(1) is produced by mutation. In a multinucleated cell of mycelium, any nuclear gene mutation can make the cell become heterokaryon, but the probability of mutation is very low and it is not easy to be found. (2) When hyphae with different genetic properties meet, they may repel each other or combine with each other, resulting in nuclear transfer or exchange. Heterokaryons were formed by pairing different Verticillium dahliae strains on 2% water agar medium plates, and the process of mycelium fusion could be observed (Puhalla et al., 1974). Two opposite hyphae appear from the corresponding sides, and the protrusions meet and fuse to form an anastomotic bridge. At this time, the mycelium is H-shaped. The nucleus on one side can be transferred to the hyphae on the other side through pons, and different groups of fungal hyphae form heterokaryons in different ways, so it is not necessary to produce pons. For example, Rhizoctonia hyphae have two types of complete fusion: the first fusion site is obvious, its pore size is smaller than the diameter of hyphae, and the cell wall dissolves; The second fusion site is not obvious, and the cell wall is digested, and its pore size is equivalent to the diameter of mycelium (

Fig. 1 1- 1 mycelium fusion of Rhizoctonia solani

)