The proportion of overrepresented identical codon pairs can be variable even at genus level.

<p>Two phylogenetic trees of <i>Mycobacterium</i> and <i>Rickettsia</i> species were constructed separately, based on molecular data. For each species, its genome size, the total numbers of overused identical codon pairs and overused non-identical codon pairs in the secondary-favored group were shown. A: Genus <i>Mycobacterium</i> represents an example with overall high level of codon pairing pattern. The most reduced genome, found in <i>M. leprae</i>, has the lowest level of overrepresented codon pairs in the genus. B: Genus <i>Rickettsia</i> represents an example that has an overall low level of codon pairing. The early-diverging lineage of <i>R. bellii</i> has the highest level of overrepresented codon pairs, while other species in the genus, with reduced genomes to variable extents, also possess variant levels of overrepresented codon pairs.