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Abstract

Colour patterns are prominent features of many animals. They show a high variability and, serving in camouflage and communication, they are under natural and sexual selection. The zebrafish, Danio rerio, is an important vertebrate model organism for bio-medical and basic research. The fish display a very characteristic and stereotyped pattern of horizontal light and dark stripes on the flanks and the anal and tailfins. This pigment pattern is generated by three different types of pigment cells, dark melanophores, orange xanthophores and light reflecting iridophores. Other Danio species, closely related to zebrafish, can be bred in captivity; they possess the same three types of pigment cells but most of them show patterns that are very different from zebrafish. For the time being we concentrate on two additional species: Danio aesculapii and Danio albolineatus. Danio aesculapii, the closest sister species of zebrafish, has a pattern of vertical bars anteriorly that changes into an irregular ‘snake skin’ pattern more to the posterior. Danio albolineatus has almost no pattern, just a short posterior remnant of one horizontal stripe. Hybrids with zebrafish are viable but sterile; they produce patterns of horizontal stripes similar to zebrafish. We have begun to analyse the genetic and molecular bases for the pattern differences in these species. Starting from our detailed knowledge of the patterning process in zebrafish we have identified the first genes that are differentially required between zebrafish and Danio aesculapii and might therefore be the basis for the rapid evolution of pigmentation patterns in the Danio clade.