Theorem of the Polyphyletic Origin of tRNA Genes

The arguments regarding the tRNA split genes of N. equitans can be summarized in a theorem on the nonmonophyletic origin of tRNA genes:

"If the half genes ofN. equitans tRNAs represent the plesiomorphic form of tRNA genes, then the mere observation ofthe existence of such genes in an organism implies that the hypothesis of the monophyletic origin of tRNA is false and the polyphyletic origin oftRNA genes is true."

The proofofthis theorem is straightforward. Ifthe monophyletic hypothesis is true, from the evolutionary stage ofLUCA onward we should be able to observe, by definition, only complete tRNA genes and no half genes, because a monophyletic origin would not allow the existence of two kinds of tRNA genes, continuous and split, at

Figure 13.3. Two alternative evolutionary sequences for tRNA genes (IS = intron). a) The hypothesis that evolution progressed from split to unsplit genes and b) The alternative possibility that evolution progressed from unsplit to split genes.1,15

any time after the LUCA stage. Therefore the observation oftRNA half genes after the LUCA stage and the evidence that these half genes represent the ancestral form, imply that the monophyletic hypothesis is false.1,15 It follows that tRNA genes are polyphyletic in origin and that LUCA must have contained half tRNA genes.

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