A natural strain of Paramecium bursaria lacking symbiotic algae

Paramecium bursaria contains hundreds of algae in its cytoplasm as symbionts and retains them over many generations. Recently, aposymbiotic cells of P. bursaria were collected from a pond, To know whether the natural aposymbiotic strain (Ysa2) lost ability to associate with algae, infection experiments were performed using symbiotic algae isolated from green paramecia. Results showed that Ysa2 establish symbiotic association in the same way as artificially obtained aposymbiotic cells (control). However, clustered algae appeared in some Ysa2 cells after infection. Algal clusters differ in number and size from cell to cell. Isolation line culture was performed to pursue the fate of cells having algal clusters. Within several cell divisions, some descendants retained large clusters and such cells were sometimes misshapen, ceasing proliferation and eventually dying. At cell division of cluster-bearing cells, unequal distribution of clustered algae occurred because many algal clusters tend to locate in the posterior of the host cell. Most extremely, all algae were distributed to the posterior daughter cell, whereas the anterior daughter cell contained no algae. This is one possible mechanism of production of Ysa2 in nature. From these results, we conclude that Ysa2 can make symbiotic association with potentially symbiotic algae, but they have some incompatibilities with the algae.