Fig. 7.

Comparison of the roles of nuclear receptor signaling during photoreceptor development in mouse and zebrafish; high levels of nuclear receptor signaling favor increases in expression of longer wavelength-sensitive opsins in both cases. (A) In mouse, protocones (not yet expressing opsin genes) influenced by high levels of nuclear receptor signaling, express high levels of M-opsin and low levels of S-opsin, while those not influenced by nuclear receptor signaling express high levels of S-opsin and low levels of M-opsin (TRbeta2 or RXRgamma; Ng et al., 2001; Roberts et al., 2005). (B) In zebrafish, cone photoreceptors are structurally as well as molecularly specialized, and a single cone type has not been demonstrated to have the capacity to express more than one opsin (but see Cheng and Novales Flamarique, 2004). In the presence of high levels of nuclear receptor signaling, rods and red cones increase expression of rod opsin and red cone opsin, while blue and UV cones decrease expression of blue and UV cone opsin.