Post-fusion somatic incompatibility in plasmodia of Physarum polycephalum.

High-resolution autoradiography has been used to establish that during the incompatibility reaction that follows fusion between plasmodia of a 'killer' and a 'sensitive' strain of the myxomycete Physarum polycephalum, the nuclei of the sensitive strain are selectively damaged, enclosed in vacuoles and eliminated from the cytoplasm. This damage is visible as increased chromatin condensation and nucleolar segregation. Nuclear envelopes of both strains show blebbing, and there is an increase in the size and frequency of cytoplasmic vesicles of endoplasmic reticulum. Multiple nuclear fusions are seen between all combinations of genetically like and unlike types of nuclei throughout the course of the incompatibility reaction. After the reaction, mean nuclear diameters increase over 2-3 days to give nuclei an order of magnitude greater in volume than the controls; the population size range returns to normal in 4-5 days. Fusions between incompatible plasmodia carried out when the killer strain is at or very near to mitosis do not produce an immediate incompatibility reaction, but give plasmodia that are neutral and act as neither killers nor sensitives; these heterokaryons convert to killer phenotypes after a few days.