CONICET | Buscador de Institutos y Recursos Humanos

Estimating the age of species or their component lineagesbased on sequence data is crucial for many studies in avianevolutionary biology. Although calibrations of themolecular clock in birds have been performed almost exclusively usingcytochrome b (cyt b), they are commonly extrapolated to other mitochondrialgenes. The existence of a large, standardizedcytochrome c oxidase subunit I (COI) library generated as a result ofthe DNA barcoding initiative provides theopportunity to obtain a calibration for thismitochondrial gene in birds. In this study we compare the evolutionary rate ofCOI relative to cyt b across ten different avian orders. We obtained divergenceestimates for both genes from nearly 300phylogenetically independent pairs of species through theanalysis of almost 5000 public sequences. For each pair of specieswe calculated the difference in divergence between COIand cyt b. Our resultsindicate that COI evolves on average 14%slower than cyt b, but also reveal considerable variation both among andwithin avian orders, precluding the use of thisvalue as a standard adjustment for the COI molecularclock for birds. Our findings suggest that this variation is partiallyexplained by a clear negative relationship between thedifference in divergence in these genes and the age of species.Distances for cyt b are higher than those for COI for closely relatedspecies, but the values become similar as the divergencebetween the species increases. This appears to be theresult of a stronger pattern of negative time-dependency in the rate ofcyt b than in that of COI, a difference that could be relatedto lower functional constraints on a small number of sites incyt b that allow it to initially accumulate mutations morerapidly than COI.