DEVELOPMENTAL PATTERNING OF ROD AND CONE PHOTORECEPTORS IN EMBRYONIC ZEBRAFISH

Cone photoreceptors in the zebrafish retina are arranged in a crystall ine lattice, with each spectral subtype at a specific position in the array; rod photoreceptors are inserted around the cones. Patterning ev ents and developmental mechanisms that lead to the formation of the co ne mosaic are not known. To begin investigating this issue, we examine d the initial stages of opsin expression in zebrafish embryos by in si tu hybridization with goldfish opsin cRNA probes to determine how and when the cone mosaic pattern arises. We found both differences and sim ilarities in the spatiotemporal patterns of rod and cone development, which suggest the following: 1) Expression of opsin message (including rod opsin, blue and red cone opsins) was initiated at 50-52 hours pos tfertilization by a few photoreceptors which were consistently found i n a ventral patch of retina located nasal to the choroid fissure. 2) T he cone mosaic pattern was generated by a crystallization-like process initiated in the precocial ventral patch and secondarily in nasal ret ina, which then swept like a wave into dorsotemporal retina. 3) The pa ttern of differentiation of rods in the ventronasal patch differed sub stantially from that in the remainder of the retina, suggesting that t hese precocial rods might differ from typical rods. 4) Developmental m aturation of rods in zebrafish, as reflected by expression of opsin, m ay be accelerated compared to cones, which are thought to become postm itotic before rods. These data are consistent with a model in which la teral inductive interactions among differentiating photoreceptors lead to patterning of the array. (C) 1995 Wiley-Liss, Inc.