Waking the dead: the value of population genetic analyses of historical samples

Archived scales and otoliths constitute a unique source of DNA that potentially enables extension of the temporal scale of genetic studies of fish populations by decades and even centuries. We review recent insights into fish population and conservation genetics obtained using analysis of DNA from archived samples. This involves both new knowledge about demographic parameters and population structure in wild populations and insights into consequences of anthropogenic pressure resulting from over-harvesting, habitat degradation and stocking. We show that the latter category of studies have led to significant changes of management practices. Ongoing improvement of genetic methods will undoubtedly further expand the ability to utilize historical DNA samples. We envisage that temporal comparisons of large numbers of coding genes will lead to novel insights into selective responses of fish populations to anthropogenic challenges, particularly fisheries-induced selection and global warming. However, both acquisition and storage of historical DNA samples can be hurdles to temporal genetic analyses, while degradation and low copy number in historical DNA samples render genetic data from such sources prone to technical artefacts. We summarize recommendations for storage of samples and DNA extraction and provide checklists for validation of genotyping results. Finally, we stress that validation procedures also involve documentation of the time and population of origin of historical samples, and the inferences drawn should account for the technical and statistical uncertainties associated with historical DNA analysis.