SOMATIC RECOMBINATION IN THE CULTIVATED MUSHROOM AGARICUS-BISPORUS

Agaricus bisporus, the cultivated button mushroom, has a mostly second arily homothallic life cycle. This mode of sexual reproduction could l imit outcrossing and recombination among homokaryons in natural popula tions and also creates difficulties in mushroom breeding. An alternati ve source of recombinant genotypes is from somatic pairings of heterok aryons. In this study, two homokaryon x heterokaryon and three heterok aryon x heterokaryon pairings were made to examine the possibility of somatic recombination. Four subcultures from the confrontation zone of each pairing were taken for analysis of restriction fragment length p olymorphisms at 18 nuclear loci representing seven linkage groups and two regions of mitochondrial DNA. A strikingly high frequency of somat ic recombination was observed. Five of the eight subcultures from the two homokaryon x heterokaryon pairings and five of the 12 subcultures from the three heterokaryon x heterokaryon pairings were recombinant. No loss of marker alleles was detected in any of the self-self pairing s of the six original strains. No recombination was observed between t he two regions of mtDNA examined in this study. The recombination amon g nuclear loci involves nuclear reassortment, exchange of genetic mate rial between nuclei, and, in one case, crossing over between markers l ocated on the same chromosome. While the mechanism of somatic recombin ation in A. bisporus is not known, heterokaryons might be used in pair ings with other heterokaryons or with homokaryons to produce abundant recombinant genotypes for mushroom breeding.