Many studies have investigated the ecological factors that affect behavior in Micrathena gracilis, a diurnal orb-weaving spider that forages exclusively on flying insects during the day. However, none yet have considered how the temporal distributions of prey and predator occurrences shape their daily behavioral rhythms, especially web construction which involves a heavy energetic investment well in advance of potential nutritional benefit. Recently, other spider species have been found to express significant circadian plasticity, suggesting that circadian clock-controlled rhythms may play a larger role in niche partitioning than once thought. Despite the appearance of significant insect abundance in the evenings, M. gracilis individuals stop foraging, take down their webs, and retreat before they can capitalize on this opportunity. Is the nutritional benefit of this forfeited prey significant compared to what they collect during the day, and if so, what potential cost might justify opting out of this potential gain? To investigate, sticky traps for prey collection and a camera array for recording predator activity were used at a local field site to survey what risks and rewards these spiders face throughout the 24-hour day. Spider activity in a lab environment and web captures in the field were also used to confirm behavioral patterns and nutrient uptake throughout the day. It was found that significant prey biomass is given up shortly after the time that spiders typically retreat, suggesting that the spiders truly forfeit this prey and do not simply retreat due to a gradual decrease in overall prey availability. Spiders reliably cease foraging in the early evening and show agitation throughout the night when not comfortably hidden, suggesting that significant extension of foraging behavior may be harshly punished. However, recorded predation events from the camera array were much rarer than anticipated, and no predation was confirmed in the evening. These results support the notion that these spiders’ circadian rhythms are shaped by factors other than prey availability, but more work is necessary to identify these factors

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.

Circadian rhythms are nearly ubiquitous and are responsible for timing biological processes and allowing for anticipation of regular changes in the environment. The internal clocks of most organisms have a period very close to 24 hours with little variation. Spiders, however, do not seem to follow this pattern. Both the fastest (18 hours) and slowest (29 hours) naturally-occurring clocks are found in spiders, and variation within a species can be orders of magnitude larger than that of previously studied animals. Circadian rhythms are assumed to be adaptive, yet little is known about their heritability in arthropods. Heritability is defined as the amount of phenotypic variation that can be attributed to genetic variation passed down from parent to offspring. Phenotype can be influenced by many complex factors including environmental effects, dominance of genetic sequences, and gene interactions. Because of these influences, the phenotypic characteristics of an individual can vary greatly, and it is often difficult to precisely identify what is truly heritable. Using spiders as a model system, we can exploit the extreme variation in circadian rhythms to investigate the potential contribution from heritability. Strong heritability would suggest that wide variation in circadian rhythms likely reflects high genetic variability in the species. Alternatively, the environment may have a greater contribution in this variation relative to the effects of heritability. To test this, we chose Parasteatoda tepidariorum, a common cobweb spider with a relatively short circadian period of 21.7 hours and intraspecific variation of more than 4 hours. To estimate the heritability of circadian rhythm, adult females were gathered with accompanying egg cases, and juveniles were raised from those cases. Six fundamental parameters of circadian rhythms were measured from the locomotor activity of adults and juveniles. Of those six, only one parameter differed between adults and juveniles: the onset of locomotor activity during the first five days when light cycles were present (Mann-Whitney U= 1814, p= 0.04). When all six circadian parameters were compared by regression of adults to respective offspring, none showed significant correlation. This indicates that variation in circadian rhythms was likely not caused by parental genetics, and that environmental factors, such as artificial light at night, may be the source of the extreme circadian rhythms seen in spiders. Another possible cause for this variation may be the presence of weak molecular circadian oscillators that are more sensitive to environmental factors than those in most other circadian systems.

Circadian rhythms are outputs of the internal clock that regulates the daily functions of almost all living organisms. Circadian rhythms are typically 24 hours because they are synchronized by external cues such as the natural light/dark cycles of the environment. When external cues are removed, the circadian rhythm “free-runs,” thus revealing the organism’s endogenous circadian period. Recent studies have found that the trashline orbweaving spiders Cyclosa turbinata and Allocyclosa bifurca have abnormally short circadian rhythms of approximately 19 and 18 hours, respectively. Trashline orbweavers construct a line of debris made of prey carcasses in the center of their web and then remain undetectable by being cryptic within their trashline. Despite similar circadian rhythms and web-building behaviors, recent genetic findings indicate that these species actually are not closely related. In fact, both genetic and morphological data now suggest A. bifurca is more closely related to Cyrtophora citricola, the Tropical Tent-web spider. This would suggest that trashline behavior and exceptionally short circadian clocks evolved independently in C. turbinataand A. bifurca. This study analyzed the circadian rhythm of C. citricola and compared it to the circadian rhythm of A. bifurca. If C. citricola has an abnormally short clock like A. bifurca, this would indicate that the evolution of the short clock preceded the divergence of these species’ lineages. However, if C. citricola has a more typical clock, this would suggest that the unusually short clock evolved in the A. bifurca lineage and may be more ecologically linked to the trashline behavior. Thirty-two female C. citricola were collected in Southern Florida and had their locomotor activity measured over four days of 12:12 light/dark cycles followed by complete darkness to determine their circadian free-running periods (FRP). Cyrtophora citricola was found to have a more typical FRP of 24.0 + 0.43 hours. Despite being closely related, C. citricola and A. bifurca differ significantly in their circadian rhythms, suggesting that short circadian rhythms may be ecologically linked with trashline behavior.

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MSc(Med. Pharmaceutical Affairs), Faculty of Health Sciences, University of the Witwatersrand, 2009

Chez la majorité des être vivants des systèmes d’horloge circadiennes (=environ 24h) se sont développés afin d’anticiper les variations journalière de l’environnement. Ces horloges endogènes reposent sur des oscillateurs moléculaires régulant l’expression circadienne de nombreux gènes. Récemment il a pu être montré que des régulations post-transcriptionnelles jouaient un rôle majeur dans la rythmicité de nombreux gènes.Dans la lignée cellulaire hypophysaire GH4C1, nous avons étudié un mécanisme post-transcriptionnel impliquant des corps nucléaires appelés paraspeckles. L’élément principal des paraspeckles est le long ARN non codant (lncRNA) Neat1 auquel s’associe des protéines de liaison aux ARNs. Il a été montré que les paraspeckles ont la capacité de retenir dans le noyau des ARNs, en particulier ceux qui présentent des motifs IRAlu dans leur région 3’ non codante. Nous avons montré que dans les GH4C1, les paraspeckles se forment avec une rythmicité circadienne. Grâce à la mise au point d’une méthode dite de RNA pull-down, permettant l’étude des cibles ARNs d’un lncRNA, nous avons montré que les paraspeckles s’associent et induisent la rétention nucléaire circadienne d’ARNs hypophysaires. Chez ces ARNs nous avons montré l’absence de motifs IRAlu. L’étude d’ARN cibles des paraspeckles a montré que la reconnaissance par les paraspeckles pouvait se faire par des motifs localisés hors de la région 3’ non codante, et que plusieurs séquences dans un même ARN pouvaient participer à la liaison aux paraspeckles.Cette étude nous a donc permis d’identifier un nouveau mécanisme post-transcriptionnel régulant l’expression circadienne de gènes dans les cellules hypophysaires. / Most living organisms have developed circadian (=close to 24h) clock to face daily changes in their environment. Those clocks rely on molecular oscillators to drive the circadian expression of many genes. In the pituitary cell line GH4C1, we studied a post-transcriptional mechanism involving nuclear bodies called paraspeckles. Paraspeckles main element is the long non-coding RNA Neat1 to which several RNA binding proteins are associated. Paraspeckles have been shown to retain RNAs in the nucleus, in particular RNAs that display an IRAlu element in their 3’ translated region (3’UTR).In GH4C1, we showed that paraspeckles display a circadian rhythm of formation. We created the RNA pull-down method, which allows to purify a lncRNA with all its RNA targets, This allowed us to demonstrate that paraspeckles induce the circadian nuclear retention of several endogenous RNAs. Finally, we showed the absence of IRAlu elements in those RNAs. The study of tree target RNAs of paraspeckles showed that elements localized out of the 3’UTR could be involved in the recognition by paraspeckles, but also that several elements could be necessary to induce the RNA retention by paraspeckles.To conclude, this study allowed us to identify a new post-transcriptional mechanism regulating the circadian expression of pituitary genes.