April 2007
Written by Jay Fitzsimmons

Tags: technology, barcode, forensics

Genetics and Forensic Entomology
If you’ve watched C.S.I. (Crime Scene Investigation), then you might be familiar with forensic entomology, the use of insects in legal investigations. The most common use of forensic entomology is in figuring out how long a person has been dead based on the maggots on them. If you can figure out how old the maggots are, the person has likely been dead at least that long. Forensic entomology has been used in several death investigations across Canada.

How do you figure out how old a maggot is?
Well, you need to know a few basic things first:

• What species of fly the maggot is (maggots are fly larvae).
• What kind of weather (especially temperature) has the body been in lately
  (the environment influences how fast maggots grow).
• How long the maggot is, as maggots get longer with age.

Once you have the basic info, you compare it to previous research. For example, if you find a maggot from a body is the species Calliphora vicina, and the body has been indoors at 21oC, and the maggot is 6 mm long, then previous research suggests the maggot is at least 2 days old. Therefore, the person has likely been dead for at least two days.

So, how do you find out what species of fly a maggot is?
Most forensic entomologists take the maggots from the body and raise them in jars with rotting meat until they grow up to become adult flies. A forensic entomologist can then use characteristics such as the number of veins in a fly’s wings and the shape of male genitalia to determine what species of fly it is. They have to raise the maggots up to adult flies because, even to an expert, most species of maggot look the same.

Genetics is changing how forensic entomologists identify maggot species
Every species of fly has different DNA. Therefore, reading the genetic sequence of a maggot’s DNA, will tell you the species of fly. This has several advantages over the traditional method of raising maggots to adult flies:

1. Speed: Grinding up a maggot and analyzing its DNA takes only a day or two instead of days to weeks to raise a maggot to an adult fly. The faster police can get an estimate of how long a person has been dead, the more precisely they can focus their investigation.
2. Availability: DNA sequence-reading facilities are common, while fly experts are rare and often far away.
3. Specimen types: You can get DNA from dead maggots, or parts broken off maggots in shipment from the police, but in order to raise a maggot to adulthood it needs to be alive and healthy.
4. Courtroom evidence: The type of evidence courts are most likely to accept is the kind that is based on numbers and probabilities as opposed to an expert’s personal opinion. Using DNA to determine what species a maggot is will result in a percent probability estimate that can be presented in court (e.g., “The maggot had a 99.3% probability of being the species Calliphora vicina based on its DNA sequence”), while using the anatomy of an adult fly does not provide any probability estimate.

More research is required before forensic entomologists can use DNA to identify maggots instead of the traditional method of raising them to adult flies. For instance, researchers are finding out which genes are best to identify fly species. The perfect genes are those that differ among individuals of different fly species, but are the same for individuals of the same fly species.

Why are researchers looking for more than one gene for species identification?
One approach to identifying animal species is to use ‘DNA barcoding,’ which is essentially reading the sequence of a single mitochondrial gene that should be different for every species. DNA barcoding identifies the correct species most of the time, but 'most of the time' is not optimal in forensics when lives hang in the balance. Some forensic entomologists are advocating using DNA from more than one gene to identify species, using DNA barcoding as a first step towards sequencing additional genes.

Sometimes a fly of one species mates with another species, and the offspring are a genetic mix of both. Some genes of the offspring ‘look like’ DNA from the mother’s species, and other genes ‘look like’ DNA from the father’s species.

Like a dirty family secret, this genetic mash-up lingers in the fly’s family for generations. Even when the great-great-great grandchildren have stuck to mating with only one species and most of their DNA is from that species, there will still be a small remnant of DNA from the other species.

If a forensic entomologist were to read only one gene to determine what species a maggot is, they might read one of those rare genes from the ‘wrong’ species. If they do, then the forensic entomologist would calculate the wrong minimum time since death, and that could lead to major problems for the police investigation. That’s why forensic entomologists are looking for more than one gene to use for species identification – the more genes used, the more reliable the conclusion.

Genetics & Forensics
Human genetics has been used in forensic science for years, but only recently have forensic scientists used genetics of non-human animals. Some people are grossed out at the sight of maggots, but when a forensic investigator sees a maggot, they see evidence that can help solve a crime.