ausblenden:
Schlagwörter:
local adaptation; reproductive timing; lunar rhythm; biological clocks; biological rhythms; sympatric speciation; gene flow; mitochondrial haplotypes; population genomics; Chironomidae; Clunio marinus; marine ecology
Zusammenfassung:
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. The spring low tides occur at location specific times on days around the full moon and new moon. C. marinus populations at the European Atlantic coast are locally adapted to the day time and lunar phase of the spring low tides. This timing is achieved through the combination of an circadian and circalunar rhythm. While the circadian rhythm is controlled by a transcriptional-translational feedback loop, the molecular workings of the circalunar rhythm are not understood yet. As tides are almost neglectable in the Baltic Sea, the local Clunio populations have adapted to lay the egg clutches in the open sea instead of an exposed intertidal substrate. This simultaneously removed the selective pressure to time the reproduction to the lunar phase and 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. C. marinus changed from circadian-circalunar-controlled emergence to circatidal rhythms in polar day conditions. The adults emerge every day at every low tide throughout the mating season. In my thesis, I investigated these cases of lunar-arrhythmicity in Northern European Clunio populations. By exploring the genetic features linked to the evolution of the here described ecotypes of C. marinus, we step further towards understanding the enigmatic circalunar rhythms. My investigations resulted in one published article, one published preprint and an additional chapter. The first article had two aims: First, I investigated the ability of short mitochondrial fragments to recover the whole mitochondrial biogeography of geographically distinct populations. DNA barcodes are short, conserved genomic fragments and commonly used to reconstruct the biogeography of species. With my Clunio populations as example I wanted to point out what issues can arise from blindly using those highly conserved DNA fragments. The second aim was to get the basic mitochondrial biogeography of all distinct population as a foundation to the investigations into the evolution of lunar-arrhythmic ecotypes. My second chapter is separated into two parts. At first I take a look at the evolution of lunar-arrhythmicity in the studied populations. Population structure and admixture analyses in addition to the mitochondrial biogeography were combined to identify the historical scenario which lead to the evolution of lunar-arrhythmic populations. Secondly, I used direct genomic comparisons to find differentiated regions and adaptive loci between rhythmic populations from the Atlantic coast and the arrhythmic populations from the Baltic Sea specifically. Established laboratory cultures of two sympatric populations were crossed for further insight into the nature of the maintenance of both populations under gene flow. In my article I identify genetic variants differentiated between lunar-rhythmic and lunar-arrhythmic populations. Genetic clusters affected by those genetic variants comprise genes for the control of circadian rhythms, neuronal development, mating behavior, responses to hypoxia and sodium ion transport. In my third chapter I performed a crossing experiment to identify putative genotypes linked to lunar-rhythmic phenotypes. By crossing two sympatric populations with differing ecotypes, I was able to raise an F2 generation with a mix of rhythmic and arrhythmic phenotypes. With the use of PCR-primers designed specifically for differentiated regions between the grandparent genomes I obtained genotypes for six distinct loci per chromosome of 237 individuals. The QTL analysis revealed multiple significant loci on all chromosomes with nine investigated phenotypes linked to lunar-rhythmicity. 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 the sympatric Bergen 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.
