Experts search for best way to present phylogenetic findings to a jury
In 1994, in Sweden, phylogenetic analysis was first used in court to help obtain a conviction in a case of HIV transmission. Since then, phylogenetic analysis—sometimes given the misnomer “HIV fingerprinting”—has been used repeatedly in world courts to try cases of deliberate HIV transmission—cases where a person is charged with deliberately infecting another with the HIV virus, either through unprotected sex, dirty needles, or blood exchange.
As with most scientific evidence, the best way to present phylogenetic findings to a jury is still evolving. Experts themselves don’t always agree.
Molecular phylogenetics is a means by which the genealogical pedigree of micro-organisms can be reconstructed from information found mostly in their DNA. The most common forensic use is for tracing or dating transmission of viruses. If organisms can be shown to share a common ancestor, this information may be forensically useful, especially in cases involving intentional HIV transmission.
“Many viruses have high rates of evolution, so there are enough genetic changes in the viruses that they can be followed through time and across transmission events,” said David Hillis, a professor of Integrative Biology, University of Texas.
Forensic phylogenetic analysis works like this. Blood samples are collected from individuals in question, as well as from a control group. The samples are coded, blinding the study to investigators. Viruses are extracted from the blood and DNA and RNA sequences are obtained from the viruses. The analysis, which takes a few days, is usually performed by an expert in phylogenetic analysis and computational biology. That expert, who only knows which samples are the controls, determines if one of the test samples can be identified as being consistent as the transmission source. This is done by determining if a recipient shows a phylogenetic subset of the sequence variation present in the source. Once a determination is made, identities are decoded and the results found to support or refute charges.
In a 2010 paper (Proc Natl Acad Sci 2010 Dec 14), Hillis said phylogenetic methods are ideally suited for determining the HIV pattern of descent in cases of suspected transmissions, and that phylogenetic analysis can also provide evidence about the direction of transmission. Direction of transmission is supported by a phenomenon called paraphyletic relationship—where a subset of source viral sequences is more closely related to all recipient sequences than to other source sequences.
"Strongly supported paraphyly can provide evidence to infer direction of transmission between pairs of epidemiologically related individuals," Hillis said. "However, a lack of paraphyly cannot be used to refute a possible transmission route."