Calibrating the molecular clock beyond cytochrome b: assessing the evolutionary rate of COI in birds

LAVINIA OBLANCA PABLO DAMIÁN; KERR KEVIN C. R.; TUBARO PABLO LUIS; HEBERT PAUL D. N.; LIJTMAER DARÍO A.

Guelph

Congreso; 6th International Barcode of Life Conference; 2015

University of Guelph

Background: Estimating the age of species or their component lineagesbased on sequence data is crucial for many studies in avian evolutionarybiology. Although calibrations of the molecular clock in birds have beenperformed almost exclusively using cytochrome b (cytb), they are commonlyextrapolated to other mitochondrial genes. The existence of a large, standardized cytochrome c oxidase subunit I (COI) library generatedas a result of the DNA barcoding initiative provides the opportunityto obtain a calibration for this mitochondrial gene in birds. Results: COIevolves on average 14% slower than cytb, but considerable variation bothamong and within avian orders was observed, precluding the use of thisvalue as a standard adjustment for the COI molecular clock for birds.Distances for cytb are higher than those for COI for closely related species,but the values become similar as the divergence between the speciesincreases. This pattern appears to result from the comparison of a gene(COI) with a relatively constant rate and one (cytb) with a negatively time dependent rate, a difference that could be related to lower functionalconstraints on a small number of sites in cytb that allow it to initiallyaccumulate mutations more rapidly than COI. Significance: Since mostspecies pairs do not fall into the ?danger zone? of low divergences, bothCOI or cytb could be used to estimate species ages with similar results inmost cases. However, when a sole calibration is used to assess the age ofdifferent nodes in phylogenetic analyses COI could be more appropriatethan cytb because its evolutionary rate appears relatively more uniform.