Genetic evidence of American and European' type symbiotic algae of Paramecium bursaria Ehrenberg

Paramecium bursaria is composed of a host ciliate and a symbiont green alga. Based upon physiology, DNA hybridization and virus infection, two types of symbionts, called American type and European type, have been reported to date. Here, we determined the 18S rDNA and internal transcribed spacer 2 (ITS2) regions for both American and European types. Sequence features clearly separated into two lineages; NC64A (USA), Syngen 2-3 (USA), Cs2 (Chinese), MRBG1 (Australian), and Japanese strains belong to the American, whereas PB-SW1 (German) and CCAP 1660/11 (British) strains belong to the European. In American 185 rDNA, three introns were inserted in the same positions as for previously described Japanese symbionts. In European 18S rDNA, a single intron occurred in a different position than in the American. Between the types, sequence differences were seven or eight nucleoticles (0.39%) in the 18S rDNA exon, and more than 48 nucleoticles (19.2%) in ITS2 regions. We subsequently sequenced the host 18S rDNA. As a result, two groups: Cs2, MRBG1, and Japanese strains, and PB-SW1 and CCAP 1660/11 strains, were separated (with 23 substitutions and 4 insertions or deletions between the groups). The congruent separations between hosts and symbionts may imply that the type of symbiont depends on the host type.