Abstract
The genetic composition of the A-mating locus in bipolar mushrooms such as “nameko” mushrooms has not been known for a long time. In this paper, I present our findings on the mating system of bipolar basidiomycetes. The A-mating locus of the bipolar mushroom, Pholiota maicrospore, contains a pair of homeodomain protein genes [<i>hox1 (HD1), hox2 (HD2)</i>], a mitochondrial intermediate peptidase gene (<i>mip</i>), a methylmalonic acid semialdehyde dehydrogenase gene (<i>mmsd</i>), a low molecular weight phosphotyrosine protein phosphatase gene (<i>lmwppp</i>), an ammonium transport protein gene (amtp), and a glycine dehydrogenase gene (<i>glydh</i>). In order to determine whether only the homeodomain protein gene can drive clamp formation, I introduced a homeodomain protein gene derived from an <i>A4</i> homokaryon strain into an <i>A3</i> homokaryon strain of the mushroom, and examined whether clamp cells were formed. I concluded that sufficient expression of the homeodomain protein gene is required to induce true clamp cell formation in bipolar mushrooms, since two nuclei per cell are present, as in heterokaryons. Fruiting body development and the formation of true clamp cells are also observed in the homokaryon strain of <i>Mycoleptodonoides aitchisonii</i>. These results strongly suggest that heterokaryotization of mushrooms is not essential for fruiting body formation and clamp cell formation; however, heterokaryotization may be necessary for the "efficient" or "high frequency" formation of fruiting body and true clamp cells.
