The use of forward, genetic mutant screens has proved to be one of the more powerful strategies in biology for working out what genes are involved in a given process (Mullins et al., 1994). In fact, some might say that it is the only reasonable strategy when one knows nothing about a particular biological event. In the world of screening, Drosophila and mouse are “king”, both of which have certain obvious advantages and disadvantages. The one alternative vertebrate model system to the mouse is zebrafish. These animals have a particular niche advantage over the mouse, especially when one is interested in examining processes involved in early embryo development. The reasons for this are clear, in that fish produce large numbers of transparent embryos, which develop outside of the female: a major advantage for studying the early stages of development. A large number of research groups have made use of this particular advantage and consequently, there has been considerable progress in the analysis of mechanisms involved in early embryogenesis using fish (Geisler et al., 2007). Perhaps rather surprisingly, this model system has not been used anywhere nearly as extensively to study other biological problems. There are a few forays into the field of cancer biology, some drug screening, a little sleep research, and
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of course the topic of this chapter, research on circadian clocks (Stern et al., 2003; Tamai et al., 2005; Prober et al., 2006). This volume will contain several additional articles on clock biology in zebrafish, and so there will be some inevitable overlap, though the opinions of each author may interestingly differ. To reduce repetition to a minimum, I will focus on aspects of fish chronology especially relating to clocks in early embryos, the significance of light exposure in these early stages, and how light has been shown to influence the core clock mechanisms using zebrafish embryonic cell lines.