Forensic soil DNA analysis using high-throughput sequencing: A comparison of four molecular markers

Soil analysis, such as mineralogy, geophysics, texture and colour, are commonly used in forensic casework to link a suspect to a crime scene. However, DNA analysis can also be applied to characterise the vast diversity of organisms present in soils. DNA metabarcoding and high-throughput sequencing (HTS) now offer a means to improve discrimination between forensic soil samples by identifying individual taxa and exploring non-culturable microbial species. Here, we compare the small-scale reproducibility and resolution of four molecular markers targeting different taxa (bacterial 16S rRNA, eukaryotic18S rRNA, plant trnL intron and fungal internal transcribed spacer I (ITS1) rDNA) to distinguish two sample sites. We also assess the background DNA level associated with each marker and examine the effects of filtering Operational Taxonomic Units (OTUs) detected in extraction blank controls. From this study, we show that non-bacterial taxa in soil, particularly fungi, can provide the greatest resolution between the sites, whereas plant markers may be problematic for forensic discrimination. ITS and 18S markers exhibit reliable amplification, and both show high discriminatory power with low background DNA levels. The 16S rRNA marker showed comparable discriminatory power post filtering; however, presented the highest level of background DNA. The discriminatory power of all markers was increased by applying OTU filtering steps, with the greatest improvement observed by the removal of any sequences detected in extraction blanks. This study demonstrates the potential use of multiple DNA markers for forensic soil analysis using HTS, and identifies some of the standardisation and evaluation steps necessary before this technique can be applied in casework.