Rods and cones have inner and outer segments. The outer segments are composed of a series of stacked bilipid membranous disks that contain visual pigment called photopigment. There are two main ways in which these disks can be presented: at the apical end of the cell or in cilia formed on that end. These cellular arrangements have been thought to divide protostomes and deuterostomes, respectively, but the photoreceptor phylogeny turns out to be rather more complex and even raises questions about the nature of chordate-nonchordate eye homologies, or the ease with which similar arrangements of photoreceptors can re-evolve (Arendt and Wittbrodt 2001), a problem we have visited several times (e.g., sea urchin larval stages, Figure 8-9). Based on differences between message transduction pathways that are used,Arendt and Willbrodt suggest that the early bilateran ancestor species had both cellular types.

The photopigment is composed of an opsin protein and a chromophore called retinal, derived from vitamin A. The opsin and chromophore are bound together and embedded in transmembrane receptors on the outer disks. Light energy is captured in these disks and, through a series of chemical reactions that take place in the photopigment, is transduced into receptor potential and sent, via the optic nerve, to the brain where it is interpreted as light and color. Alternative chromophores used by some taxa modify the spectral response. Bird, amphibian, and reptile photoreceptor pigments have an overlying oil droplet that filters light and modifies their spectral sensitivities. As a result, different taxa can use different pigment and chro-mophore combinations to achieve a similar spectral response.

When light is received by a photoreceptor, the chromophore all-trans retinal converts to 11-cis retinal, which changes the conformation of the opsin protein. This in turn modifies its intracellular domain (the opsin must then be recharged by a new chromophore molecule).

In vertebrates this "bleaching" releases the chromophore from the opsin; the chromophore is then regenerated by neighboring cells reversing its conformation to the all-trans form so it can be reused. In invertebrates, the chromophore does not leave the opsin before being reversed.

Invertebrates and vertebrates use homologous opsins. These are coded by yet another branch of the 7TMR gene family, although the two major animal groups use unrelated second messengers to relay the signal. Vertebrate photoreceptors are coupled to the G protein transducin. Absorption of light causes a conformational change in the receptor, leading to increased binding of GTP by the a-subunit of transducin. This activates cGMP phosphodiesterase, which degrades cGMP, causing cGMP-gated Na+ channels to close, hyperpolarizing the cell and inhibiting neuro-transmitter release. This in turn propagates signal to the brain. Thus, neurons are inhibited in the dark, when neurotransmitter is released at a high rate. By contrast, the invertebrate second messenger is inositol triphosphate, and their photorecep-tors respond by depolarizing rather than hyperpolarizing (Hardie and Raghu 2001; Nilsson 1996; Ranganathan et al. 1995).

Invertebrate photoreceptor cells also have layers of membranes filled with rhodopsin, a photoreceptor pigment, and the photoreceptor collects light in the same way as in vertebrates (Yarfitz and Hurley 1994). Each photoreceptor cell in the Drosophila eye has a structure called a rhabdomere that contains ~60,000 microvilli. Millions of rhodopsin molecules associated with the downstream light transduction cascade reside in the microvilli.

Opsin Evolution and Function

The number of opsin genes varies among species (Pichaud et al. 1999). Although the basic structure of opsins is conserved, each gene has a distribution of wavelengths to which it responds, a distribution with a peak wavelength sensitivity, and diminishing sensitivity at surrounding wavelengths (Figure 14-3C).The spectral sensitivity can be evaluated experimentally and is determined by the amino acid

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