The mechanism of variation in Penicillium heterokaryons

Jinks (1952) has demonstrated that the proportion of the two kinds of nuclei in heterokaryons of Penicillium cyclopium varies adaptively according to the medium on which the heterokaryon is growing. A cytological investigation of the fungus was made to gain information on where and how the readjustment takes place. It is shown that older cells of hyphae contain from one to three nuclei, whereas cells im­mediately at the tips of young growing hyphae contain as many as fifteen nuclei, or more. All the mitoses observed were found to occur in these same cells at the growing tips. Nuclear division was simultaneous for a number of adjacent nuclei in homokaryons and stable heterokaryons. From the cytological, and from genetical, evidence it is concluded that readjustment of the nuclear ratios, and therefore of genetic balance, takes place within the tip cells, and arises by differential rates of division of the two kinds of nuclei. The control of the division rates must be under the same cytoplasmic control that produces simultaneous divisions of adjacent nuclei in homokaryons and stable heterokaryons. The genetic balance established in this way at the tips could operate in older cells through the cytoplasm which the older cells inherit, or by intercell co-operation between nuclei through the medium of cytoplasmic channels between the cells.