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 strains heterozygous for a single heterokaryon incompatibility (het) gene are viable, but grow at a significantly inhibited rate. Strains heterozygous for het-6 or het-c were examined microscopically for evidence of cell death; approximately 15% of cells randomly distributed within such colonies were dead or dying. Electron microscopy revealed extensive organelle degradation and plasmolysis. Ultimately, the cytoplasm fragmented into small membrane-bound bodies. Hyphal regrowth into dying cells from adjacent healthy cells was common. Ultrastructure and cell size measurements indicated no differences in death processes between incompatibility caused by het-6 and het-c. Linear growth rate was the only measured parameter which correlated with the observed macroscopic differences in colony morphology between het genes. The ultrastructural changes in dying cells were consistent with descriptions of apoptosis in plants and animals. However, designating vegetative incompatibility as apoptosis is premature without further study. Copyright 1998 Academic Press.