MICROSCOPIC AND ULTRASTRUCTURAL EXAMINATION OF VEGETATIVE INCOMPATIBILITY IN PARTIAL DIPLOIDS HETEROZYGOUS AT HET LOCI IN NEUROSPORA-CRASSA

Vegetative (or heterokaryon) incompatibility is often characterized by cell death after anastomosis. In Neurospora crassa, partial diploid s trains heterozygous for a single heterokaryon incompatibility (het) ge ne are viable, but grow at a significantly inhibited rate. Strains het erozygous for het-6 or het-c were examined microscopically for evidenc e of cell death; similar to 15% of cells randomly distributed within s uch colonies were dead or dying. Electron microscopy revealed extensiv e organelle degradation and plasmolysis. Ultimately, the cytoplasm fra gmented into small membrane-bound bodies. Hyphal regrowth into dying c ells from adjacent healthy cells was common, Ultrastructure and cell s ize measurements indicated no differences in death processes between i ncompatibility caused by het-6 and het-c. Linear growth rate was the o nly measured parameter which correlated with the observed macroscopic differences in colony morphology between het genes. The ultrastructura l changes in dying cells were consistent with descriptions of apoptosi s in plants and animals. However, designating vegetative incompatibili ty as apoptosis is premature without further study. (C) 1998 Academic Press.