The human DNA content in artifacts deposited by the blowfly Lucilia cuprina fed human blood, semen and saliva
Introduction
Blowflies and other insects are known to consume decomposing bodies at crime scenes and subsequently leave deposits, termed artifacts, through regurgitation and excretion. While it is likely that DNA in any biological material consumed by flies would be broken down by digestive processes, at least in part, human DNA has been successfully extracted and profiled at STR loci from the excreta of an adult human crab louse [1], and the gut contents of fly larvae [2], [3], [4], [5], [6] and mosquitoes [7], [8], [9]. Furthermore, DNA profiling kits are becoming increasingly sensitive and more able to derive forensically useful human profiles from low level and degraded DNA [10], [11], [12]. As a consequence of these developments, the amount of biological material required for amplification has decreased and DNA is routinely collected from previously ineffectual sources, such as bitten food [13], fingerprints [14] and items touched only briefly [15]. It is, therefore, feasible that human DNA profiles could be obtained from by-products of the regurgitation and excretion processes of flies.
Flies readily consume a number of bodily fluids including blood and semen [pers.obs.]. Should fly artifacts deposited after flies have fed on human biological material contain human DNA, they could interfere with crime scene investigations and laboratory analysis. For example, in some instances it can be difficult to distinguish between artifacts and bloodspatter due to their similar morphology [16], [17], [18], [19], [20], [21], [22]. Consequently, it would be possible for an investigator with limited training in bloodspatter patterns, while obtaining samples for DNA analysis, to collect fly artifacts in the mistaken belief that it is blood. In addition to being a contamination risk, fly artifacts could prove to be a source of DNA in instances where the source of the biological material is no longer present, such as when a body has been moved, or where an offender has attempted to clean up at a crime scene. For these reasons, it is important to know if blowfly artifacts contain human DNA and, if so, whether the level of DNA extracted is dependent on the biological material consumed by the fly, and the number of artifacts required to generate a forensically relevant profile. Also, given that the human DNA extracted from fly artifacts would potentially be degraded, and that digestive enzymes may be active in the feces or regurgitant for some time after deposition, it is important that the period over which the DNA remains detectable and also typeable, is investigated.
Section snippets
Flies
Lucilia cuprina blowflies were used in all experiments. L. cuprina is a forensically relevant blowfly, being found in carrion or live human wounds in countries such as New Zealand, Australia, South Africa, Thailand, Japan and the United States [23], [24], [25], [26], [27], [28]. The temperature in the laboratory was maintained at 25 °C and flies were subjected to a 12:12 light:dark daily cycle during rearing and experiments. Before each experiment, groups of 20 adult L. cuprina blowflies were
Experiment Series 1: quantity and quality of human DNA from variable numbers of artifacts deposited after meals of blood, semen or semen/saliva
These experiments assessed whether or not the number of artifacts collected per sample had an impact on the amount of human DNA that could be extracted. Fig. 1 shows the amount of extracted human DNA in each 100 μL sample in each artifact-number group, for each food source. All samples derived from semen/saliva contained more than 2.50 ng of human DNA, regardless of artifact number in each sample. For samples containing semen-based artifacts, increasing the number of artifacts collected from 1 to
Discussion
The presence of typeable human DNA in fly artifacts derived from various biological materials demonstrates that these deposits can be potential contaminants at crime scenes or in forensic laboratories, in addition to being valuable sources of DNA. In the reconstruction of a crime, DNA profiles can be used to determine what area(s) of a crime scene a particular person visited. If human DNA is present in fly artifacts mistaken for biological fluid at a scene, it can make it difficult to
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