Mitochondrial nucleotide composition within the genus Paramecium. A) Guanine + cytosine content at 1st, 2nd and 3rd codon positions of the mitochondrial cox1 gene. B) Phylogenetic tree based on 776 bp of the cox1 gene. The numbers represent bootstrap values for 2,000 pseudoreplicates in the Neighbor Joining analysis and the scale bar equals 0.05 substitutions/site. The arrow indicates the phylogenetic position of the G+C shift.