Euglena: A Paradoxical Eukaryotic Species Serving as a Unique Tool in Research

The complex and unique nature of Euglena makes it a useful experimental tool for research. It is unique because it is a protist that shares both animal-like and plant-like characteristics that scientists make use of to study the evolutionary aspects and biological behaviors of both plants and animals. Many experimental studies benefit from the animal-like behavior of Euglena in the dark and its plant-like behavior in the light. In the light, Euglena is actively photosynthetic and green. When conditions are altered to become dark, Euglena is rendered colorless. However, re-exposure to light will slowly cause Euglena to revert back to its green color resuming its photosynthetic activity. The experiments geared towards studying the molecular basis for these changes could be used to understand the processes of “photosynthesis, phototropism, vision and communication” due to the cellular and structural features present in Euglena (1).

When observing euglena through a microscope, one could expect to see a splendidly green spindle-shaped unicellular organism exhibiting peculiar wormlike contractions, popularly known as ‘euglenoid movements’. Euglena moves due to the presence of a flagellum that projects from its anterior end and runs along the side of its body. This structure is made up microtubules that enable euglena to move forward and rotate, very often following a corkscrew path. The green color is caused by the presence of chlorophyll containing chloroplasts, which are photosynthetic plastids dispersed throughout the cytoplasm. However, there are many variants of euglena that are also colorless, red, yellow or brown (1).

Another fascinating structural aspect of euglena biology is the location and function of a photoreceptive organ called the eyespot or stigma that appears as an orange-red region near the flagellum. This orange-red stigma is what gives this species its name Euglena, which literally translates to true eyeball. The flagellum and the eyespot act together as a singular unit in the detection and movement towards light for the photosynthetic activity of this organism, thereby helping it synthesize carbohydrates from carbon dioxide and water, an autotrophic mode of nutrition, like any green plant (1).

However, euglena also has the capacity for heterotrophic means of obtaining its food, similar to animal-like protists known as peranema, by engulfing particles of food found in its habitat such as still pools and ponds, where they often give a greenish color to the water. As well as being able to move and feed like animals, Euglena also lack a cell wall, a very well-known characteristic of plants. Instead their bodies are surrounded by a flexible structure called a pellicle.

Euglena gracilis and Euglena viridis are commonly studied species of Euglena. However, over thousand species of Euglena have been identified. Euglena can yield useful research information as it is able to adapt to variable environmental conditions such as changes in the temperature, chemical content in its environment and light or dark conditions (1).

It serves as a model organism for the study of the use of light by living systems because molecular studies utilizing its chloroplasts are feasible. Its reaction to light makes it a highly sensory cell and helps answers questions about the relationship between the receptor (the eyespot) and the effector (its flagellum).The mechanism of the eyespot and flagellum is analogous to the reflex action. Experiments involving the eyespot have revealed that the energy absorbed by Euglena is directly proportional to its mobility (1).

Other studies have shown that the pigment present in the eyespot of Euglena gracilis has similarities to rhodopsin (2). Consequently, deeper analysis of the eyespot pigments compared to the visual pigments in the retinas of animal cells may help shed light on the photoreceptor systems of animals. Similarities between the phototactic behavior of Euglena and the visual process in animals may also exist (1).

Euglena in research also has the potential to provide significant evolutionary information about introns of chloroplasts and transitions from plant like characteristics to animal like characteristics.

Introns are non-coding regions of DNA. The evolution of chloroplast introns and twintrons (occurrence of introns within introns) gives valid genetic information about the intron evolution theory. The phenomenon of twintrons may have occurred later in evolution by the insertion of one or more introns into existing introns. The chloroplasts for Euglena gracilis has been identified as the richest source of introns and are used to study the proliferation of group II and group III introns. By looking back in history about specific introns and twintrons using the Euglena plastid lineage it is possible to find out if introns are ancestral or derived traits (3).

Different theories of evolution have been put forward regarding plants and animals. One popular theory believes that an early stem organism with the ability to photosynthesize could have been the precursor of plants and animals. Its ability to photosynthesize would have given it the advantage to derive nourishment in conditions of organic food scarcity. After, the establishment of plant life, when organic foods became plentiful the same organism may have transformed to incorporate animal like characteristics similar to those seen in euglena under dark conditions by loss of its chlorophyll. A different theory suggests that Euglena is representative of a group of organisms containing both colorless and colored forms, from which plants, fungi and animals evolved separately. The evolution of plants may indicate a chance encounter progressing into a symbiosis (1).

Regardless of whether Euglena is closer to animals or plants, the possession of characteristics belonging to both plants and animals show its potential in research as a useful organism giving information about chloroplasts in photosynthesis, pigment synthesis, visual process in animals and its cellular contents pertaining to growth and functional physiology (1). Euglena studies shows that at a molecular level, animals and plants share a lot of similarity forming a common basis for living processes. However, it also emphasizes to all scientists that the answers to the evolutionary process do not easily come by and requires detailed analysis and use of unique organisms like Euglena as research tools.

References

1. Jerome J. Wolken, Euglena. An Experimental Organism for Biochemical and Biophysical Studies. Rutgers (New Jersey) 1961. Rutgers University Press.

2. James T.W., Crescitelli F., Loew E.R., McFarland W.N. The eyespot of euglena gracilis: a microspectrophotometric study. Vision Research.1992; 32: 1583-1591.

3. Thompson MD, Copertino DW, Thompson E, Favreau MR, Hallick RB. Evidence for the late origin of introns in chloroplast genes from an evolutionary analysis of the genus Euglena. Nucleic Acids Res. 1995; 23: 4745–4752.