Bilateral visual projections exist in non-teleost bony fish and predate the emergence of tetrapods

In most vertebrates, camera-style eyes contain retinal ganglion cell neurons that project to visual centers on both sides of the brain. However, in fish, ganglion cells were thought to innervate only the contralateral side, suggesting that bilateral visual projections appeared in tetrapods. Here we show that bilateral visual projections exist in non-teleost fishes and that the appearance of ipsilateral projections does not correlate with terrestrial transition or predatory behavior. We also report that the developmental program that specifies visual system laterality differs between fishes and mammals, as the Zic2 transcription factor, which specifies ipsilateral retinal ganglion cells in tetrapods, appears to be absent from fish ganglion cells. However, overexpression of human ZIC2 induces ipsilateral visual projections in zebrafish. Therefore, the existence of bilateral visual projections likely preceded the emergence of binocular vision in tetrapods.This work was supported by Programme Investissements d’Avenir IHU FOReSIGHT (ANR-18-IAHU-01) (A.C. and F.D.B.), INSERM cross-cutting program HuDeCA 2018 (A.C.), NIH R01OD011116 (I.B.), and UQ Amplify Fellowship (R.S.)Peer reviewe

Expression pattern of <i>cabp1a</i>.

<p>(<b>A–D′</b>) mRNA expression of <i>cabp1a</i> in lateral (A–A′) and dorsal (B,B′) views of a 2dpf and 4dpf zebrafish embryo with higher magnifications (C–D,C′–D′). (<b>E–H</b>). Cross-sections of retinae in the Tg(Ptf1a:GFP) transgenic line show staining of <i>cabp1a</i> in the amacrine cell layer.(E) Epifluorescence picture of a sectioned retina. Scale bar: 50 µm (<b>F–H</b>) Confocal images of the area selected in E. (F) <i>In situ</i> signal in bright field, (G) GFP signal restricted to amacrine cells, (H) DAPI nuclear stain. Scale bar: 20 µm. HR: heart, NC: notochord, ACL: amacrine cell layer.</p

Expression pattern of <i>caln2.</i>

<p>(<b>A–D′</b>) mRNA expression of <i>caln2</i> in lateral (A,C–A′,C′) and dorsal (B,D–B′D′) views of 2dpf (A–D) and 4dpf (A′–D′) larvae with zoom in (C,D–C′,D′). The onset of <i>caln1</i> expression takes place within the first two days of embryonic development. (A–D) At 2dpf the <i>caln1</i> transcript localizes to the forebrain at the level of the subpallial region and in the caudal midbrain. (A-‘D’) The same expression profile seems to be maintained until day 4 of development. (E) Transverse section of the retina showing signal of <i>caln1</i> antisense probe in a subset of retinal ganglion cells in the central part of the ganglion cell layer. Strongly stained cells are present in the region of the telencephalic pallial domain (F). Scale bar: 50 µm. P: pallium; DT: dorsal thalamus; T: midbrain tegmentum; GCL: ganglion cell layer; Sd: dorsal division of subpallium; Sv: ventral division of subpallium.</p

Expression pattern of <i>cabp2a</i>.

<p>(<b>A–D</b>) <i>In situ</i> hybridisation showing expression of <i>cabp2a</i> in 5dpf larvae. Lateral views (A,C) , dorsal views (B, D). <i>Cabp2a</i> antisense probe exhibits diffuse retinal localisation; a strong staining is also present along the notochord. (<b>E–H</b>) Cross-section of a retina in the Tg(Vsx2:GFP) transgenic line. (E) Epifluorescence image of a sectioned retina, showing prominent expression of <i>cabp2a</i> in the bipolar cell layer. Scale bar: 50 µm. (F–H) Confocal images of the area selected in E. (F) <i>In situ</i> signal in bright field, (G) GFP signal localized in bipolar cells, (H) DAPI. Scale bar: 20 µm (I) Cross-section exhibiting <i>in situ</i> staining in the notochord. Scale bar: 100 µm. INL: inner nuclear layer, NC: notochord.</p

Expression pattern of <i>cabp7b</i>.

<p>(<b>A–D</b>) <i>In situ</i> localisation of <i>cabp7b</i> transcript in 5dpf larvae. Lateral (A,C) and dorsal views (B, D). A wide distribution of <i>cabp7b</i> mRNA in the brain was observed. (<b>E,F</b>) Transverse sections. Strongly stained cells are present in the midbrain, probably in the region of the nucleus of medial longitudinal fascicle (E) and in the hindbrain area anterior to the medulla oblongata most likely at the level of the reticular formation(F). Scale bar: 50 µm. M2: migrated posterior tubercular area; MO: medulla oblongata; N: nucleus of medial longitudinal fascicle; RF: reticular formation; T: midbrain tegmentum; TeO: tectum opticum.</p

Expression pattern of <i>cabp2b</i>.

<p>(<b>A–D</b>) <i>In situ</i> hybridisation signal of <i>cabp2b</i> in 3dpf zebrafish embryos. Lateral (A) and dorsal (B) views of the embryo with higher magnification (C–D). Staining is restricted to hair cells in the inner ear and to the neuromasts. (F) View of stained neuromasts in the lateral line and (E) higher magnification of the selected area; arrowheads in E show the neuromasts. (<b>G–L</b>) Transverse section of the ear in the Tg(Brn3C:memGFP) transgenic line. (L) Epifluorescence image, showing strong expression of <i>cabp2b</i> in a subset of cells in the inner ear. Scale bar: 100 µm. (G) Higher magnification picture. (H–I) Confocal images. (H) DAPI signal, (I) GFP staining in the same domain as <i>cabp2b</i>. Scale bar: 20 µm. Nm: neuromast, IE: inner ear.</p

Comparison of CaBP family member expression domains in mouse and zebrafish.

*<p>inner nuclear layer.</p>**<p>curnu ammonis.</p

Expression pattern of <i>cabp5b</i>.

<p>(<b>A–D′</b>) <i>Cabp5b</i> expression in 2dpf (A–D) and 5dpf (A′–D′) zebrafish embryos: lateral (A,C–A′,C′) and dorsal (B,D–B′,D′) views. An intense staining is present in the inner ear and in the notochord at both stages of development. In 5dpf embryos, in addition, a strong signal is detectable in the retinal inner nuclear layer. (<b>E–H</b>) Sections of the ear in Tg(Brn3C:memGFP) transgenic line embryos. Epifluorescence image of a sectioned ear (E) and confocal images (F–H) showing strong expression of <i>cabp5b</i> in a subset of cells in the inner ear. Picture in bright field (F), DAPI (G), GFP (H). Scale bar: 20 µm. (<b>I–L</b>) Cross-sections of a retina in the Tg(Vsx2:eGFP) transgenic line. Epifluorescence image of a sectioned retina (I) and magnification (J), showing <i>cabp5b</i> antisense probe signal in the bipolar cell layer. Scale bar: 50 µm. (<b>K–L</b>) Confocal images. GFP signal localized in bipolar cells (K), DAPI (L). Scale bar: 20 µm. Ov: otic vesicle, IE: inner ear, INL: inner nuclear layer.</p

Expression pattern of <i>cabp4b</i>.

<p>(<b>A–D</b>) <i>In situ</i> localisation of <i>cabp4b</i> transcript in 5dpf larvae. Lateral (A–C) and dorsal views (B–D). A strong staining in the photoreceptor layer is detected. (<b>E–F</b>) Cross section of a retina (E) with zoom in to the photoreceptor layer (F). Scale bar: 20 µm. Pr: photoreceptors.</p

Expression pattern of <i>caln1</i>.

<p>(<b>A–D′</b>) <i>In situ</i> signal of <i>caln1</i> transcript in 2dpf (A–D) and 4dpf (A′–D′) larvae. Lateral (A,C–A′,C′) and dorsal views (B,D–B′D′). (E) Higher magnification shows specific staining in the dorsal spinal cord probably in Rohon Beard cells. (A′–D′) 4dpf whole mount embryos show diffuse signal in the brain and a specific expression in what is most probably the head of the pronephric ducts. (F) A transverse section through the retina shows expression in the retinal ganglion cell layer. Scale bar: 50 µm. SC neurons: spinal cord neurons, P: pallium; DT: dorsal thalamus; PD: pronephric duct; GCL: ganglion cell layer.</p