olMG cells re-enter the cell cycle after injury but do not generate neurogenic clusters.

(A–A’’) Cryosection of a needle-injured hatchling medaka retina of the transgenic line rx2::H2B-eGFP. PH3 stainings (magenta) on the hatchling medaka retinae 3 days post needle injury show mitotic cells present in the central retina (arrowhead), co-localizing with the rx2 nuclear reporter expression (green). (n = 4 fish, data obtained from two independent experiments). (B–B’’) Cryosection of a needle-injured hatchling medaka retina of the transgenic line rx2::H2B-eGFP. A 3-day pulse of BrdU marks proliferating cells in the central retina after needle injury (arrowheads). BrdU staining (magenta) co-localizes with rx2 nuclear reporter expression (green), indicating that olMG cells re-entered the cell cycle. (n = 6 fish, data obtained from three independent experiments). (C, C') Cryosection of a needle-injured hatchling medaka retina. BrdU-positive (magenta) single cells are present in the INL and ONL. (n = 6 fish, data obtained from two independent experiments). (D, D') Cryosection of a needle-injured zebrafish retina. BrdU-positive (magenta) neurogenic clusters are present in the INL. Additionally, BrdU-positive proliferating cells can be detected in the ONL (n = 3 fish, data obtained from two independent experiments). Scale bars are 10 μm.

Rod photoreceptor density is increased during postembryonic growth of zebrafish but not medaka.

(A–B''') Cryosection of wild-type hatchling zebrafish and adult zebrafish retina. The ONL of the zebrafish retina is comprised of two nuclear layers, which contain only Zpr-1-positive (green) PRCs. The ONL of the adult zebrafish retina is comprised of four nuclear layers: one Zpr-1-positive (green) layer and three Recoverin-positive (magenta) layers. (C–D''') Cryosection of wild-type hatchling medaka and adult medaka retina. The ONL of the hatchling medaka retina is comprised of two nuclear layers: one Zpr-1-positive and one Recoverin-positive (magenta) layer. The ONL of the adult medaka retina is comprised of two nuclear layers: one Zpr-1-positive and one Recoverin-positive (magenta) layer. Scale bars are 20 μm.

Sox2 is present in MG cells of the hatchling medaka and zebrafish retina but not maintained after injury in medaka.

(A–A’’’) Cryosection of an uninjured hatchling medaka retina. Sox2 (green) labeled cells with round nuclei are present in the INL and the GCL. Sox2-labeled cells with round nuclei are ACs present in the INL and the GCL (asterisks). Sox2-positive cells with elongated nuclei are present in the INL (arrowheads). Co-labeling with GS (magenta) proves that cells with elongated nuclei are olMG cells. Additional staining, which is likely unspecific staining since sox2 mRNA cannot be detected there (Reinhardt et al., 2015), can be detected in the ONL. (B–B’’’) Cryosection of an uninjured zebrafish retina at 9 dpf. Sox2 (green)-positive cells with round nuclei are present in the INL and the GCL. Sox2-labeled cells with round nuclei are ACs present in the INL and the GCL (asterisks). Sox2-positive cells with elongated nuclei are present in the INL (arrowheads). Co-labeling with GS (magenta) proves that cells with elongated nuclei are drMG cells. Scale bars are 20 μm. (C–C’’) Cryosection of an injured hatchling medaka retina at 3 dpi. BrdU (magenta, arrowheads) labeled cells are not co-labeled with Sox2 (green, arrowheads). Sox2-positive cells with elongated nuclei, indicating non-proliferative olMG cells, are found in the INL (open arrowheads). Sox2-positive cells with round nuclei are ACs present in the INL and the GCL (asterisks) (n = 3 fish, data obtained from two independent experiments). (D) Quantification of the amount of Sox2-positive and negative proliferating cells of BrdU-positive cells at 3 dpi in medaka (74 cells in 3 retinae). ****p<0.0001. Box plots: median, 25th and 75th percentiles; whiskers show maximum and minimum data points. (E–E’’) Cryosection of an injured zebrafish retina at 3 dpi. BrdU-(magenta) and Sox2-(green) double positive cells can be detected in the INL (arrowheads). BrdU-positive Sox2-negative cells can rarely be detected (open arrowhead). Sox2-labeled cells with round nuclei are ACs present in the INL and the GCL (asterisks) (n = 3 fish, data obtained from two independent experiments). Scale bars are 10 μm. (F) Quantification of the amount of Sox-positive and negative proliferating cells of BrdU-positive cells at 3 dpi in zebrafish (68 cells in 3 retinae). ****p<0.0001. Box plots: median, 25th and 75th percentiles; whiskers show maximum and minimum data points.