Fig. 4.—

An evolutionary tree for quinolinate synthase indicates separate origins of eukaryotic sequences. The unrooted Bayesian tree shows posterior probabilities above the branches and PhyML bootstrap values below the branches. Thickened horizontal lines represent 1.0 Bayesian posterior probability. Trees with significantly lower log-likelihoods were obtained when monophyly was enforced for all sequences from Eukaryota, from Viridiplantae plus Cyanophora paradoxa, from Bacteria, or from Archaea (supplementary table S1, Supplementary Material online).