Abstract
Mitochondrial DNAs (mtDNAs) from eight and unknown syngens of Paramecium caudatum, including stocks of different geographic origin, were investigated for their evolutionary relationships using restriction endonucleases. Thirty-six mtDNA variants as to the restriction patterns were identified within 61 natural stocks. Pair-wise comparison of the ratios of shared restriction fragments were used to construct an evolutionary tree, in which mtDNAs were genealogically classified into several distinct groups. Collecting localities of stocks in each mtDNA group were overlapped with those in other groups. Distribution of the mtDNA groups was also overlapped among syngens, i.e., each group included several syngens shared with other groups. Classification of mtDNAs thus did not conform to that of syngens. The possible reasons are natural hybridization among syngens resulting in the introgression of mtDNAs and/or mutations changing the mating-type specificities similar to those of other syngens, which lead to the multiple origin of the syngens. These two possibilities are discussed in relation to the situation of syngens in this species.
