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The phototransduction components of the third - or parietal - eye of lizards, which mediates the detection of dawn and dusk, have been identified.

The third, or parietal, eye of lizards and other lower vertebrates mediates the detection of dawn and dusk instead of conventional image-forming vision. The parietal eye photoreceptors resemble their lateral eye counterparts (rods and cones) in morphology, but could produce a hyperpolarizing light response most sensitive to blue light, as well as an unusual depolarizing light response most sensitive to green light — a curious phenomenon known as chromatic antagonism. Writing in Science, Chih-Ying Su and colleagues reveal the molecular machinery that underlies these pathways of light detection in the parietal eye.

By screening a parietal eye cDNA library from the side-blotched lizard, the researchers identified pinopsin, the blue-sensitive pigment in rods and cones that is known to drive the hyperpolarizing response, as well as a new pigment, which they named parietopsin. Double immunostaining showed that pinopsin and parietopsin co-localized in the same photoreceptor outer segment, where phototransduction takes place.


A side-blotched lizard, which has a third, or parietal, eye containing the unusual photoreceptors. Image courtesy of J. Finn and K.-W. Yau, Johns Hopkins University, Maryland, USA.

The hyperpolarizing response of rods and cones in lateral eyes is mediated by the G protein transducin- alpha . Interestingly, Su et al. found that photoreceptors in the parietal eye do not contain transducin- alpha , but express its close relative gustducin- alpha — the G protein that mediates bitter and sweet transduction in taste buds — as well as G alpha ₀, the alpha -subunit of G₀. Co-localization studies showed that gustducin- alpha and G alpha ₀ are also expressed in the same photoreceptor outer segment.

The researchers next examined the signalling pathways of light detection in the parietal eye using single-cell recordings. Similar to transducin- alpha , gustducin- alpha activates a cyclic GMP (cGMP) phosphodiesterase and closes cGMP-gated cation channels in response to a flash of green light, thereby hyperpolarizing the membrane. By contrast, G alpha ₀ inhibits the phosphodiesterase and opens the same ion channels in response to a flash of blue light, thereby depolarizing the membrane.

The pinopsin–gustducin- alpha pair is similar to the signalling components in vertebrate rods and cones, whereas the parietopsin–G alpha ₀ coupling resembles that in some invertebrate photoreceptors. Therefore, the researchers suggest that the third eye is more primitive, and that image-forming lateral eyes might have evolved from their parietal ancestor but inherited only one G-protein pathway.

Jane Qiu

References

Su, C. -Y. et al. Parietal-eye phototransduction components and their potential evolutionary implications. Science 311, 1617–1621 (2006) | Article | PubMed |