Equivalence of the A and B mating type factors in the tetrapolar basidiomycete Agrocybe aegerita.

In the progeny of crosses involving the geographical races BR and ST of Agrocybe aegerita mating reactions were observed which could not be explained by the action of the A and B factors according to a commonly accepted tetrapolar incompatibility mechanism. As a result of a comprehensive genetic analysis it became evident that in these two races of Agrocybe the incompatibility genes A and B are located reciprocally transposed. Therefore, as expected, recombinants were found having either two A or two B genes. Consequently a physiological specificity for A and B, as generally accepted for higher basidiomycetes, must be ruled out for Agrocybe aegerita. In this fungus the tetrapolar mating reaction is by contrast controlled by the action of two genes exhibiting an equivalent function during the morphogenetic sequence leading to dikaryosis and the formation of clamp connections.