Comparison of two ancient DNA extraction protocols for skeletal remains from tropical environments

Abstract

Objectives: The tropics harbor a large part of the world’s biodiversity and have a long history of human habitation. However, paleogenomics research in these climates has been constrained so far by poor ancient DNA yields. Here we compare the performance of two DNA extraction methods on ancient samples of teeth and petrous portions excavated from tropical and semitropical sites in Tanzania, Mexico, and Puerto Rico (N=12). Materials and Methods: All samples were extracted twice, built into double-stranded sequencing libraries, and shotgun sequenced on the Illumina HiSeq 2500. The first extraction protocol, Method D, was previously designed for recovery of ultrashort DNA fragments from skeletal remains. The second, Method H, modifies the first by adding an initial EDTA wash and an extended digestion and decalcification step. Results: No significant difference was found in overall ancient DNA yields or post-mortem damage patterns recovered from samples extracted with either method, irrespective of tissue type. However, Method H samples had higher endogenous content and more mapped reads after quality-filtering, but also higher clonality. In contrast, samples extracted with Method D had shorter average DNA fragments. Discussion: Both methods successfully recovered endogenous ancient DNA. But, since surviving DNA in ancient or historic remains from tropical contexts is extremely fragmented, our results suggest that Method D is the optimal choice for working with samples from warm and humid environments. Additional optimization of extraction conditions and further testing of Method H with different types of samples may allow for improvement of this protocol in the future. 1 bioRxiv preprint doi: https://doi.org/10.1101/184119; this version posted December 16, 2017. The copyright holder for this preprint (which was not certified by peer review) is the author/funder, who has granted bioRxiv a license to display the preprint in perpetuity. It is made available under a CC-BY-NC-ND 4.0 International license. Ancient DNA (aDNA) is a low quality and low quantity source of genetic material, which is highly susceptible to external contamination (Gilbert et al., 2006; Hofreiter et al., 2001; Pääbo, 1989). Due to the variety of taphonomic and diagenetic processes that take place after death, DNA decays exponentially once cell repair functions cease in biological tissues (Hofreiter et al., 2001). …