Sensing light is an important function for most organisms, which is mediated by photoreceptors such as opsins and cryptochrome. Although nematodes do not have eyes and known photoreceptors, worms can react to light and show avoidance behavior. Recently, a new photoreceptor LITE-1 has been identified in the nematode Caenorhabditis elegans. However, LITE-1 is only conserved in the genus Caenorhabditis, and the mechanism of light-sensing in other nematodes is unknown. To elucidate the mechanism of light-sensing, we use the nematode Pristionchus pacificus, which has been established as a satellite model organism for comparison with C. elegans and does not have LITE-1 and opsins. We established a light-avoidance assay in P. pacificus, based on C. elegans studies. Similar to C. elegans, illumination of the short-wavelength light induced avoidance behavior in P. pacificus. To reveal the mechanism of the light avoidance behavior, we investigated mutants of six neurotransmitter-related genes. GABA and glutamate mutants had a defect in light avoidance, indicating that GABA and glutamate are required for light avoidance in P. pacificus. To identify genes involved in light-sensing in P. pacificus, we performed a forward genetic screening of the light avoidance behavior and isolated three UV light-unresponsive mutants. By next-generation sequencing, we found that two mutants have mutations in
Ppa-tax-4 and
Ppa-daf-11, respectively. TAX-4 (a cyclic GMP-gated channel) and DAF-11 (a guanylate cyclase) are components of phototransduction in C. elegans. To confirm these genes are responsible for the light-unresponsive phenotype, we knocked out
Ppa-tax-4 and
Ppa-daf-11 by the Co-CRISPR method we have established previously in P. pacificus. These mutants decreased the light avoidance, suggesting that
Ppa-tax-4 and
Ppa-daf-11 are required for the light avoidance behavior and a part of the light-sensing mechanism is conserved between C. elegans and P. pacificus. For the other mutant, we will narrow down a candidate region by SNP mapping to identify a responsible gene. Future analysis of these mutants may promote an understanding of the mechanism for light detection in nematodes.