The human DNA content in artifacts deposited by the blowfly Lucilia cuprina fed human blood, semen and saliva

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Abstract

Adult flies of some species are known to be attracted to crime scenes where they feed on the proteinaceous decomposition products of dead bodies. The flies leave deposits through excretion and regurgitation, and these artifacts often appear morphologically similar to bloodstains. To date, little consideration has been given to the possibility of the fly artifacts containing forensically useful levels of human DNA, or of flies as vectors of human DNA. In the present study, groups of artifacts collected after the adult blowfly Lucilia cuprina fed on biological fluids were examined and found to contain human DNA sufficient for profiling. Random samples from each group of artifacts were then subjected to human DNA profiling. Of the samples analysed, full or partial human DNA profiles were found in 57% of samples deposited by flies after blood meals, 92% after semen meals, 46% after saliva meals, 93% after blood/semen meals, 58% after blood/saliva meals and 95% after semen/saliva meals. DNA from artifacts deposited after flies were fed blood, semen, saliva, blood/semen, blood/saliva or semen/saliva was extracted at various time points up to 750 days, and the human DNA component quantified. The human DNA extracted from blood- and semen-based fly artifacts demonstrated a clear trend in which the amount of DNA extracted increased over the first 400 days, and full human DNA profiles were still obtained 750 days after artifact deposition. Saliva- and blood/saliva-based samples were tested at intervals up to 60 days and generated partial profiles at this final time. Blood/semen- and semen/saliva-based samples generated full profiles at 250 days. The presence of human DNA in fly artifacts has considerable forensic significance. Fly artifacts could potentially compromise crime reconstruction, and/or contaminate DNA evidence, up to at least two years after their deposition. Alternatively, fly artifacts may be a useful source of DNA if an offender has attempted to clean up a crime scene.

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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