Steindal, Inga Angelica Frøland; (2021) Light and Clocks in Dark Places. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

The sun is the premise of our existence. Most animals and all plants use the predictability of the sunrise and sunset to drive a lot of their biological processes, which in turn gives these organisms a fitness advantage. However, there are a subset of animals that live in dark environments, hidden away from the light, yet we know very little about how these animals organise the biology that is normally light or clock driven in other animals. In this thesis, I explore how light impacts the biology of three very different species of fish and explore some aspects of how adaptation to a dark environment many have occurred. 1. The zebrafish, a well-developed model system that uses and depends heavily on sunlight for its survival. In this first data chapter I address how different wavelengths of light impact clock and light inducible genes in different ways. Furthermore, I present data that supports the hypothesis that the different organs of fish have different wavelength sensitivities. 2. I then move on to a relatively “recent” dark adapted animal, the Mexican blind cavefish, a species of fish that has been isolated in over 30 subterranean caves some million years ago, and show extreme adaptations to the dark, such as loss of pigment and eyes. In this second data chapter I show how all strains of cavefish show a delay in the onset of its clock in development, and how different cave populations show slightly different expression patterns in response to light/dark cycles, hinting at different evolutionary adaptations. 3. Finally, I dive deep, into the deep-sea, an environment that shows a strange duality of being pitch black but with animals that possess some of the most impressive visual systems on earth. In this last chapter I present work on two species of deep-sea hatchetfish that have evolved over the past 200 million years in the deep-sea. In this final chapter I present the first molecular study on light and circadian rhythms in deep-sea vertebrates. I explore how two species of hatchetfish, one rhythmic and one arrhythmic, have adapted to a light in the open ocean, and find surprising evidence of in vitro light-sensitivity.

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