POLLEN STRUCTURAL DIVERSITY AND ITS APPLICATION TO FORENSIC INVESTIGATION.

TITLE : POLLEN STRUCTURAL DIVERSITY AND ITS APPLICATION TO FORENSIC INVESTIGATION.

ABSTRACT

The study of pollen grains is referred to as palynology. It is one of the most effective tools which can be used to reconstruct past environment. Pollen grains represent the male portion of the reproductive process in plants and trees. In this experiment, pollen from three different plant species were observed under a light microscope and compared to a control sample which comprised of a mixture of the pollen grains. The plant species whose pollen was viewed were of the families Malvaceae, Asteraceae and Rosaceae. Images of the pollen grains were taken from the microscope at a magnification of *400. A notable difference in size, shape, ornamentation and structure was evident. All three types of pollen grains were present in the control sample but an image capturing the mixture could not be taken, possibly due to the difference in size of the pollen grains and hence resolution of the microscope at one magnification. Application of palynology in forensic investigations is explored in the report.

INTRODUCTION

Pollen grains are the tiny male reproductive bodies of flowering plants. Different types flowering plants have evolved different methods of ensuring that these male reproductive bodies reach their appropriate female counterpart. One of these methods is windblown distribution. Because the wind randomly distributes the pollen grains, the plant must produce a huge amount of extra pollen to ensure that at least some of it reaches its intended destination. This extra pollen, called pollen rain, is eventually deposited on the ground where, if conditions are favourable, it may persist for thousands of years. These are microscopic structures that vary in size and shape. Some are tiny orbs, while others are egg-shaped. Although too small to see individually, they can be seen by the naked eye in large quantities. A hard coating is present on the grains and it offers great protection from harsh outdoor conditions. This is important because inside this tough shell lie two cells: the tube cell, which will eventually become the pollen tube, and a generative cell, which contains the male sperm nuclei needed for fertilization (Kurman and Doyle, 1994).

There are three main components of a pollen grain. The inside of the grain is made up of cytoplasm. This fluid medium houses the aforementioned living cells, keeping them moist and alive. The outer shell is made up of two layers. The inside layer is aptly named the intine , which is the thick interior. It is composed partly of cellulose, a common component in the cell walls of plant cells. When pollen falls on regular soil, it tends to migrate downwards with the water percolating through the soil. By mixing modern and ancient pollen together, this creates a more difficult, but not impossible, situation for the palynologist to evaluate (Renfrew and Bahn, 1991).

Pollen grains provide clues as to the source of the items and the characteristics of the environments from which the material on them is sourced. Their usefulness lies in a combination of their abundance, dispersal mechanisms, resistance to mechanical and chemical destruction, microscopic size, and morphology. Their often complex morphology allows identification to an individual parent plant taxon that can be related to a specific ecological habitat or a specific scene. Pollen assemblages characterise different environments and scenes and can easily be picked up and transported away from scenes of interest without providing any visual clue to a suspect as to what has occurred. Pollen grains are microscopic, they remain unseen, silent witnesses. Furthermore, if they were visible, unlike fingerprints, they would be nearly impossible to eliminate from a crime scene (www.sciencedirect.com).

AIM OF EXPERIMENT

To observe structural diversity of pollen from three different plant species under a light microscope, while comparing to the control sample, and determine its application to forensic investigation.

OBJECTIVES

To carry out the air mount mounting technique in pollen structural studies.
To appreciate other techniques used in pollen structural studies.
To examine and appreciate the diversity of pollen size, shape and ornamentation.
To relate pollen size and structure to taxa.

MATERIALS AND METHODS

In this experiment, flowering parts from three different plant species were provided. The anthers were teased in each case so as to allow access to pollen grains. The pollen dust was placed on a slide, separately for each flower. A cover slip was placed on top of the pollen grains and sealed on the side with highly viscous nail polish. The slides were then placed under a light microscope and pollen grains were viewed. Reference was made to the pollen mixture control sample which was provided.

RESULTS

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DISCUSSION

When identifying pollen, there are three characteristics to think of. These are size, shape and structure. The sizes can range from very small to giagantic. Shapes of pollen grains can either be oblate, spherical, prolate or other. On structure, the characteristics under view include exine sculpturing and apertures. The pollen grains under study showed variation in size. At a magnification of *400, the pollen grains of Malvaceae where relatively much larger compared to those of Asteraceae and Rosaceae, as captured in the images. In terms of shape, variation is noted. The pollen grains of Rosaceae appeared to be prolate at the orientation shown while the pollen grains of Malvaceae appeared spherical. In contrast, the pollen grains of Asteraceae appeared to be oval rounded. The diversity in shape and size, as noted, is a powerful tool which can be used in forensic investigations (Kurman and Doyle, 1994).

The tough outer layer of pollen grains is known as the exine, which appears as a thick exterior. This highly sophisticated and complex outer layer is rich in a compound known as sporopollenin. It is waterproof, resistant to deterioration and very stiff making this shell is one of nature's most advanced polymers. It ensures that the tender cells inside have a strong chance of survival. Pollen exines are amazingly diverse, sometimes even to the species level, and their production is generally seasonally and often geographically restricted, thus their presence can point to a specific season, sometimes even a specific location, in which a crime was committed Often times the exine has folds, creases and spikes rising from its surface. Like extra armor, these features add to the protective nature of this layer. They also play an important role in the mobility of the grains, making it more likely that they will stick to the legs of insects as well as catch the wind. The exines of the pollen which was studied showed variation. Rosaceae pollen grain exine appeared verrucate while that of Malvaceae appeared echinate and of Asteraceae appeared techtate. This variation in the sculpturing of the exines is due to genetic adaptations and environmental adaptations. Based on this data, use of pollen exines in identification of pollen can pin point location and seasonality for forensic investigations (www.microbehunter.com )

When identifying pollen grains, the first characteristic to consider is the aperture. An aperture is a thin or missing part of the exine, which is independent of the patterning of the exine. Two different types of apertures can be distinguished, namely pores and fissures (colpi). The latter are more primitive, they are elongated with pointed ends. Pores are usually isodiametric. They can also be slightly elongated but, in contrast to colpi, they have rounded ends. In some pollen grains, the exine around the apertures is either thicker or thinner. Pollen grains with pores are porate and those with colpi are colpate. If both pore and colpus are combined in the same aperture, the pollen grain is colporate. Fenestrate pollen grains form a further group. They have large window-like spaces, where the tectum is missing. The Asteraceae ,among others, belong to this group. In the experiment, the aperture of Rosaceae pollen appeared to be colpate. That of Malvaceae appeared to be porate while that of Asteraceae was viewed as tricolpate. This distinctive feature was apparent and this information would be useful in a forensic investigation (www.sciencedirect.com)

CONCLUSION

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REFERENCES

http://www.sciencedirect.com/science/article/pii/0034666767900413
http://www.microbehunter.com/making-mounts-of-pollen-grains/
Kurman, M.H. and Doyle,J.A. (1994). Ultrastructure of Fossil Spores and Pollen. Royal Botanical Gardens, Kew, England.
Renfrew, C. and Bahn,P. (1991). Archaeology: Theories, Methods, and Practice. Thames and Hudson Publishing. New York.

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