Effects Of The Symbiont On Host In Paramecium Bursaria-Chlorella System

The effects of Chlorella symbionts on the ultrastructure, thermal tolerance and mitochondria! genomic structure of the host Paramecium bursaria in the symbiotic system was studied. The results in cellular and genie level add fundamental data to the knowledge of the symbiotic system.The electron microscopic observations revealed that the Chlorella-containing P.bursaria contains more mitochondria than Chlorella-free Paramecium, which accumulate in the region of highly metabolism activity such as peri-basal body. It is inferred that, in the long-period dark culture, the photosynthesis suspension resulted in the disappearance of the Chlorella from the host. The P. bursaria without Chlorella has to obtain nutrition individually hence the number of mitochondria increased dramatically to meet the energy requirement.The effects of Chlorella symbionts on the thermal tolerance of the host paramecium were observed. It is demonstrated that survival ratio of Chlorella-containing P. bursaria was higher than that of Chlorella-free P. bursaria in the range of 35 to 40 degrees centigrade. At 41 "C, Chlorella-containing P. bursaria had longer time of thermal tolerance than Chlorella-free P. bursaria. Recoverd P. bursaria is of similar thermal tolerance with Chlorella-free P. bursaria. It is suggested that symbiotic Chlorella improved tthe thermal tolerance of Chlorella-containing P. bursaria. The thermal tolerance also be improved when adding the photosynthesis product of maltose to the culture medium of chlorella-free P. bursaria, which showed that the presence of Chlorella enhanced the thermal tolerance of Chlorella-containing P. bursaria. Compared PCR product of hsp 70 , it is showed that the band of Chlorella-containing P. bursaria was more intensive than that of chlorella-free P. bursaria. The results indicated that symbiotic Chlorella may effect the hsp70 gene, then improv the thermal tolerance of the host paramecium. This also indicated that HSP70 highly expressed and rescued the denatured proteins so as to reduce the damage of the host. However, no similar phenomenon was observed in the Chlorella-free P. bursaria when environmental temperature enhanced.The results of RAPD-PCR showed that total 64 fragments were amplified by 11 random primers using mtDNA as templates, raging from 150 to 1500bp. In these fragments, the mtDNA s of Chlorella-containing P. bursaria and Chlorella-free P. bursaria shared 27 fragments, with similarity value of 88.52%. There are both 3 specific fragments of Chlorella-containing and chlorella-free P bursaria respectively. The result showed that the symbiotic Chlorella altered the mtDNA structure of the host paramecium. MtDNA tend to mutate against nuclear DNA due to its unique structure and physiological characteristics, which inferred that the symbiotic Chlorella may stimulate the mtDNA of host directly tomutate ; alternatively, the nuclear DNA of host was activated to modify its structure by symbiosis, which inversely acted on the mtDNA to alter its structure. Since mitochondria is a semi-autonomous organelle and regulated by nuclear and mitochondrial gene , the structural alternations of nuclear DNA effected the mitochondrial genome to a large extent, and the mitochondrial genome has feedback regulation on nuclear genome.