Thesis (Ph. D.)--Ohio State University, 2006. / Title from first page of PDF file. Includes bibliographical references (p. 140-154).

Thesis (Ph. D.) -- Lincoln University, 2007. / Also available via the World Wide Web.

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In this study, the locomotor activity of a nocturnal prosimian, the slow loris (Nycticebus coucang), was monitored under an experimentally varied light regime. During the process of reversing their day-night cycle, activity was monitored for a light-dark cycle roughly correlating to the external day-night, a free-running period in total darkness followed by a reversed day- night light regime. Under constant conditions an endogenous cycle was shown to be present. During the artificial light regimes, locomotor activity became synchronized to the period of darkness.

Virtually nothing is known about how internal circadian clocks interact with daily environmental cycles in nature. Previous work has shown that temperature and light are both able to successfully entrain (synchronize) circadian rhythms in eclosion (adult emergence) and locomotor activity in Sarcophaga crassipalpis when applied independently. However, much less work has been done to evaluate the relative strength of these Zeitgebers (time cues) when applied simultaneously. In nature, light and temperature cycles generally maintain a fixed relationship with one another, with peak soil and air temperature occurring about three hours after peak brightness each day. By manipulating the relationship between these Zeitgebers this project aims to evaluate the effects of conflicting environmental information on eclosion and locomotor activity rhythms in S. crassipalpis. We measured locomotor and eclosion activity under three temperature/light cycle regimes: 1) in-phase temperature and light cycles, with light and thermophase (warm-period) onset occurring simultaneously, 2) thermophase-delayed, beginning six hours after the onset of photoperiod (light-period), or 3) out-of-phase, with the beginning of photophase corresponding to the end of thermophase. In all experiments, eclosion times are very close to thermophase onset, while locomotor activity does not always hold the same phase position. In fact, in the out-of-phase experiment, locomotor activity is almost entirely synchronized with photophase while eclosion appears to anticipate thermophase onset. These findings suggest that eclosion and locomotor activity rhythms are controlled by different circadian oscillators. This fits with the ecological context of these vital life events. Timing of eclosion is critically important to wing development and the survival of the adult. This process is initiated after being underground, with minimal to no light input, for two weeks – therefore, the most reliable Zeitgeber would be daily soil temperature cycling. As these flies are diurnal, one could reasonably expect light to be the primary Zeitgeber for adult activity, as it is more consistent than temperature cycling. Overall, this implies that an organism’s life history and natural environment must be considered when investigating the circadian clock.

Almost all organisms have approximately a 24-hour circadian rhythm that enables them to anticipate their environment’s daily rhythmicity. Anticipation increases their likelihood of success in foraging, reproduction, predation, and other life events. Therefore, a disruption of their endogenous clock results in detrimental physiological consequences that significantly impact organisms’ fitness. Surprisingly, we have found numerous spider species with free-running periods that deviate greatly from 24 hours. Free-running period (FRP) is a standard measurement of the period of an organism’s circadian rhythm found by measuring periodicity of behaviors or physiology under constant conditions (e.g., constant darkness and temperature). So far, these extreme spider FRPs have only been observed in the superfamily Araneoidea, but we have only limited sampling of species outside this clade. Therefore, we want to fill this data gap of non-araneoid spiders to deepen our understanding of the evolution of circadian clocks in spiders. Also, we will observe if significant deviation from 24 hours and wide variation in FRP are common to all spider species or are only characteristics of araneoid spiders. Here, we describe the FRPs of four non-araneoid spider species belonging to the RTA clade: Schizocosa avida, Phidippus audax, Agelenopsis pennsylvanica, and Mecaphesa celer. We detected significant free runs (mean + SD) at p<0.001 using Lomb-Scargle periodograms in three out of the four species: S. avida (23.84 ± 1.03 h); P. audax (22.67 ± 0.36 h); and A. pennsylvanica (23.97 ± 0.32 h). However, M. celer was found to be arrhythmic under constant conditions. These findings of near 24-hour FRPs with low deviation among the RTA species, along with previous data, strongly suggest that extreme FRPs are confined to the Araneoidea clade. Thus, we have phylogenetically localized a major evolutionary change in the circadian system of spiders occurring in the Araneoidea clade, approximately 170 million years ago.

No description available.

Circadian clocks are responsible for scheduling many behavioral and physiological processes to occur at the most appropriate time of day. The resulting daily rhythms also synchronize (entrain) to external environmental cues, known as zeitgebers. This phenomenon of entrainment enables organisms to anticipate daily changes in environmental conditions such as sunrise/sunset, temperature variations, availability of prey, etc. Given the critical nature of entrainment to survival, it is no surprise that the mechanism is conserved across taxa. The misalignment of the intrinsic clock with the external environment results in a plethora of negative consequences, made apparent by studies involving shift work and jet lag. The focus of the present study is to investigate the chronobiology of Larinioides cornutus (Araneidae), a nocturnal orb-weaving spider, with an emphasis on its entrainment to light:dark cycles. Because spiders have received scarce attention with respect to their chronobiology, it is instructive to compare the properties of spider circadian systems with those of the more established circadian model systems, such as Drosophilaand Murines. We found that both lights-off and lights-on are equally influential zeitgeber cues for (determines the phasing of) both activity onset and offset. Locomotor activity typically begins within a half hour after nightfall, continues throughout the night, and ceases just prior to dawn. Phase shifting experiments show that these spiders can re-entrain within 2 days to a light/dark cycle shifted by 6 hours, and within 3 days when shifted by 12 hours. These rates of re-entrainment occur at an extremely accelerated rate compared to mammals, which readjust to a time shift at a rate of around 1 day/ 1 hour of phase shift. In other words, spiders have a minimal jet-lag response. This suggests an increased level of plasticity in the spider circadian clock that has yet to be observed in other organisms. Typical of circadian rhythms in nearly all organisms, activity also persisted (free-runs) under constant conditions. However, in constant darkness (DD), a drastic change in periodicity was revealed in 66% of individuals, from 23.4 to 25.2. This particular phenomenon is rare and likely indicates the possible interaction of multiple oscillators. Further evidence to support this interpretation is the consistent periods of the rhythm displayed before and after the change. In contrast, under constant light (LL) conditions, 65% of spiders were arrhythmic, with 4 individuals ceasing activity completely. Significant periods detected in LL were normally distributed over an unusually broad range, from 16.7 to 34.9 hours, suggesting a high sensitivity to light. Because of the unusual rates of re-entrainment to light/dark cycles, the spontaneous changes in free-running period under DD, and arrhythmicity in LL, we propose that spiders are valuable comparative model organism for elucidating fundamental mechanisms of circadian clocks.