Understanding Blow Fly Genetic Variation May Reduce Error Rates in Time of Death Estimates

Forensic entomologists use age estimates of insects, such as blow flies on human corpses, to determine the time of death. Texas A&M University entomologist Aaron Tarone, supported by an NIJ grant, noted that although current insect-based estimates are useful, “it is also clear that there are ways to improve both the accuracy and precision of estimates with blow fly age through the use of genetic approaches.”

The research goals of the project were to obtain quantitative and functional genetic information for the blow fly commonly used as a forensic indicator, Cochliomyia macellaria Fabricius. The information resulting from the research can, according to Tarone, “be used to develop both short- and long-term strategies for using genetic tools to account for uncertainty in forensic estimates of blow fly age.”

Tarone noted that “little is known about the consequences of genetic variation in blow fly traits of forensic relevance.” … “There is not much information regarding the impact of genetic variation on error in forensic entomology,” he said. Given that forensic entomologists rely on estimates of the age of blow flies on a corpse to determine time of death, understanding the consequences of genetic variation is an important step in reducing error rates.

Tarone conducted a selection experiment on development time of blow flies that allowed him to “observe the full distribution of development times for starting populations and to observe the change in means and variances in development time over tens of generations of selection.”

“This project has advanced our understanding of the role of genetics in uncertainty in forensic estimates of insect age,” he concluded. His work showed that natural genetic variation has the potential to drive average development time differences of up to six days at 25 Celsius, or 77 Fahrenheit. “That time difference occurred in a controlled experiment and is not what is found in nature,” he said. But it does indicate that genetic variation does influence development time.

“Temperature also likely plays a role in development time,” he said; and while his experiment was done at a single temperature, “we have begun investigating the nature of thermal plasticity in the selected lines [of blow flies].” Results from the selection experiment indicate “that there is high potential for thermal interactions with development time genotypes,” and that these “are likely to affect other traits, including size, which is forensically informative.”

About this Article

The work described in this article was performed under cooperative agreement number 2012-DN-BX-K024 awarded to Texas A&M University.

This article is based on the final grant report Genomic Tools to Reduce Error in PMI Estimates Derived from Entomological Evidence (pdf, 29 pages) by Aaron M. Tarone, Christine Picard, and Sing-Hoi Sze.

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