Endosymbiotic Chlorella variabilis reduces mitochondrial number in the ciliate Paramecium bursaria

Abstract

Extant symbioses illustrate endosymbiosis is a driving force for evolution and diversification. In the ciliate Paramecium bursaria, the endosymbiotic alga Chlorella variabilis in perialgal vacuole localizes beneath the host cell cortex by adhesion between the perialgal vacuole membrane and host mitochondria. We investigated whether host mitochondria are also affected by algal endosymbiosis. Transmission electron microscopy of host cells showed fewer mitochondria beneath the algae-bearing host cell cortex than that of alga-free cells. To compare the density and distribution of host mitochondria with or without symbiotic algae, we developed a monoclonal antibody against Paramecium mitochondria. Immunofluorescence microscopy with the monoclonal antibody showed that the mitochondrial density of the algae-bearing P. bursaria was significantly lower than that of the alga-free cells. The total cell protein concentration of alga-free P. bursaria cells was approximately 1.8-fold higher than that of algae-bearing cells, and the protein content of mitochondria was significantly higher in alga-free cells than that in the algae-bearing cells. These results corresponded with those obtained by transmission electron and immunofluorescence microscopies. This paper shows that endosymbiotic algae affect reduced mitochondrial number in the host P. bursaria significantly.

Characterization of Crystals in Ciliate Paramecium bursaria Harboring Endosymbiotic Chlorella variabilis

“…(Kitatani and Kodama, unpublished data). A previous study demonstrated that the number of host mitochondria and trichocysts was significantly reduced with increasing numbers of endosymbiotic algae [ 24 , 25 , 34 ]. Similarly, algal reinfection may decrease the number of host crystals.…”

“…1 N cells [ 24 ]. Strain 1N was identified as C. variabilis by rbcL gene analysis [ 25 ]. Moreover, P. multimicronucleatum strain YM-25 (syngen 2, mating type III or IV [cycler]) was used.…”

“…Crystals were isolated using a previously described method [ 25 ]. Approximately 300 ml of both alga-free and algae-reduced P. bursaria cells were strained through two layers of Kimwipes to remove large debris.…”

Cellular interactions and evolutionary origins of endosymbiotic relationships with ciliates

“…Algae-free P. bursaria cells are smaller in size compared to algae-bearing cells, and their proliferation rate significantly weakens [ 67 , 69 , 71 ]. The presence of endosymbiotic Chlorella also significantly reduces the number of host mitochondria, accompanied by a lower content of mitochondrial proteins compared to algae-free host cells [ 72 ]. Notably, separated P. bursaria and Chlorella can readily form new symbiotic combinations by mixed together.…”