The circadian clock of the Mexican blind cavefish, Astyanax mexicanus

The prevalence of circadian clocks in most, if not all, organisms on the planet implies they are an adaptive trait. However, only a few studies provide evidence to support this. To address this issue directly, we examined circadian clock function in the Mexican cavefish, Astyanax mexicanus. This species has spent a significant period of its evolutionary history in constant darkness, away from the daily cycles of the surface environment. Many cavefish populations have adapted for life in the dark by reducing eyes and pigmentation and enhancing sensory processes relevant in darkness, such as mechanosensation. Have cavefish retained the circadian clock architecture in the absence of the primary entraining cue, light? This thesis presents evidence for: i. The presence of a light-entrainable molecular circadian clock in Astyanax cavefish. Robust circadian rhythms of per1 expression are exhibited in cavefish, but with reduced amplitude and altered phase relative to their surface fish ancestors. ii. Alterations in the light input pathway, especially the raised expression of a putative clock repressor, per2b. We propose that this dampens the amplitude of core clock gene expression rhythms observed in cavefish. iii. Absence of circadian rhythmicity in wild cavefish. iv. A developmental delay in the appearance of light-detection in cavefish embryos v. Upregulation of two DNA repair genes, CPD phr and ddb2, in cavefish. This effect is accentuated in the wild, leading us to propose that dampening of the circadian clock in cavefish may be a by-product of selection for raised expression of DNA repair family genes and light-induced genes. By examining multiple aspects of the circadian biology of A. mexicanus surface and cavefish, the data presented in this thesis supports the hypothesis that the clock gives a fitness advantage through the provision of internal temporal order but suggests the clock may be partially reduced in aperiodic environments.