosity, high genetic diversity, and differentiated populations. Analyses of the mitochondrial haplotypes suggest that some of the wild populations are seriously contaminated by cultivars. The isolates are systematically assessed for their cultural performances, their morphology, their cap colour, and are eventually tested for their resistance against diseases and for their fragrance. The cap colour varies from brown to white, however, only about 3 % of the wild strains have a white cap, resulting from the presence of recessive alleles at the PPC1 locus located on chromosome VIII. These data are used to manage the collection and to choose the parents. The absence of germplasm resources and the particular life cycle of A. bisporus are not handicaps for its genetic improvement any more.