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Detection of light by the sea lamprey Petromyzon marinus
1 Department of Integrative Biology and Physiology, 2129 Terasaki
Life Sciences, 610 Charles E Young East, University of California
Los Angeles, Los Angeles, California 90095-7239
This work consisted of three separate but interrelated parts. (1) Morshedian A & Fain GL (2015). Single-Photon Sensitivity of Lamprey Rods with Cone-like Outer Segments. Current Biology 25: 484-487. Most vertebrates have a duplex retina containing rods for dim light vision and cones for bright lights and color detection. Photoreceptors like cones are present in many invertebrate phyla as well as in chordata, and rods evolved from cones; but the sequence of events is not well understood. Since duplex retinas are present in both agnatha and gnathostomata, which diverged more than 400 million years ago, some properties of ancestral rods may be inferred from a comparison of cells in these two groups. Lamprey have two kinds of photoreceptors, called “short” and “long”, which seem to be rods and cones; but the outer segments of both have an identical cone-like morphology of stacks of lamellae without a continuous surrounding plasma membrane. This observation and other aspects of the cellular and molecular biology of the photoreceptors have convinced several investigators that “the features of ‘true’ rod transduction in jawed vertebrates, which permit the reliable detection of single photons of light, evolved after the separation of gnathostomes from lampreys.” To test this hypothesis, we recorded from photoreceptors of the sea lamprey Petromyzon marinus and show that their rods have a single-photon sensitivity similar to that of rods in other vertebrates. Thus photoreceptors with many of the features of rods emerged before the split between agnatha and gnathostomata, and a rod-like outer segment with cytosolic disks is not necessary for high-sensitivity visual detection. (2) Morshedian A & Fain GL (2017). Evolution of Adaptation in Vertebrate Photoreceptors. In preparation. We previously showed that adult Petromyzon marinus has a duplex retina: rods respond to single photons, have a longer integration time, and are 80 times more sensitive than cones, much as in other vertebrates. Do lamprey photoreceptors also have mechanisms of light and dark adaptation like jawed vertebrates? To answer this question, suction-electrode recordings were made from rods and cones in maintained background light and after bright bleaches. Responses to maintained steps of light decay as in other vertebrates with two time constants (taus in rods of 8s and 26s, in cones 800ms and 7.8s). Flash responses superimposed on steady backgrounds show decreases in sensitivity and changes in waveform in both rods and cones, also typical of other vertebrates. Backgrounds produce a decrease in maximum flash-response amplitude and an increase in the flash intensity necessary to produce a detectable response, with characteristic shifts of response-intensity curves along the intensity axis. Sensitivity as a function of background intensity decreased by Weber’s Law in both rods and cones; rods showed incremental saturation, and cones began to adapt near the intensity at which rod saturation occurred. Bright bleaching light produced an equivalent background, with opsin in rods 7.5 x 10-6 times as effective in stimulating the cascade as Rh*. The decreases in sensitivity and acceleration of response decay in stably bleached photoreceptors can be nearly completely reversed with exogenous 11-cis retinal. Thus lamprey rods and cones adapt to backgrounds and bleaches with a phenomenology nearly identical to that of other vertebrates including mammals. Our experiments taken together with previous results show that primitive vertebrates before the divergence of jawed from jawless vertebrates had a duplex retina with rods and cones like those of other vertebrates. (3) Toomey MB, Morshedian A, Pollock G, Fredericksen R, Enright JE, McCormick S, Cornwall MC, Corbo JC & Fain GL (2107). A Cambrian origin of the rhodopsin/porphyropsin switch. In preparation. Lamprey are anadromous, migrating between freshwater and marine environments during their life cycle. In the 1950s, Wald detected vitamin A1-based rhodopsin in juvenile lamprey (as in marine vertebrates) but red-shifted vitamin A2-based porphyropsin in adults (as in fresh-water fishes, amphibians and reptiles). Here, we sought to ask whether lampreys use the same mechanism of A1-to-A2 conversion as fresh-water jawed vertebrates. We show that responses of juvenile lamprey rods and cones resemble those of adults. Rods respond to single photons, have longer integration times, and are 70-80 times more sensitive than cones. Spectral sensitivity and microspectrophotometry show that both juveniles and adults have only one spectral class of rod and one cone with best-fitting λmax’s for juveniles of 504 nm (rods) and 551 nm (cones); and for adults of 522 nm (rods) and 592 nm (cones). HPLC shows that vitamin A2 is present in adult eyes but not in those of juveniles. Quantitative PCR for expression of the vitamin A1 3,4-dehydrogenase CYP27C1 gene indicates that levels are significantly higher in the adult RPE compared to juvenile. In situ hybridization shows that the Cyp27c1 transcript localizes to the adult RPE and is not detected in the juvenile eye. Thus lamprey rods and cones respond to light in a nearly identical fashion in juvenile and adult forms, with one spectral class of rod and one cone having predominantly A2-based pigments in fresh-water adults and A1-based pigments in marine juveniles. Lamprey convert vitamin A1 to vitamin A2 with the same enzyme used by jawed vertebrates, the 3,4-dehydrogenase CYP27C1. Moreover in both lamprey and jawed vertebrates, this enzyme is localized to the RPE. We conclude that porphyropsin and the mechanism of converting A1 to A2 were present in the retina before jawed and jawless vertebrates diverged, not long after the origin of chordates during the Cambrian radiation.