Researchers have successfully sequenced the genome of the black blow fly, a common insect referred to as “forensic timekeepers.” With an excellent sense for smelling recently dead tissue, black blow flies are usually the first insects to colonize a human body, usually within minutes after death. Females lay eggs on recently deceased corpses, setting a "clock" that enables forensic investigators to estimate the postmortem interval, or minimum time since death.
There is nothing special about black blow flies—which is what drew Christine Picard to them in the first place. It took Picard, a biology professor and forensic scientist at Indiana University-Purdue University Indianapolis, and her team five years to sequence the fly’s genome.
It’s lack of uniqueness is exactly what makes it unique.
"If you are interested in studying a particular human disease, for instance, you don't start by studying people with the disease,” explained Picard. “You start by studying healthy individuals, and then you look for differences between the healthy and the sick to make sure any differences that you observe are actually due to the disease and not due to other factors. The first step is to figure out what the normal is. That's why I have been studying this fly for a decade.”
Thus, Picard has established a baseline of sorts for other forensic scientists and/or entomologists. The genome provides a resource for studying other insects that have unusual or dangerous characteristics.
Through genome sequencing and processing, Picard calculated the female genome of the black blow fly to contain 8,312 genes, while the male genome totaled 9,490 genes. Additionally, the genomes appeared to contain few mobile genetic elements, an almost complete absence of short interspersed nuclear elements, or SINEs, and most of the landscape comprised simple repetitive sequences.
In addition to their role in forensic analysis, black blow flies also have environmental and medical uses. They feed on decaying flesh and help consume dead vertebrates throughout the environment; and their larvae are used medically to debride human wounds, as the insects physically remove dead tissue while simultaneously excreting antimicrobial compounds into the wound.
Picard said mapping of the black blow fly genome will help researchers gain better insight into insecticide sensitivity and resistance. Knowledge of the genome will advance understanding of the antimicrobial compounds secreted by these insects, as well.
"Now that we have described the genome, I plan to continue working toward a better understanding of black blow fly population variation from location to location, and show how the variations influence postmortem interval estimates, with the goal of making these important determinations more accurate,” said Picard.
This isn’t Picard’s first insect contribution to the forensic science community. In September 2012, during a routine collection of fly samples, the forensic biologist discovered a non-native fly in Indianapolis that had never been documented to fly farther north than New Mexico.
Since it had never been seen locally before, Picard said the lack of data on the fly—Chrsomya megacephala Fabricius—could negatively impact the accuracy of forensic investigations.
C. megacephala breeds in the decomposing flesh of animals or discarded organic materials and has the potential to carry disease.
“This discovery tells us as researchers that there is a new fly we have to consider, especially when we’re processing casework samples,” Picard said at the time.