Being bigger is better: Size-dependent advantages of plasmodial fusion in a multinucleated slime mould

Larger body size often enhances survival and reproduction. While most organisms grow to achieve larger sizes, some can rapidly increase size through fusion. Fusion decisions are influenced by genetic relatedness and environmental factors, balancing potential benefits and costs. In Physarum polycephalum, a multinucleated slime mould, fusion is thought to occur mainly between genetically identical or highly related individuals. However, the frequency of fusion among close relatives, and the drivers and benefits of fusion, remain unclear. This study explored fusion frequency, morphology, and benefits among clonal and closely related individuals of Physarum. We also assessed the impact of abiotic stress on fusion and whether genetic relatedness between fusion partners influences survival during exposure to stress. Our results revealed variation in fusion frequency and morphology among different plasmodial lines and pairs of close relatives. Abiotic stress increased fusion rates between both clonal and compatible non-clonal pairs. However, the benefits of fusion did not depend on the whether fusion occurred between clones or non-clones. These findings demonstrate plasmodial fusion provides size-related survival benefits. Furthermore, the promotion of fusion in stressful conditions highlights its adaptive role in responding to environmental challenges. Collectively, these results underscore the evolutionary importance of fusion in Physarum, shaping its ecological success and resilience. Competing Interests StatementThe authors declare no competing financial interests.