Genetic Analysis of the Cone Photoreceptor Mosaic in Zebrafish

Abstract

Abstract: : Purpose: To discover mechanisms responsible for determining cone photoreceptor cell fates and patterning. The four spectral classes of cone photoreceptors in zebrafish are distributed in a highly orderly planar array at the outer surface of the retina. Blue and UV cones form rows that alternate with rows of red and green double cones. We are isolating and characterizing zebrafish (Danio rerio) mutants with patterning defects in the mosaic array. Methods: The progeny of F2 families derived from ENU–mutagenized zebrafish were screened for disruptions of the cone mosaic by in situ hybridization with cone opsin probes on embryos at 72 hours post fertilization. Mutants of interest had irregular spacing of cones, but normal–sized eyes, normal retinal lamination and lacked other obvious phenotypes. Linkage mapping and genetic complementation assays are underway. Results: To date we have screened 205 F2 families and isolated 7 recessive mutant lines in which the cone mosaic is disrupted. At least two of these mutants (mi215 and mi40) are alleles of the same gene. The mutants have various abnormalities in distribution of cones, including clumping and reduced density. Simultaneous visualization of two or more opsins by double in situ hybridization or by crossing mutants into the zebrafish transgenic UV–opsin reporter line, Tg(sws1:GFP), revealed abnormal neighbor relationships between the cone spectral types. The mi215 mutation maps to chromosome 20. Conclusions: Our forward genetic screen has isolated mutants in which the cone mosaic pattern is specifically disrupted with no other obvious anatomical defects in the retina. Our data suggest that some genes critical in the pathway that determines cone photoreceptor cell fate and/or cell position function independent of other steps in retinal development. Isolation of such genes, and elucidation of the role played in retinal development, will be valuable to understanding how retinal cell fates and positions are specified.

Keywords: retinal development • mutations • gene mapping