| During the age of non-avian dinosaurs, ancestors of present-day mammals were likely small insectivores, relegated to nocturnal and subterranean niches. This nocturnal "bottle-neck" is postulated as a driving force of numerous physiological and sensory adaptations including those of the visual system. The consequences of structural mutations of opsins upon spectral sensitivity and environmental adaptation have been studied in great detail, but lacking is knowledge of the potential influence of alterations in gene regulatory networks upon the diversity of cone subtypes and the variation in the ratio of rods and cones observed in diurnal and nocturnal species. Exploiting photoreceptor patterning in cone-dominated zebrafish, we uncovered two independent mechanisms by which the sine oculis homeobox homolog 7 (six7) regulates photoreceptor development. In a genetic screen, we isolated the lots-of-rods-junior (ljrp23ahub) mutation that resulted in an increased number and uniform distribution of rods in otherwise normal appearing larvae. Sequence analysis, genome editing using TALENs and knockdown strategies confirm ljrp23ahub as a hypomorphic allele of six7, a teleost orthologue of six3, with known roles in forebrain patterning and expression of opsins. Based on the lack of protein-coding changes and an identified deletion of a conserved sequence about 40 kb upstream of six7 loci, a cis-regulatory mutation is proposed as the basis of the reduced expression in ljrp23ahub. Comparison of the hypomorphic and knock-out alleles provides evidence of two independent roles in photoreceptor development. EdU and PH3 labeling show that the increase in rod number and uniform distribution is associated with extended mitosis of photoreceptor progenitors, and TUNEL suggest that the lack of green cones is the result of cell death of the cone precursor. These data add six7 to the small but growing list of genes essential for specification and patterning of photoreceptors in non-mammalian vertebrates, and form the basis of a model that underscores the potential of alterations in transcriptional regulation as a mechanism underpinning photoreceptor variation across species. |
