The genomic basis of circadian and circalunar timing adaptations in a midge

Tobias S. Kaiser, Birgit Poehn, David Szkiba, Marco Preußner, Fritz J. Sedlazeck, Alexander Zrim, Tobias Neumann, Lam-Tung Nguyen, Andrea J. Betancourt, Thomas Hummel, Heiko Vogel, Silke Dorner, Florian Heyd, Arndt von Haeseler, Kristin Tessmar‐Raible•View original
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Picture a swarm of midges rising from the ocean surface on a single night each month, timed to the lowest tide of the lunar cycle. The adults live for only a few hours. Miss the window, and you don't reproduce. Now imagine that midges from a beach in Portugal emerge at a measurably different time of day, and a different phase of the moon, than midges from a beach in France. The question Kaiser and colleagues set out to answer is precise: what is different in their DNA that makes their clocks run differently? The system they chose is Clunio marinus, a marine midge controlled by two separate biological timekeepers. The first is a circadian clock, a roughly 24-hour internal oscillator that times daily emergence. The second is a circalunar clock, a roughly 29.5-day timer linked to the lunar cycle. Natural variation in when these events happen, called chronotypes, differs measurably between strains from different European locations. Cross-strain breeding experiments showed these timing differences are genetic. Two circadian quantitative trait loci, or QTLs, genomic regions statistically linked to variation in a measured trait, together explain eighty-five percent of the daily timing difference, while two circalunar QTLs explain the entire monthly difference. To find what lives inside those QTLs, the team sequenced the C. marinus reference genome, CLUMA_1.0, producing about eighty-six megabases of sequence with twenty-one thousand six hundred seventy-two annotated gene models, ninety-two percent anchored to a genetic linkage map. They then resequenced five geographic strains, generating over a million single nucleotide polymorphisms, individual letter-changes in the DNA, with the two most intensively compared strains, Por and Jean, sequenced at greater than 240-fold coverage. The first surprise: core circadian clock genes were absent from the QTLs. The only clock-related gene in the intervals was timeout, a timeless-related gene, and it showed only weak differentiation between strains. The strongest signal came from a different gene entirely: CaMKII.1, which encodes calcium/calmodulin-dependent kinase II, an enzyme that modifies other proteins in response to calcium signals. Within the circadian QTL region, the CaMKII.1 locus carries the highest concentration of highly differentiated variants between strains, including a one hundred twenty-five base-pair insertion present at greater than ninety-nine percent frequency in the early-emerging Por strain. The gene produces four alternatively spliced transcripts, RA through RD, that differ in the length of a linker region. RNA sequencing and targeted quantitative polymerase chain reaction showed long-linker variants are more abundant in Por, while short-linker variants are more abundant in the late-emerging Jean strain, with some differences significant at a p-value below 0.0005. The functional test was direct. In cell-based luciferase assays, CLOCK and CYCLE, the two core circadian proteins that drive rhythmic gene expression, produced a measurable signal on their own. Adding a constitutively active form of Cma-CaMKII.1 substantially increased that transcriptional output across five biological replicates. The kinase-dead version did not. That result links splice-form abundance to clock output: more long-linker CaMKII.1 means stronger CLOCK-CYCLE activity, which means a shifted circadian period. The broader implication is about the mechanism of evolutionary adaptation itself. Evolution didn't rewire the clock; it tuned the enzyme that amplifies it, through changes in how a single gene is spliced. This lecture was created by ennepō. Go to https://ennepo.ai to Discover, Create and Follow the latest research in your field. Read when you can. Listen when you want to.

Picture a swarm of midges rising from the ocean surface on a single night each month, timed to the lowest tide of the lunar cycle. The adults live for only a few hours. Miss the window, and you don't reproduce. Now imagine that midges from a beach in Portugal emerge at a measurably different time of day, and a different phase of the moon, than midges from a beach in France. The question Kaiser and colleagues set out to answer is precise: what is different in their DNA that makes their clocks run differently? The system they chose is Clunio marinus, a marine midge controlled by two separate biological timekeepers. The first is a circadian clock, a roughly 24-hour internal oscillator that times daily emergence. The second is a circalunar clock, a roughly 29.5-day timer linked to the lunar cycle. Natural variation in when these events happen, called chronotypes, differs measurably between strains from different European locations. Cross-strain breeding experiments showed these timing differences are genetic. Two circadian quantitative trait loci, or QTLs, genomic regions statistically linked to variation in a measured trait, together explain eighty-five percent of the daily timing difference, while two circalunar QTLs explain the entire monthly difference.

To find what lives inside those QTLs, the team sequenced the C. marinus reference genome, CLUMA_1.0, producing about eighty-six megabases of sequence with twenty-one thousand six hundred seventy-two annotated gene models, ninety-two percent anchored to a genetic linkage map. They then resequenced five geographic strains, generating over a million single nucleotide polymorphisms, individual letter-changes in the DNA, with the two most intensively compared strains, Por and Jean, sequenced at greater than 240-fold coverage. The first surprise: core circadian clock genes were absent from the QTLs. The only clock-related gene in the intervals was timeout, a timeless-related gene, and it showed only weak differentiation between strains. The strongest signal came from a different gene entirely: CaMKII.1, which encodes calcium/calmodulin-dependent kinase II, an enzyme that modifies other proteins in response to calcium signals. Within the circadian QTL region, the CaMKII.1 locus carries the highest concentration of highly differentiated variants between strains, including a one hundred twenty-five base-pair insertion present at greater than ninety-nine percent frequency in the early-emerging Por strain. The gene produces four alternatively spliced transcripts, RA through RD, that differ in the length of a linker region.

RNA sequencing and targeted quantitative polymerase chain reaction showed long-linker variants are more abundant in Por, while short-linker variants are more abundant in the late-emerging Jean strain, with some differences significant at a p-value below 0.0005. The functional test was direct. In cell-based luciferase assays, CLOCK and CYCLE, the two core circadian proteins that drive rhythmic gene expression, produced a measurable signal on their own. Adding a constitutively active form of Cma-CaMKII.1 substantially increased that transcriptional output across five biological replicates. The kinase-dead version did not. That result links splice-form abundance to clock output: more long-linker CaMKII.1 means stronger CLOCK-CYCLE activity, which means a shifted circadian period. The broader implication is about the mechanism of evolutionary adaptation itself. Evolution didn't rewire the clock; it tuned the enzyme that amplifies it, through changes in how a single gene is spliced. This lecture was created by ennepō. Go to https://ennepo.ai to Discover, Create and Follow the latest research in your field. Read when you can. Listen when you want to.

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