Abstract

A vast amount of research has been conducted on various insects such as crickets, flies, cockroaches, and beetles, demonstrating the existence of circadian rhythms and how these rhythms relate to insect behavior. This review defines commonly used terms, and describes the behaviors associated with the rhythms and their hosts. Timeless protein (TIM), timeless gene (tim), period protein (PER), period gene (per), pacemaker, light-dark cycles, transcription, behavior, biological clock, and allele are terms that are often used in discussing circadian rhythms, but are jargon barriers in describing the phenomenon of rhythms. Reducing jargon to a level of user friendly concepts is critical for dissemination of information. Circadian rhythms have an effect on the macro and microbiological functions of behavior. Sleep, foraging, and photosynthesis patterns are examples of circadian rhythm's effect on macro behavior, while micro behaviors are those activities at the cellular level which give rise to ultradians. Behaviors associated with circadian rhythms range from those listed above to courtship, visual capabilities, migration, phosphorylation, and photoperiodism.
 

Key words: Circadian rhythms, insects, behavior
 

Introduction

 Circadian rhythms are cycles within a living organism, that take about 24 hours in order to complete from start to finish. These cycles are currently being studied feverishly in order to explain the detailed mechanisms and intricacies involved. The basic scientific consensus about rhythms is that they are quite complex, not understood, and that they seem to effect everything imaginable. The purpose of this review is to familiarize the reader with some of the terminology used by scientists and to use current research to further explain the terms. With this in mind, the structure of this review is modeled closer to that of a glossary than a paper. The majority of the terms are in alphabetical order. Due to the complexity of the terms and their inter-relatedness, several different terms may be discussed under a particular heading.
 

Definitions

An allele is a variation of a gene. Genes are made up of thousands of base pairs (bp) of nitrogenous molecules to form DNA (deoxyribonucleic acid). The difference between one allele and another allele may be a single difference in one of the thousands of base pairs. Therefore, a single gene may have many different alleles. The allele that is exhibited (expressed) is termed dominant, while the allele that is not expressed is recessive. Recessive alleles are expressed if both alleles on the chromosome (for a specific feature i.e. color) are recessive. However, if one of the alleles is dominant or if both alleles are dominant, then the dominant allele will be exhibited. An example would be Swallow tail butterflies. Some of the female butterflies have red spotted wings (recessive) while most (dominant) have white spotted wings (Bjostad, 1999).

Behavior of an insect is often thought of as a response to a particular stimulus, while in mammals this definition is further expanded to how an animal acts or reacts. This implies that mammalian behavior may not always be dependent on a stimulus. However, insects are believed to be stimulus driven and as such, their behavior is reactionary. Stimuli may be either from an external (exogenous) or an internal (endogenous) source. Eclosion, the emergence of an insect larva from an egg, is an example of a behavior that is governed by an endogenous clock referred to as a biological clock or clock gene. A grasshopper that leaps away when touched is reacting to an exogenous stimulus.

The biological clock is an internal device that keeps the body's time by driving and (or) coordinating a circadian rhythm. Another name for a biological clock is an oscillator. In mammals, the suprachiasmatic nucleus (SCN) is believed to be the biological clock.

Circadian means about one day. Therefore, a circadian rhythm is a cycle that takes about 24 hours in order to complete.

Circadian axis is a name for a group of body parts comprised of the retinas, the pineal complex (of which the pineal gland is a photoreceptor, a circadian oscillator, and produces melatonin) and the suprachiasmatic nucleus (SCN) which is located in the region of the brain called the hypothalamus. The circadian axis is common to all life forms without a backbone (invertebrates).

Circadian organization is the rules and principles that guide the entire circadian system from the cells up. Circadian organization does not refer to activities within a cell, but how cells work together to be rhythmic. If circadian rhythms were a house, circadian organization would be the blueprints.

Circadian rhythm is a self-sustained rhythm (cycle) that repeats itself approximately every 24 hours. This term is reserved for living organisms. For example, the sleep pattern of a housefly is a circadian rhythm while traffic congestion over the course of a day is rhythmic; it is not a circadian rhythm. Daily activity rhythm is another term for a circadian rhythm. It has been demonstrated in the melon fly that specific activities, such as mating, and wing vibration only occur during a specific time during a daily activity rhythm (Miyatake, 1997). This fixed time of a behavior within a circadian rhythm has been documented in several species (De Lima,et al., 1994; Fitt, 1981ab). Miyatake (1997) suggests that there is a positive correlation between when mating occurs and the length of development that a melon fly larvae undergoes (development period) prior to eclosion. Two neurons found in the eye of a housefly (Musca), L1 and L2 are being extensively studied for their relationship with light/dark environmental cues.

Circadian oscillators are biological clocks.

Circannual is an annual rhythm. An example would be the annual migration of monarch butterflies from North America to Mexico.

A clock is a term to describe the entire biological mechanism that regulates natural occurring rhythms in an organism. This mechanism occurs in a particular gene referred to as a clock gene. There are different clocks for different rhythms. The clock gene that controls when an ant lays an egg is different than the clock gene that controls when an ant searches for food. There is some evidence that although these clock genes are different for different functions they maybe quite similar in their composition (Lee et al., 1998). Furthermore, the gene that controls egg laying in the ant maybe very similar to the gene that controls egg laying in other insects. Nature tends to be conservative. Her motto may very possibly be "if it isn't broke, don't fix it." There is increasing evidence that clock genes whether in an insect or a mammal regardless of its function are similar. These clocks can be reset (entrained) by various factors such as temperature and light (Hege et al., 1997). In vitro experiments conducted by Weber (1995), have suggested that the circadian clock in cockroach legs is located within the skin (epidermis) of the leg. This lends support to the theory that circadian rhythms and their clocks are wide spread throughout the body.

DNA (deoxyribonucleic acid) is a tightly coiled strand of genetic information that resembles a twisted ladder. DNA is the blueprint for the design and building of a living system such as a grasshopper, beetle, daisy or a human. Replication is the splitting of the two strands of DNA and each strand then acquiring a second strand to take the place of the strand that was removed. This is how DNA can make a copy of itself. DNA may be converted to a protein in two major steps, transcription and translation. Transcription is the process of a strand of DNA splitting down its center and "rewritten" as a strand of RNA (ribonucleic acid). RNA is very similar to DNA with the biggest distinctions being a different sugar used to connect the bases, and having one different base than DNA. Translation is the final step in making a protein from a strand of DNA. In this step the RNA is joined by amino acids (protein building blocks) and thus becomes a protein. This is an over simplified explanation of replication and protein construction.

The doubletime protein (DBT) is believed to be a PER kinase that assists in post- transcriptional levels of PER (Kloss et al., 1998; Price et al., 1998). DBT has also been found to attach itself to PER (Period protein) of adult fruit flies (Kloss et al., 1998). This cooperation between the two proteins may regulate the attachment, combination of, or transportation of phosphates (phosphorylation) in fruit flies (Kloss et al., 1998).

Entrain is a verb which specifically refers to synchronization when used in the context of biology. We synchronize our watches but we entrain our cellular clocks. Environmental factors such as light and temperatures can entrain (synchronize) a cellular clock. Although these are considered the normal factors for entrainment, it does not appear to be understood as to how this occurs. Yang et al. (1998), discuss entrainment occurring in fruit flies that had for all intents and purpose, no eyes. They suggest that although light can entrain a behavioral rhythm, the eyes are not a required medium for the light to cause a shift in a rhythm. This also lends credence for the theory that circadian rhythms and their components being wide spread throughout the body and occur at both the cellular and subcellular levels.

An entraining agent is that which synchronizes the internal clock to environmental cues. i.e. light, temperature, and food.

Environmental cues are external cues, which resets the circadian clock. Light is a very common cue used by circadian pacemakers to keep time. Environmental cues, entraining agents, and zeitgebers are interchangeable terms.

A free-running rhythm is a circadian rhythm that is independent of environmental cues.

An infardian rhythm is a cycle that is greater than 24 hours. An example would be the frequency of receptivity of female fire flies to male fire flies for mating.

In vitro refers to occurring outside the living body, but within an artificial environment. Combining a sperm and egg in a petri dish to make a fetus is an in vitro procedure.

In vivo occurs within the living body. For example, the production of eggs by a female water mite occurs in vivo.

Light-dark cycles are those periods in which there occurs light and then darkness. Usually the length of light and darkness are equivalent. Light-dark cycles are popularly used as an entraining agent in order to study an organism's rhythm, as the length of the cycle can be easily manipulated within a laboratory. The naturally occurring light-dark cycle is approximately 24 hours in length, which corresponds to the length of circadian rhythms.

Neurons are nerve cells that make up a large part of the nervous system. They transport chemical messages (neurotransmitters) from one part of the body to another.

A neurotransmitter (NT) is a chemical signal, which is released by a neuron to find a corresponding neuron and deliver a particular message. For example, the brain might send a signal (NT) to an arm that says, "The stove is hot, remove your hand!"

Pacemaker is an internal mechanism, which keeps time for the circadian clock by either coordinating or constructing rhythms.

Period (PER) protein has been found in various cells and tissues in the fruit fly (Drosophila). Liu et al., (1992) discovered PER operates within the Drosophila's nuclei. Hege et al. in,1997, found that the PER (period) protein continues to cycle up to three days after a fly's head has been removed, whether or not the corpse is exposed to periods of light and dark. This suggests that the brain does not control all circadian rhythms. Sauman et al (1996) found that PER is critical for egg hatching in silkmoths.

period (per) gene is a gene that operates from within the pacemaker, and is associated with maintaining rhythms. In the fruit fly (Drosophila), the per gene produces and maintains the fly's rhythms (Liu et al., 1992). per also influences development and as such has led to the speculation that there is a relationship between the biological clock and development period (Kyriacou et al., 1990).

A phase response curve is the direction and the amount of shifting of an organism's circadian rhythms plotted on the "y-axis" and the specific environmental cue the organism is exposed to being plotted on the "x-axis".

Phase shift is the resetting of the internal clock due to an entraining agent. Depending on when an organism is exposed to the environmental cue (entraining agent) the clock can be set forward, set back, or not changed at all.

Postmortem refers to after (post) death (mortem) while ante-mortem is before (ante) death (mortem). For instance, people are given physicals while they are alive (ante-mortem), and after they die (postmortem), they undergo an autopsy.

A pulsatile is a regular rhythmic movement. The beating of a heart is pulsatile.

Rhythm refers to a pattern of reoccurring events. In biology, rhythms can be loosely thought of as a cycle. An example would be a sleeping pattern.

Rhythm desynchronization and rhythm disruption are the result of a living system's physiological rhythms being out of sync with the environmental cycles. Jet lag is a good example of rhythm desynchronization. Jet lag occurs when the clock on the wall and the position of the sun indicate that it is evening (environmental cue), but the body believes (physiological rhythm) that the time is closer to midnight.

The timeless (TIM) protein is a critical part of the fruit flies circadian pacemaker. Relatively little is known about TIM or the timeless gene (tim). TIM, PER, tim, and per appear to be working hand in hand in the function of circadian rhythms. Suri et al., (1999) found that TIM has an effect on per mRNA levels in the fruit fly. It is of interest to note that light has a negative effect on TIM protein, causing the protein to vanish (Suri et al., 1998; Yang et al., 1998; Zeng et al., 1996). This phenomenon led scientists to hypothesize that this decrease in TIM by the presence of light was a mechanism for resetting the biological clock (Hunter-Ensor et al., 1996; Lee et al., 1996). The timeless gene (tim) has been found to alter the period protein (PER) and further establishes that TIM protein works with PER as a pacemaker (Rutila et al., 1996).

Ultradian rhythms are those which cycle in less than 24 hours. These rhythms are a reflection of the "beating" or pulsing of individual cells. One ultradian rhythm equals one activity cycle (not to be confused with a daily activity rhythm). Daydreaming, urination, and hunger are all examples of ultradian rhythms. Activity cycles are positively correlated to the size of the organism. The larger the animal the greater the activity cycle (Smith 1998).

Zeitgeber is the term for a cue that resets the circadian clock. Light is a very common cue used by circadian pacemakers to keep time. Zeitgber is also known as an environmental cue, and an entraining agent.

Conclusion

 Rhythms are intrinsic to possibly all life forms and occur at all levels of the life form. The digestive tract may be controlled by a rhythm, as is a single cell, and as does a part of a cell such as a nucleus. There are various types of rhythms: Circadian, infardian, ultradian, and circannual. One organism will have multiply rhythms operating at a given time. These rhythms appear to be complex and highly specific for what they control, but they are also seem to be very similar in how they work to control and produce a rhythm..
 
The key to understanding any area is knowing the key terms and jargon associated with the topic. It has been said that if five essential terms are known, then a conversion with an expert in that field can be held (Gary Smith, personal conversation, Dept. of Animal Sciences, Colorado State University, 1998).
 
 

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