Microscopic and Ultrastructural Examination of Vegetative Incompatibility in Partial Diploids Heterozygous athetLoci inNeurospora crassa

Abstract

Vegetative (or heterokaryon) incompatibility is often characterized by cell death after anastomosis. InNeurospora crassa,partial diploid strains heterozygous for a single heterokaryon incompatibility (het) gene are viable, but grow at a significantly inhibited rate. Strains heterozygous forhet-6orhet-cwere examined microscopically for evidence of cell death; ≈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 byhet-6andhet-c.Linear growth rate was the only measured parameter which correlated with the observed macroscopic differences in colony morphology betweenhetgenes. 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.