Symbiotic alga Chlorella vulgaris of the ciliate Paramecium bursaria shows temporary resistance to host lysosomal enzymes during the early infection process.

Paramecium bursaria free of symbiotic Chlorella species can be experimentally reinfected with algae isolated from algae-bearing cells by ingestion into digestive vacuoles. Isolated symbiotic algae were cloned, mixed with the algae-free P. bursaria at 25 +/- 1 degrees C for 1.5 min, washed and chased, with or without fixation 3 h after mixing. Though genetically identical, a few of the algae were not digested but coexisted with the digested ones in the same vacuole after lysosomal fusion. Light microscopy showed that algal fate did not depend on cell cycle stage or location in the vacuole. Electron microscopy showed that the nondigested algae were not protected by a perialgal vacuole membrane in the digestive vacuole. Moreover, this phenomenon was also observed in the presence of cycloheximide and puromycin, which are known to inhibit algal and host protein synthesis, respectively. These observations suggest that a few algae can acquire temporary resistance to host lysosomal enzymes in order to establish endosymbiosis without algal protein synthesis.