Molecular Identification of Insects Relevant to Medicolegal Casework

This webinar originally occurred on December 2, 2021
Duration: 1 hour

Overview

Accurate insect identification is critical to their use in estimation of time of colonization (TOC) and post-mortem interval (PMI) during medicolegal death investigations. Insect specimens are currently identified by evaluating morphological characteristics as indications of particular taxonomic groups; however, this process is limited because immature life stages typically lack distinguishing morphologies. Identification may be achieved by rearing live specimens; however, this process is time-consuming, labor-intensive, and not always successful.

These challenges may be addressed through molecular identification by DNA “barcoding” wherein DNA sequences from unknown samples are matched to references. This technology enables identification of immature specimens, may be performed without specialized forensic entomology training, and requires equipment common to forensic genetics laboratories. DNA barcoding has been demonstrated in numerous entomological surveys of forensically relevant species; however, the technology has not been implemented for medicolegal death investigations. This is due in part because of challenges in the technology: no single primer set is capable of distinguishing all of the diverse species important to forensic investigations. This may be remedied by applying multiple primer sets to maximize the number of species that may be identified; however, this may be too resource-intensive for publicly funded laboratories participating in medicolegal death investigations.

The presenter will demonstrate a DNA barcoding strategy for identifying insects commonly encountered in casework at Harris County Institute of Forensic Sciences (HCIFS). The strategy encompasses analysis of a fragment amplified from the mitochondrial COI locus from which taxonomic identification may be statistically supported. Targeted species include those that have been previously encountered in our agency’s medicolegal death investigations, in particular members of blow-fly genera Lucilia, Calliphora, Chrysomya, Phormia, and Cochliomyia, the flesh-fly genus Blaesoxipha, and the scuttle fly genus Megaselia. The strategy is advantageous over previous methods in that all target species may be amplified using a single primer set. Identification is demonstrated for specimens whose species-level identification was known, i.e., colony-bred specimens or wild flies that have been identified by morphology. This is additionally demonstrated for larva and pupa collected during past HCIFS medicolegal death investigations for which species-level identification was undetermined by morphology. We describe a database of COI sequences produced from local specimens which provide additional statistical analyses, including phylogenetic analysis for direct sequence comparisons and inter/intraspecific sequence variations for comparisons to local populations.

Detailed Learning Objectives

1. Attendees will be able to understand the benefits of analyzing entomological evidence during medicolegal death investigations.
2. Attendees will be able to identify the resources and techniques required for implementing molecular identification.
3. Attendees will observe identification of casework-type samples using comparisons to published references and local populations.

Presenter

• Sam Kwiatkowski, Ph.D. | DNA Method Development and Validation Coordinator, Harris County Institute of Forensic Sciences Forensic Genetics Laboratory