Mutation and evolutionary rates in adélie penguins from the antarctic

Abstract

Precise estimations of molecular rates are fundamental to our understanding of the processes of evolution. In principle, mutation and evolutionary rates for neutral regions of the same species are expected to be equal. However, a number of recent studies have shown that mutation rates estimated from pedigree material are much faster than evolutionary rates measured over longer time periods. To resolve this apparent contradiction, we have examined the hypervariable region (HVR I) of the mitochondrial genome using families of Adélie penguins (Pygoscelis adeliae) from the Antarctic. We sequenced 344 bps of the HVR I from penguins comprising 508 families with 915 chicks, together with both their parents. All of the 62 germline heteroplasmies that we detected in mothers were also detected in their offspring, consistent with maternal inheritance. These data give an estimated mutation rate (micro) of 0.55 mutations/site/Myrs (HPD 95% confidence interval of 0.29-0.88 mutations/site/Myrs) after accounting for the persistence of these heteroplasmies and the sensitivity of current detection methods. In comparison, the rate of evolution (k) of the same HVR I region, determined using DNA sequences from 162 known age sub-fossil bones spanning a 37,000-year period, was 0.86 substitutions/site/Myrs (HPD 95% confidence interval of 0.53 and 1.17). Importantly, the latter rate is not statistically different from our estimate of the mutation rate. These results are in contrast to the view that molecular rates are time dependent.

Figure 1. A plot of the frequency of the non-majority nucleotide bases across the 344 bp region of the HVR I for all adult penguins examined in this study.

Only individuals that showed no heteroplasmies were used. The sites that showed transitional mutations are shown. When more than one transition was recorded at a site, the number is shown. The position of the single transversional mutation that was recorded is shown (triangle).

Figure 2. The estimates of the frequency of observed heteroplasmies (μ _o) with the mean and 95% confidence intervals are shown.

The distribution, mode and confidence intervals of the mutation rate (μ) derived from modeling the inter-generational persistence of the heteroplasmies are also shown. The posterior probability densities of the evolutionary rate (k), estimated from 162 ancient penguin bones of known ages, are shown below the line. The median and 95% intervals are given.

Figure 3. The distribution of differences between the two haplotypes in heteroplasmic mothers and their chicks is shown, as determined by DNA trace peak heights.

The curved line represents the prediction of the model, given the estimated number of mitochondria transmitted between mothers and chicks (N = 36.5).

Figure 4. Location of the study site at Cape Bird, Ross Island, Antarctica.