A protocol for obtaining DNA barcodes from plant and insect fragments isolated from forensic-type soils

Soil is often collected from a suspect's tire, vehicle, or shoes during a criminal investigation and subsequently submitted to a forensic laboratory for analysis. Plant and insect material recovered in such samples is rarely analyzed, as morphological identification is difficult. In this study, DNA barcoding was used for taxonomic identifications by targeting the gene regions known to permit discrimination in plants [maturase K (matK) and ribulose 1,5-biphosphate carboxylase (rbcL)] and insects [cytochrome oxidase subunit I (COI)]. A DNA barcode protocol suitable for processing forensic-type biological fragments was developed and its utility broadly tested with forensic-type fragments (e.g., seeds, leaves, bark, head, legs; n, 213) isolated from soils collected within Virginia, USA (n, 11). Difficulties with PCR inhibitors in plant extracts and obtaining clean Sanger sequence data from insect amplicons were encountered during protocol development; however, the final protocol produced sequences specific to the expected locus and taxa. The overall quantity and quality of DNA extracted from the 213 forensic-type biological fragments was low (< 15 ng/μL). For plant fragments, only the rbcL sequence data was deemed reliable; thus, taxonomic identifications were limited to the family level. The majority of insect sequences matched COI in both GenBank and Barcode of Life DataSystems; however, they were identified as an undescribed environmental contaminant. Although limited taxonomic information was gleaned from the forensic-type fragments processed in this study, the new protocol shows promise for obtaining reliable and specific identifications through DNA barcoding, which could ultimately enhance the information gleaned from soil examinations.