Forensic palynology and the search for geolocation: Factors for analysis and the Baby Doe case

Use of pollen assemblages as forensic evidence in non-seasonal high-altitude soils

Forensic palynology is a tool in criminalistics that uses spores and pollen grains to link a certain geographical location with a crime scene. The comparison of the pollen assemblage of a crime-scene soil and that of footwear of suspects and victims proved to be very useful as judicial evidence in multiple environments with marked seasonality. However, its usefulness in non-seasonal high-altitude soils has not been experimentally evaluated to the same extent. For this reason, the present study addressed this information gap by undertaking a palynological study in areas with high crime rates in the city of La Paz, Bolivia. To do this, we carried out multiple experimental samplings in three locations with different types of soil and different degrees of urbanization. Specifically, we compared whether the vegetation present at the time of taking the reference samples, was reflected in the pollen rain. Results showed that the vast majority of the species present in the vegetation were found in the pollen rain, with the exception of some plant species with entomophilous pollination syndrome. We also show that the transfer between assemblages from pollen rain to footwear happened effectively, which helped identify their geographical origin, and unveiled a great number of useful indicator species.

Using plants in forensics: State-of-the-art and prospects

The increasing use of plant evidence in forensic investigations gave rise to a powerful new discipline - Forensic Botany - that analyses micro- or macroscopic plant materials, such as the totality or fragments of an organ (i.e., leaves, stems, seeds, fruits, roots) and tissue (i.e., pollen grains, spores, fibers, cork) or its chemical composition (i. e., secondary metabolites, isotopes, DNA, starch grains). Forensic botanists frequently use microscopy, chemical analysis, and botanical expertise to identify and interpret evidence crucial to solving civil and criminal issues, collaborating in enforcing laws or regulations, and ensuring public health safeguards. The present work comprehensively examines the current state and future potential of Forensic Botany. The first section conveys the critical steps of plant evidence collection, documentation, and preservation, emphasizing the importance of these initial steps in maintaining the integrity of the items. It explores the different molecular analyses, covering the identification of plant species and varieties or cultivars, and discusses the limitations and challenges of these techniques in forensics. The subsequent section covers the diversity of Forensic Botany approaches, examining how plant evidence exposes food and pharmaceutical frauds, uncovers insufficient or erroneous labeling, traces illegal drug trafficking routes, and combats the illegal collection or trade of protected species and derivatives. National and global security issues, including the implications of biological warfare, bioterrorism, and biocrime are addressed, and a review of the contributions of plant evidence in crime scene investigations is provided, synthesizing a comprehensive overview of the diverse facets of Forensic Botany.

Forensic botany: who?, how?, where?, when?

Plants are a good source of biological forensic evidence; this is due to their ubiquity, their ability to collect reference material, and their sensitivity to environmental changes. However, in many countries, botanical evidence is recognised as being scientifically. Botanical evidence is not mostly used for perpertration, instead it tends to serve as circumstantial evidence. Plant materials constitute the basis, among others, for linking a suspect or object to a crime scene or a victim, confirming or not confirming an alibi, determining the post-mortem interval, and determining the origin of food/object. Forensic botany entails field work, knowledge of plants, understanding ecosystem processes, and a basis understaning of geoscience. In this study, experiments with mammal cadavers were conducted to determine the occurence of an event. The simplest criterion characterising botanical evidence is its size. Therefore, macroremains include whole plants or their larger fragments (e.g. tree bark, leaves, seeds, prickles, and thorns), whereas microscopic evidence includes palynomorphs (spores and pollen grains), diatoms, and tissues. Botanical methods allow for an analysis to be repeated multiple times and the test material is easy to collect in the field. Forensic botany can be supplemented with molecular analyses, which, although specific and sensitive, still require validation.

The forensic utility of reworked geological materials in soil

Geological materials such as rock fragments, microfossils and mineral grains are continuously being entrained (i.e. reworked) into soil during natural weathering processes. Distinctive reworked rock types in soil, and specific components of them such as palynomorphs (organic microfossils), can prove extremely useful in forensic investigations, i.e. to connect (match) people to places. If the outcrop area of a unique rock is small and well mapped, it potentially has substantial evidentiary value in soil forensic studies. Furthermore, clay minerals, geochemical data and minerals may support the presence of a suspect at a crime scene. Modern pollen and spores extracted from soil samples in forensic investigations can be invaluable in linking suspects to crime scenes. This is because the majority of localities, especially those with natural vegetation, have characteristic (often unique) floral character. Reworked (i.e. largely pre-Quaternary) palynomorphs and other microscopic fossils may co-occur with the in situ (indigenous) pollen and spores. If these reworked forms have relatively short geological ranges, they can indicate the age of the bedrock, thereby further helping to place a person at a location. However, stratigraphically recycled palynomorphs in the soil can be somewhat rare and sporadic, and many rock units are entirely or virtually devoid of palynomorphs. Furthermore, glacial sediments such as till can provide highly mixed reworked palynomorph associations due to their typically heterogenous nature. These diverse assemblages are frequently highly distinctive hence can potentially provide very powerful forensic evidence. The potential of geological materials derived from bedrock in soil for forensic investigations is absolutely clear. Hence, the use of reworked microfossils, minerals and rock should be considered in any major crime where the evidence includes soil.

Geographic source estimation using airborne plant environmental DNA in dust

Information obtained from the analysis of dust, particularly biological particles such as pollen, plant parts, and fungal spores, has great utility in forensic geolocation. As an alternative to manual microscopic analysis of dust components, we developed a pipeline that utilizes the airborne plant environmental DNA (eDNA) in settled dust to estimate geographic origin. Metabarcoding of settled airborne eDNA was used to identify plant species whose geographic distributions were then derived from occurrence records in the USGS Biodiversity in Service of Our Nation (BISON) database. The distributions for all plant species identified in a sample were used to generate a probabilistic estimate of the sample source. With settled dust collected at four U.S. sites over a 15-month period, we demonstrated positive regional geolocation (within 600 km² of the collection point) with 47.6% (20 of 42) of the samples analyzed. Attribution accuracy and resolution was dependent on the number of plant species identified in a dust sample, which was greatly affected by the season of collection. In dust samples that yielded a minimum of 20 identified plant species, positive regional attribution was achieved with 66.7% (16 of 24 samples). For broader demonstration, citizen-collected dust samples collected from 31 diverse U.S. sites were analyzed, and trace plant eDNA provided relevant regional attribution information on provenance in 32.2% of samples. This showed that analysis of airborne plant eDNA in settled dust can provide an accurate estimate regional provenance within the U.S., and relevant forensic information, for a substantial fraction of samples analyzed.

Massively parallel sequencing is unlocking the potential of environmental trace evidence

Massively parallel sequencing (MPS) has revolutionised the field of genomics enabling substantial advances in human DNA profiling. Further, the advent of MPS now allows biological signatures to be obtained from complex DNA mixtures and trace amounts of low biomass samples. Environmental samples serve as ideal forms of contact trace evidence as detection at a scene can establish a link between a suspect, location and victim. Many studies have applied MPS technology to characterise the biodiversity within high biomass environmental samples (such as soil and water) to address questions related to ecology, conservation, climate change and human health. However, translation of these tools to forensic science remains in its infancy, due in part to the merging of traditional forensic ecology practices with unfamiliar DNA technologies and complex datasets. In addition, people and objects also carry low biomass environmental signals which have recently been shown to reflect a specific individual or location. The sensitivity, and reducing cost, of MPS is now unlocking the power of both high and low biomass environmental DNA (eDNA) samples as useful sources of genetic information in forensic science. This paper discusses the potential of eDNA to forensic science by reviewing the most explored applications that are leading the integration of this technology into the field. We introduce novel areas of forensic ecology that could also benefit from these tools with a focus on linking a suspect to a scene or establishing provenance of an unknown sample and discuss the current limitations and validation recommendations to achieve translation of eDNA into casework.