A BEHAVIORAL SCREEN FOR ISOLATING ZEBRAFISH MUTANTS WITH VISUAL-SYSTEM DEFECTS

Optokinetic and phototactic behaviors of zebrafish larvae were examine d for their usefulness in screening for recessive defects in the visua l system, The optokinetic response can be reliably and rapidly detecte d in 5-day larvae, whereas the phototactic response of larvae is varia ble and not robust enough to be useful for screening. We therefore mea sured optokinetic responses of mutagenized larvae as a genetic screen for visual system defects, Third-generation larvae, representing 266 m utagenized genomes, were examined for abnormal optokinetic responses, Eighteen optokinetic defective mutants were identified and two mutants that did not show obvious morphological defects, no optokinetic respo nse a(noa) and partial optokinetic response a (pea), were studied furt her, We recorded the electroretinogram (ERG) to determine whether thes e two mutations affect the retina, The b-wave of noa larvae was grossl y abnormal, being delayed in onset and significantly reduced in amplit ude, In contrast, the ERG waveform of poa larvae was normal, although the b-wave was reduced in amplitude in bright light, Histologically, t he retinas of noa and poa larvae appeared normal, We conclude that noa larvae have a functional defect in the outer retina, whereas the oute r retina of poa larvae is likely to be normal.