Breeding and strain protection in the button mushroom Agaricus bisporus.

The button mushroom Agaricus bisporus is one of the most widely cultivated edible mushroom species in the world. Being the main species cultivated in the Western hemisphere, its popularity also increases in Eastern Countries such as China and Korea. The world production level for 2009 is estimated at ca. 4 million tons with an economic value of ca. 4.7 $ billion. Despite its economic relevance, it is surprising to see that breeding effort in this species is low. The main reasons for this low effort are the typical life cycle that hampers breeding and the difficulty to protect strains. The complete life cycle of A. bisporus was unravelled in the early 70-ties of the previous century. After an apparently normal meiosis, predominantly bisporic basidia are produced, each containing two non-sister post meiotic nuclei. Upon germination these spores generate heterokaryotic mycelia. Only spores from the rare four spored basidia contain one haploid nucleus and can be used to generate hybrids in breeding programs. Release of the first commercial hybrid dates back to 1980. Subsequent commercial hybrids were identical or very similar to the first hybrid. It is clear that some “new” varieties were generated by making copies of the first hybrid via tissue cultures of mushrooms. It is, however, unclear how other varieties were generated with apparently identical genotype but nevertheless some clear differences in phenotypes. Recent research indicates that offspring can be generated without recombination between homologous chromosomes. A pairing of non-sister nuclei on bisporic basidia will thus result in a redistribution of homologous chromosomes over the constituent nuclei. This redistribution appears to have phenotypic influence. This phenomenon thus allows for a relatively easy way of generating new varieties. This paper will present opportunities of this typical meiotic behaviour for breeders but also addresses what consequences it has for strain protection.