Heterocaryons between Strains of Neurospora Crassa with Different Cytoplasms.

ITCHELL and MITCHELL (1952) and MITCHELL, MITCHELL and TISSIERES M (1953) have shown that two strains of Neurospora crassa, poky and mi 3, of independent origin, differ from wild type and from each other in growth rate and cytochrome content. The differences are inherited maternally and thus appear to be associated with the cytoplasm. The object of this work has been to show what type of cytoplasm results when heterocaryons are made between strains having poky and mi 3 cytoplasms or between either and wild type. It was hoped to show in particular (1) whether poky and mi 3 cytoplasms interact to give wild type, (2) whether intermediate states of cytoplasm are to be found, and (3) whether either mutant, poky or mi 3, is “dominant” over wild type in a way which would suggest infection. MITCHELL et al. (1953) report growing mixed cultures of poky and mi 3 strains and obtained from the mixture no strain with a growth rate faster than either; but there is no certainty that heterocaryon formation occurred in these cultures. In the present work, the components of all heterocaryons were marked with biochemical mutant nuclear genes, usually pantothenicless (5531) in one component and p-aminobenzoicless (1633) in the other, so that when the two strains were grown together on minimal medium the heterocaryon might be a t an advantage. It will be seen that heterocaryons were in fact formed, that is mixture of nuclei occurred and there was opportunity for mixture of cytoplasms.