DNA Preservation in Vertebrate Samples: Insights from Forensic and Ancient Contexts

Sample preservation is a critical factor in the success of genetic analyses of biological material, particularly in ancient and forensic contexts where DNA degradation and contamination are prevalent. Analytical outcomes are influenced by multiple factors, some unavoidable, whereas others, including sample collection, handling, preservation, and downstream analyses, can be optimized. This study evaluates current DNA preservation strategies for human and non-human vertebrates, focusing on both biological samples (before DNA extraction) and extracted DNA. The goal was to identify common methodologies, limitations and challenges, and finally to propose a standardized workflow that might improve data quality and reproducibility. After evaluating a broad group of references, 202 of them were chosen as the most representative, with the idea of reporting a minimum quantity of information for each category of biological samples and preservative methods. At the beginning, a brief introduction regarding ancient DNA and forensic genetics, and an explanation about the process of DNA degradation and its related causes, explain the context. Subsequently, the main section of the review contains an analysis of the preservation methods used for each sample type, followed by extracted DNA. Some of these methods (freezing, ethanol, formalin, formaldehyde) have been traditionally used to preserve any kind of sample. Nonetheless, their limitations must be considered, especially when dealing with damaged DNA, which might require other procedures to be stored without inducing further damage. According to our evaluation, freezing and dry storage (especially at RT) remain the most reported approaches overall, even though their performance is strongly sample-type-dependent, with RT-based methods addressing freezing-related problems. This appears to apply to both biological samples and extracted DNA. Combining multiple techniques can be particularly effective, in fact the addition of liquid storage solutions is frequent. Data is insufficient for secretions, touch DNA, and swabs, even though dry storage appears quite common; however, among them, urine and saliva are comparatively better characterized. Considerations regarding the phase prior to storage (sampling, handling and transportation) and post-storage (downstream applications) are also discussed, since they will eventually affect the choice of the storage solution. It was not possible to define a particular difference among storage methods for human samples and other vertebrate animals, apart from the fact that human studies are much more present in the literature.