Combining physiology and genetics in the zebrafish retina.

The zebrafish has recently joined the ranks of Drosophila and C. elegans as a tractable model for genetic screens (Fishman, 1999). Zebrafish grow fast, can be kept in large numbers in a small space, and are efficiently mutagenized and screened. Genomic resources are made available at an increasing pace. These days, a mutation can be mapped and cloned in a matter of months. Because a mutant hunt is intrinsically unbiased in terms of the classes of genes that will be tagged, it holds the unique potential to discover novel genes or, in our era of genome sequencing, to identify novel functions for known genes. Zebrafish display dozens of innate behaviours in response to light, of which the optomotor and the optokinetic responses are the most widely studied (Brockerhoff et al. 1995; Easter & Nicola, 1996). Their retinae are crisply layered following the typical vertebrate pattern, and the retinal layers are tiled in an almost crystalline fashion by mosaics of different cell types. Electroretinograms are recorded routinely and therefore, not surprisingly, zebrafish are now also being used for a genetic approach to the visual system.