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

LAVINIA OBLANCA, P. D,; KERR, K. C. R.; TUBARO, P. L.; HEBERT, P. D. N.; LIJTMAER, D. A.

Guelph

Congreso; 6th International Barcode of Life Conference; 2015

Biodiversity Institute of Ontario, University of Guelph

Background: Estimating the age of species or their component lineages based on sequence data is crucial for many studies in avian evolutionary biology. Although calibrations of the molecular clock in birds have been performed almost exclusively using cytochrome b (cytb), they are commonly extrapolated to other mitochondrial genes. The existence of a large, standardized cytochrome c oxidase subunit I (COI) library generated as a result of the DNA barcodingin itiative provides the opportunity to obtain a calibration for this mitochondrial gene in birds. Results: COI evolves on average 14% slower than cytb, but considerable variation both among and within avian orders was observed, precluding the use of this value 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 species increases. 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 functional constraints on a small number of sites in cytb that allow it to initially accumulate mutations more rapidly than COI. Significance: Since most species pairs do not fall into the "danger zone" of low divergences, both COI or cytb could be used to estimate species ages with similar results in most cases. However, when a sole calibration is used to assess the age of different nodes in phylogenetic analyses COI could be more appropriate than cytb because its evolutionary rate appears relatively more uniform.