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Post-fusion somatic incompatibility in plasmodia of Physarum polycephalum
Abstract:
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.
Insights
In Physarum polycephalum, incompatible cell fusions trigger selective nuclear damage and elimination of sensitive strain nuclei. This process involves nuclear fusion and leads to enlarged nuclei in the surviving population.
Area of Science:
- Cell Biology
- Mycology
- Genetics
Background:
- Cell fusion in myxomycetes can lead to incompatibility reactions.
- The 'killer' and 'sensitive' strain system in Physarum polycephalum provides a model for studying cellular interactions.
Purpose of the Study:
- To investigate the cellular and nuclear events during the incompatibility reaction between 'killer' and 'sensitive' strains of Physarum polycephalum.
- To characterize the fate of nuclei from both strains following fusion.
Main Methods:
- High-resolution autoradiography was employed to track nuclear behavior.
- Microscopic observation was used to assess nuclear and cytoplasmic changes.
Main Results:
- Sensitive strain nuclei are selectively damaged, vacuolated, and eliminated from the cytoplasm.
- Nuclear damage manifests as chromatin condensation and nucleolar segregation.
- Nuclear envelopes exhibit blebbing, and endoplasmic reticulum vesicles increase.
- Multiple nuclear fusions occur between genetically similar and dissimilar nuclei.
- Mean nuclear diameter increases significantly post-reaction, with population size normalizing later.
- Fusion near killer strain mitosis results in neutral heterokaryons that convert to killer phenotype.
Conclusions:
- The incompatibility reaction in Physarum polycephalum involves targeted elimination of sensitive nuclei.
- Nuclear fusion dynamics and cellular responses are key features of this incompatibility.
- Mitotic stage influences the outcome of incompatible fusions, leading to phenotypic conversion.