Experimental evidence for cross-supergroup host switching in photosynthetic symbiosis between Mayorella viridis and Paramecium bursaria

Photosynthetic symbioses between unicellular eukaryotes and intracellular green algae (zoochlorellae) are widespread among protists and have evolved repeatedly during eukaryotic evolution. Molecular phylogenetic studies have suggested that host switching has contributed to the present distribution of zoochlorellae, but direct experimental evidence has remained scarce. Here, we examined whether zoochlorellae can be transmitted between the amoeba Mayorella viridis and the ciliate Paramecium bursaria using cross-infection experiments with isolated symbiotic algae and co-culture experiments with intact host cells. Intracellular establishment of zoochlorellae was achieved in all combinations examined. Phylogenetic analyses further demonstrated that Mayorella -associated zoochlorellae belong to multiple evolutionary lineages, supporting repeated acquisition of symbionts during evolution. Transmission electron microscopy revealed that the perialgal vacuole membrane was closely apposed to host tubular endoplasmic reticulum and mitochondria, forming membrane associations consistent with putative contact sites. These findings provide experimental evidence for host switching between distantly related protozoan hosts and suggest that conserved host–symbiont interactions may facilitate the establishment and maintenance of photosynthetic symbiosis.