Intraspecific Genetic Divergence of Paramecium bursaria and Re-construc- tion of the Paramecian Phylogenetic Tree

Paramecium bursaria, a species belonging to the genus Paramecium, is quite unique in that the species intracellularly maintains several hundreds of symbiotic algae. This species has been regarded as firstly branched-off species within this genus, and reported its higher genetic variation. We determined rDNA 5’ half regions of 13 strains of P. bursaria, and investigated their intraspecific genetic divergences with comparisons of 18S rDNA and internal transcribed spacer 2 (ITS2). The 18S rDNA divergence among the strains of P. bursaria reached 1.61% which definitely greater than that of P. caudatum, P. multimicronucleatum and P. aurelia complex, and also greater than some interspecific relations (e.g. between P. jenningsi and P. schewiakoffi). These results seemed to be caused by the strains used for the experiments; we used large number (14) of P. bursaria strains and small number of others (four P. caudatum, three P. multimicronucleatum and three P. aurelia complex strains). Comparison of the ITS2 region using 10 P. bursaria, 13 P. caudatum, 11 P. multimicronucleatum and 18 P. aurelia complex strains showed inverted results: P. multimicronucleatum and P. aurelia complex diverged greater than P. bursaria in the quantitative (number of nucleotide substitutions) as well as the qualitative sense (presence/absence of [hemi-] compensatory base change). Increasing the divergent sequence number occasionally changed the topology of the phylogenetic tree. We re-evaluated the phylogenetic relationships of Paramecium spp. as inferred from 18S rDNA with our 20 added paramecian sequences. The trees indicated that paramecia divided into four lineages: i) P. bursaria, ii) P. duboscqui and P. putrinum, iii) P. calkinsi, P. jenningsi, P. nephridiatum, P. polycaryum, P. woodruffi, and P. aurelia complex, and iv) P. caudatum, P. multimicronucleatum and P. schewiakoffi; in strict correspondence with the latest subgeneric concept of „Chloroparamecium”, „Helianter”, „Cypriostomum” and „Paramecium”, respectively. „Chloroparamecium” and „Helianter” of which there are similar characteristics were claded, as were „Cypriostomum” and „Paramecium”.