Circadian clocks in blind cavefish: entrainment in the laboratory and the field

The majority of animals and plants possess an internal biological clock. This clock, when correctly set, organises biological processes in the body such that they occur at the optimal time of day. The light we experience during the day is the most important environmental cue for setting this clock. What then happens in the case of animals that have evolved in complete darkness, within deep caves. Do these animals possess a clock with which to tell the time of day, and, if so, how is it set? We intend to explore these questions by looking at the molecular mechanism of the clock in a blind, eyeless species of cavefish. We will examine the genes involved in the clock mechanism and see how they have changed (evolved) after about 100,000 years in the dark. Have critical elements of the clock changed after evolving for so long in the dark? Can these cavefish clocks still detect light? What environmental signals set these cavefish clocks? We hypothesise that either daily temperature changes, or feeding signals, are the critical cues. Our favoured idea at this time, is that feeding cues are the critical signal, as temperature has been reported to be very constant in these caves in Mexico. In fact, we think that it is the faeces from the bat population that enter the water every morning, which sets these cavefish clocks.

Most animals and plants possess endogenous circadian clocks. These clocks are typically set or entrained by light, but other rhythmic environmental cues have also been shown to function as zeitgebers. Some species, however, that are thought to have evolved under constant conditions, i.e. in deep sea environments or within caves, are believed not to have a requirement for biological clocks. Though to date not well examined, this is probably not true, and there is limited data to support the idea that some cave species do possess circadian pacemakers. We intend to explore the molecular aspects of the circadian clock in a cave species (the eyeless cavefish, Astyanax mexicanus) for the first time. We have already cloned a number of key clock genes in this species, and intend to complete this task in the timeframe of this proposal. Furthermore, we will explore how this clock is set in the laboratory. Can these cavefish clocks still detect light after about 100,000 years in the dark? Our preliminary data suggests that this is the case. However, what are the real, natural zeitgeber cues that can set this clock? We will explore the role of two likely cues, temperature cycles and feeding schedules on clock function. We are in a unique position to examine clock function in the native, cave environment in collaboration with Dr Yamamoto. We intend to explore clock function and entrainment within a series of diverse Mexican caves, making this one of very few studies on the natural entrainment of circadian clocks. In addition, the existence of multiple geographically isolated cave populations offers us a unique opportunity the evolutionary divergence of key clock genes over a relatively short and defined geological period.