Functional abnormalities in mzbbs1−/− mutants at 5 dpf despite normal eye development and retinal morphology.(a) Representative images of semi-thin plastic sections stained with Richardson solution in wildtype (top) and maternal zygotic mutants (bottom). Note comparable lamination and layer thickness for OS, ONL, OPL and INL. b–e Immunostaining on cryosections with zpr1 marking red/green cones (b), DiO labelling membranes (c), 4D2 recognizing opsins in rods and some cones (d) and SV2 overlay with DiO to identify the OPL (e), indicates normal differentiation of retinal cellular subtypes and unaffected organization of the retinal layers in mutants. Staining with 4D2 (d) does not show opsin mislocalization (Insert: magnification of central area). f, g Transmission electron microscopy shows the normal retinal organization and photoreceptor ultrastructure with neat stacking of membrane discs in mzbbs1−/− mutants (right image), similar to their sibling controls (left image). h Bar plots of the maximum b-wave amplitude by electroretinography (ERG) shows a significantly decreased response to light in mzbbs1−/− mutants for all light intensities (log0 to log-4). Unpaired two-tailed multiple T test; Sig: **FDR(q value) < 0.01; ***FDR(q value) < 0.001, ****FDR(q value) < 0.0001; Sample size (n = 10 WT, n = 28 Mut larvae); Error bars show standard deviation around the mean. For more detailed statistics, please see Supplementary Data 6. i Average ERG response curve after a 100 ms light flash at log-2 intensity (Light On/Off line below the ERG curve) for mutant and wildtype. Scale Bars: (a) 10 µm, (b–e) 50 µm (insert: 10 µm), (f, g) 5 µm, Abbreviations: RPE retinal pigment epithelium, OS outer segment, ONL outer nuclear layer, OPL outer plexiform layer, INL inner nuclear layer, M mitochondria.