The Survival of the Kiss: Presence and Persistence of Salivary Male DNA in Mixed Samples

Abstract

Background/objectives: The study of DNA transfer and persistence has become increasingly significant, driven by advancements in DNA detection sensitivity and the need for reliable forensic evidence. In forensic investigations, saliva and saliva-stained materials are recognised as valuable DNA sources, particularly in cases of homicide, sexual assault, and burglary, where saliva can be transferred between individuals during the criminal act. The time between the crime and sample collection is a critical factor that can influence the success of the analysis. The value of the specimens collected from the victim's skin or mouth (perilabial and labial sites, teeth and tongue) after the crime has not been investigated with currently used highly sensitive and specific molecular methods.

Methods: On the assumption that a significant loss of DNA occurred, in our study, 10 voluntary pairs were tested at different time points after intense kissing and samples were taken from the above-mentioned sites to assess the presence of the donor's DNA. Extracted DNA was quantified using the Plexor HY System kit (Promega), and both autosomal STRs and Y-STRs were analysed.

Results: The results reveal a greater persistence of male DNA on the female partner, particularly in the labial and perilabial regions, even up to 120 min after contact, in terms of both concentration and duration.

Conclusions: This study emphasises the forensic importance of salivary DNA as a solid source of evidence, particularly in investigations involving mixed DNA profiles.

Keywords: DNA persistence; Y-STR; autosomal STR; kissing; mixed profiles; qPCR; saliva.

Figure 1

Amount of Y-chromosome DNA present on each female collection site (n = 10). Y-axis: values of Y-DNA are shown in a logarithmic scale (Log); x-axis: time points.

Figure 2

Schematic representation of the variation in male DNA recovered on female sites over time. Y-axis: values of Y-DNA/Total DNA ratio are shown in a logarithmic scale (Log); x-axis: time points.

Figure 3

Percentage of male autosomal alleles detected on female partner (a) and female autosomal alleles detected on male partner (b) in all collection sites (n = 10) at each time interval considered (min). Results of ANOVA for repeated measures (ANOVA = analysis of variance). vs = versus.

Figure 4

Percentage of Y-chromosome alleles recovered on female partners present on each collection site (n = 10) at all time intervals considered (min). Results of ANOVA for repeated measures (ANOVA = analysis of variance). vs = versus.