Abstract
Circadian rhythms are ubiquitous among taxa and are essential for coping with recurrent daily events, leading to selection on the properties of the clock underlying these rhythms. To quantify this selection in the wild, we need, however, to phenotype wild individuals, which is difficult using the standard laboratory approach for which individuals need to be kept under constant conditions. To overcome this problem, we explored the possibility to link the variation in a key clock property, circadian period (Tau), to genetic variation. We measured Tau in 152 captive great tits (Parus major). We further linked Tau to two circadian phase markers, the onset of activity in the Light:Dark cycle, and the first onset in constant conditions (Dim:Dim), directly after entrainment. We did a genome-wide association study using a 650k SNP chip, and we linked genetic polymorphisms of a set of twelve candidate genes, to Tau and the two circadian phase markers. In line with earlier studies, Tau was heritable (h2 =0.48 ± 0.22). Despite this genetic variation, we did not find any significant associations at the genome-wide level with the measured traits and only one candidate gene showed association with onset of activity in the Light:Dark cycle. Identifying the genetic base of circadian timing for wild species thus remains challenging. Including alternative molecular methods such as epigenetics or transcriptomics could help to unravel the molecular basis of the biological clock in great tits.
Highlights
Circadian rhythms are ubiquitous to both uni- and multi-cellular organisms, and govern numerous physiological and behavioral functions such as cell division, gene expression, hormone secretion, and rest-activity cycles (Takahashi et al, 2001)
Our results demonstrate that Tau is significantly heritable in our natural bird population, albeit lower than in a previous study done with great tits and with low evolvability (IA)
Onset phase and DD phase shift were not heritable in our birds showing slight positive correlation with Tau. Given this heritable variation in Tau it is well possible that there are evolutionary forces acting on variation in Tau in the wild, but to assess this we need to obtain Tau phenotypes of wild birds and associate these with fitness
Summary
Circadian rhythms are ubiquitous to both uni- and multi-cellular organisms, and govern numerous physiological and behavioral functions such as cell division, gene expression, hormone secretion, and rest-activity cycles (Takahashi et al, 2001). In the presence of a natural light-dark cycle, individuals will synchronize their endogenous circadian pacemaker to the external day-night rhythm (a process called entrainment). In the absence of time cues, organisms continue to express activity in a regular pattern. The duration of this free running rhythm is determined by the endogenous circadian period (Tau), a key property of the clock underlying this rhythm (Aschoff, 1989).
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