Temporal responses of a community of Antarctic seals to local environmental change
Understanding the impacts of environmental change on polar species, and especially their responses to these changes, is among the most pressing questions in evolutionary ecology. Analysing ancient DNA from the mummified remains of seals from the Victoria Land coast of Antarctica, a region that has been in a state of environmental flux over the last 10,000 years, provides a powerful, temporally-explicit approach to addressing these questions. The remains of approximately 700 seals from four species were identified in the field. The weathering of each carcass was scored, and for a subset of ~225 individuals ancient DNA was extracted and the samples were radiocarbon dated. We amplified short mitochondrial and nuclear DNA fragments via quantitative PCR to examine the relationship between carcass age, carcass erosion, and ancient DNA preservation. We then conducted targeted enrichment and next-generation sequencing for full mitochondrial genomes of these ancient samples as well as contemporary individuals for each species.
Overall, we found a weak but significant relationship between the ages of the samples and their weathering stage. There was no significant relationship between DNA preservation and sample age, though, which may indicate that microclimate or sampling site on the carcass leads to variation in success. The concentration of nuclear DNA in each sample was less than that of mitochondrial DNA, but there was a significant positive correlation between the two, indicating that mitochondrial DNA concentration may predict nuclear DNA concentration. We were able to successfully sequence more than half of the mitochondrial genome (~8,000 bp) for most samples. Earlier analyses of a short mitochondrial region (240 bp) for one species, the southern elephant seal (Mirounga leonina), suggested that genetic diversity and population size increased during warm periods when breeding beaches were released from ice, but then decreased and became extirpated when the ice returned. The results of our analyses of the extensive data set we are generating for the southern elephant seal and three additional species, which differ in their ecological characteristics, will provide further insights into how the community of seals have responded to environmental change.