The zebrafish (Danio rerio) is a small tropical freshwater fish which lives in rivers of northern India, northern Pakistan, Nepal, and Bhutan in South Asia. The characteristic stripes running along the body and the fins give its name to this species, which belongs to the family of Cyprinidae. In contrast to many other fish species, zebrafish
adults are only approximately 3-5 cm long, so that they can be easily managed in large numbers in the laboratory (Kishi et al, 2003). Zebrafish have short generation times of approximately three to five months.
The development of the zebrafish is very similar to the embryogenesis in higher vertebrates, including humans, but, unlike mammals, zebrafish develop from a fertilised egg to an adult outside the female in a transparent egg. Moreover, the embryos themselves are transparent during the first few days of their lives (Wixon, 2000).
The embryonic development of zebrafish is very rapid: in the first 24 hours after fertilisation, all major organs are developed and within three days the fish hatch and start looking for food. After three to four months, zebrafish are sexually mature and can generate new offspring. A single female can lay up to 200 eggs per week (Stern and Zon, 2003).
Recently the whole genome of zebrafish has been sequenced (Sanger Institute). It is even more complex than the human
genome, as zebrafish have two more pairs of chromosomes than humans. This difference arose during evolution in teleosts, when the whole genome was duplicated, which did not occur in mammals. Many of these duplicated genes were lost again and only a small proportion remains today. Functions of these duplicated genes changed in several cases (Hill et al, 2005).