Sexual diploids of Aspergillus nidulans do not form by random fusion of nuclei in the heterokaryon

The sexual stage of Aspergillus (Emericella) nidulans consists of cleistoth ecia containing asci, each with eight ascospores. The fungus completes the sexual cycle in a homokaryotic or a heterokaryotic mycelium, respectively. The common assumption for the last 50 years was that different nuclear type s are not distinguishable,lc when sexual development is initiated. When cul tured on a medium limited for glucose supplemented with 2% sorbitol, sexual development of A. nidulans is slowed and intact tetrads call be isolated. Through tetrad analysis we found that unlike haploid nuclei fuse preferenti ally to the prezygotic diploid nucleaus. When heterokaryons are formed betw een nuclei of different genetic backgrounds, then recombinant asci derived from opposite nuclei are formed exclusively. Strains in the same heterokary on compatibility group with moderate differences in their genetic backgroun ds can discriminate between the nuclei of a heterokaryon and preferentially form a hybrid diploid nucleus, resulting in 85% recombinant tetrads. A. ni dulans strains that differ at only a single genetic marker fuse the haploid nuclei at random for formation of diploid nuclei during meiosis. These res ults argue for a genetically determined "relative heterothallism'' of nucle ar recognition within a heterokaryon and a specific recruitment of differen t nuclei for karyogamy when available.