Multiple origins of a frameshift insertion in a mitochondrial gene in birds and turtles

BACKGROUND: During evolutionary history, molecular mechanisms have emerged to cope with deleterious mutations. Frameshift insertions in protein-coding sequences are extremely rare because they disrupt the reading frame. There are a few known examples of their correction through translational frameshifting, a process that enables ribosomes to skip nucleotides during translation to regain proper reading frame. Corrective frameshifting has been proposed to act on the single base pair insertion at position 174 of the mitochondrial NADH dehydrogenase subunit 3 gene (ND3) that has been observed in several turtles and birds. However, the relatively sparse taxonomic representation has hampered our understanding of the evolution of this insertion in vertebrates. RESULTS: Here, we analyzed 87,707 ND3 sequences from 10,309 vertebrate taxa to reveal the evolutionary history of this insertion and its common genomic characteristics. We confirmed that the insertion only appears in turtles and birds and reconstructed that it evolved independently in both groups with complex patterns of gains and losses. The insertion was observed in almost all bird orders but was absent in all members of the diverse Passeriformes. We found strong conservation in the nucleotides surrounding the insertion in both turtles and birds, which implies that the insertion enforces structural constraints that could be involved in its correction. CONCLUSIONS: Our study demonstrates that frameshifts can be widespread and can be retained for millions of years if they are embedded in a conserved sequence theme.