Figure 7.

The R-R Connection through Evolution

(A) Working model of the function of R-R in different species. In species where the size of the tectum and retina is similar, the establishment of bilaterally congruent maps may rely on point-to-point molecular tagging mechanisms.

(B) In species where the size of the visual target is larger than the retina, map topography relies both on a molecular tagging mechanism and a later refinement process, dependent on waves of spontaneous activity. The presence of an R-R connection may enable the synchronization of retinal waves from each eye, driving symmetrical refinement in both hemispheres.

(C and D) ISH for Unc5c in horizontal (C) and coronal (D) sections of the zebrafish retina revealed no expression of Unc5c.

(E and E’) A 36- to 48-h post fertilization (hpf) zebrafish embryo stained with DAPI and monocularly injected with DiI shows no retinal axons entering into the contralateral retina.

(F and G) ISH for Unc5c mRNA in horizontal (F) and coronal (G) sections from P0 ferret retinas. Unc5c mRNA is expressed in the RGC layer of ventral and temporal retina.

(H) Schematic diagram representing monocular DiI anterograde labeling. Optic chiasm of a DiI-labeled P0 ferret is shown. White arrowhead points to the axons into the contralateral optic nerve.

(I) Schematic diagram representing retrograde CTB-labeling. CTB injection was performed at P1, and labeled RGCs were visualized by red fluorescence in whole-mount retinas 1 day later.

(J–J’’) ISH for Unc5c mRNA and immunohistochemistry for Zic2 in a coronal retinal section from an E34.5 ferret embryo counterstained with DAPI. Red arrowheads mark the most peripheral limit of Unc5c mRNA expression and green arrowheads the central limit of Zic2 expression. Zic2 and Unc5c mRNA expression is mostly complementary.

CTB, cholera toxin subunit B-Alexa594; RGCl, retinal ganglion cell layer. See also Figures S5, S6, and S7 and Table S1.