Natural selection against a circadian clock gene mutation in mice

Area of Science:

• Chronobiology
• Evolutionary biology
• Genetics

Background:

• Circadian rhythms are endogenous biological processes synchronized with environmental cycles.
• The circadian resonance hypothesis posits that alignment between endogenous and environmental periods is evolutionarily adaptive.
• Previous research lacked empirical testing of this hypothesis under natural conditions.

Purpose of the Study:

• To test the circadian resonance hypothesis in a mammalian model under natural conditions.
• To investigate the fitness consequences of altered circadian period length.
• To evaluate the impact of a short-period mutation in casein kinase 1ε (tau mutation) on mouse survival and reproduction.

Main Methods:

• Established six outdoor replicate populations of mice with wild-type, heterozygous, and homozygous tau mutations.
• Monitored daily activity (feeding) rhythms, survivorship, and reproduction over 14 months.
• Quantified changes in allele frequency of the tau mutation in response to natural selection.

Main Results:

• Homozygous mutant mice with accelerated circadian cycles exhibited significantly reduced survival.
• The frequency of the short-period tau allele decreased from initial parity to 20% over 14 months.
• Increased daytime activity in mutant mice correlated with reduced fitness, impacting both adult survival and juvenile recruitment.

Conclusions:

• Strong natural selection against short-period circadian genotypes was observed, supporting the circadian resonance hypothesis.
• Circadian clock period length is a critical factor for individual fitness in natural environments.
• These findings contrast with studies on mutations affecting clock function without altering period length, highlighting the importance of period resonance.