Neurospora heterokaryons with complementary duplications and deficiencies in their constituent nuclei provide an approach to identify nucleus-limited genes.

Introgression is the transfer of genes or genomic regions from one species into another via hybridization and back-crosses. We have introgressed four translocations (EB4, IBj5, UK14-1, and B362i) from Neurospora crassa into N. tetrasperma. This enabled us to construct heterokaryotic [T + N] and [Dp + Df] strains in which the mat-A and mat-a nuclei have different genotypes. Self-crosses of the heterokaryons again produced [T + N] and [Dp + Df] progeny. From conidia (vegetative spores) produced by the heterokaryotic mycelia we obtained self-fertile (heterokaryotic) and self-sterile (homokaryotic) derivative strains. [T + N] heterokaryons produced homokaryotic derivatives of both mating types, but [Dp + Df] heterokaryons produced viable homokaryons of only the mating type of the Dp nucleus. All the four [T + N] heterokaryons, and three [Dp + Df] heterokaryons, produced both self-sterile and self-fertile conidial derivatives, but the [Dp(B362i) + Df(B362i)] heterokaryons produced only self-sterile ones. Conceivably, the Df(B362i) nuclei may be deleted for a nucleus-limited gene required for efficient mitosis or nuclear division, and whose deficit is not complemented by the neighboring Dp(B362i) nuclei. Repeat-induced point mutation (RIP) was shown to occur in a Dp-heterozygous cross, therefore RIP-alteration of a translocated segment would depend on relative numbers of self-crosses undergone in [Dp + Df] versus [T + N] ancestors.