The loss of circalunar rhythms in arctic and tide-free habitats : Genomic investigations into lunar-arrhythmic populations of Clunio marinus
Biological rhythms are adaptations to periodically changing environmental conditions. The non-biting midge Clunio marinus (Diptera: Chironomidae) is known for the link between its reproduction and the tidal regime. The short-lived adults emerge when most of the intertidal habitat is exposed. C. marinus populations at the European Atlantic coast are locally adapted to the day time and lunar phase of the spring low tides on days around the full moon and new moon. While the circadian rhythm is controlled by a transcriptional-translational feedback loop, the molecular workings of the circalunar rhythm are not understood yet. Populations in the Baltic Sea have adapted to lay the egg clutches in the open sea due to the lack of tides. This allowed for lunar-arrhythmic emergence throughout the entire mating season. In arctic habitats of the Atlantic coast tides are still present. During the mating season the sun illuminates the habitat around the clock, preventing the perception of moon light. Individuals changed from circadian-circalunar-controlled emergence to circatidal rhythms in polar day conditions. My thesis takes a large step towards the understanding of the circalunar rhythms in C. marinus by comparing rhythmic to naturally occurring arrhythmic populations. I generated a comprehensive genomic resource for geographically and ecologically distant populations of the same species. Genomic screens for ecotype-adaptive loci identified a putative involvement of circadian clock genes in circalunar rhythms of C. marinus. A crossing experiment between rhythmic and arrhythmic ecotypes of sympatric populations hinted towards the involvement of multiple loci across the genome in lunar-rhythmicity. The addition of further genetic markers could identify a link of the circadian clock to circalunar rhythms as well as unravel the maintenance of sympatric ecotypes.
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