Genetic Proof of Heterokaryosis in Penicillium notatum

Abstract

IN a recent paper, Baker¹ has given an account of nuclear behaviour in Penicillium notatum. We had, independently, reached the same conclusions, summarized as follows: (a) the older 'cells' of the mycelium, the sterigmata and the conidia are usually uninucleate; (b) the 'cells' at the growing edge of the colony are usually multinucleate, containing up to a dozen nuclei each; (c) hyphal fusions occur between branches of the same hypha and also between hyphse of different origin. From her cytological work, Baker infers that P. notatum is liable to be 'heterokaryotic', that is, may carry genetically unlike nuclei in a multinucleate cell, or in different cells of a hypha. This condition, widespread in fungi, well deserves careful investigation in view of its important implications for the theories of gene action and of the evolution of genetic systems. Some of these implications have been discussed by Hansen², Dodge³, Lindegren⁴ and, especially, by Beadle and Coonradt⁵.