Strain variations in cone wavelength peaksin situduring zebrafish development

Abstract

There are 4 cone morphologies in zebrafish, corresponding to UV (U), blue (B), green (G) and red (R)-sensing types, yet genetically, 8 cone opsins are expressed. How 8 opsins are physiologically siloed in 4 cone types is not well understood, and in larvae, cone physiological spectral peaks are unstudied. We use a spectral model to infer cone wavelength peaks, semi-saturation irradiances, and saturation amplitudes from ERG datasets composed of multi-wavelength, multi-irradiance, Aspartate-isolated, cone-PIII signals, as compiled from many 5–12-day larval, and 8–18-month adult eyes isolated from WT or roy orbeson (roy) strains. Analysis suggests (in nm) a 7-cone, U-360 / B1–427 / B2–440 / G1–460 / G3–476 /R1–575 / R2–556 spectral physiology in WT larvae, but a 6-cone, U-349 / B1–414 / G3–483 / G4–495 / R1–572 / R2–556 structure in WT adults. In the roy larvae there is a 5-cone structure: U-373 / B2–440 / G1–460 / R1–575 / R2–556; in roy adults, a 4-cone structure, B1–410 / G3–482 / R1–571 / R2–556. Existence of multiple B, G, and R types is inferred from shifts in peaks with red or blue backgrounds. Cones were either high or low semi-saturation types. The more sensitive, low semi-saturation types included U, B1 and G1 cones [3.0–3.6 log(quanta·μm−2·s−1)]. The less sensitive, high semi-saturation types were B2, G3, G4, R1 and R2 types [4.3–4.7 log(quanta·μm−2·s−1)]. In both WT and roy, U- and B- cone saturation amplitudes were greater in larvae than adults, while G-cone saturation levels were greater in adults. R-cone maximal amplitudes were the largest (50–60% of maximal dataset amplitudes), and constant throughout development. WT and roy larvae differed in cone sisgnal levels, with lesser UV- and greater G-cone amplitudes occurring in roy, indicating strain variation in physiological development of cone signals. These physiological measures of cone types suggest chromatic processing in zebrafish involves at least 4–7 spectral-signal processing pools.

Thyroid hormone receptor beta mutations alter photoreceptor development and function in Danio rerio (zebrafish)

“…to previous reports for WT larvae, and consistent with LWS2 expression [18]. The missing peak, of course, was the 6BP+1 mutant, where there is no red-cone signal from either LWS2 or LWS1.…”

“…In no case where red-cone sensitivity peak could be detected was it significantly changed by the presence of a mutant gene. All peaks lay in a tight range between 566 and 569 nm, similar to previous reports for WT adults, and consistent with adult LWS1 opsin expression [18]. There are missing peaks in the 6BP+1 and 3BP mutant, in the first, because of a lack of red-cone signal, and in the second because the homozygous mutation is embryonic lethal.…”

“…The missing peak, of course, was the 6BP+1 mutant, where there is no red-cone signal from either LWS2 or LWS1. In the 6BP+1 mutant and heterozygous mutants green, blue and UV cone peaks were unaffected (Table 1) and in a range reported for WT larvae [18], corresponding to G1 (RH2-1) for green cones, B1 (SWS2) for blue cones, and U (SWS1) for UV cones. Because of low signal amplitude in mutant larvae, no green-cone opsin peak was fit, however the presence of a green cone amplitude (Vg, Fig 10C) indicates that green cones are physiologically active.…”

“…The loss of photon sensitivity at 490 nm may not be caused solely by the loss of red cones. Red and green cones are known to be equal in number in the wild type [18]. Since the 460 nm larval green cone opsin and the 556 nm red cone opsin absorb equally at 490 nm, a factor of 2 decrease in sensitivity could be attributed to the red-cone loss in the mutant.…”

“…In development there is a shift from LWS2 to LWS1 opsin [24] and a concomitant shift in physiological red-cone opsin peak from 556 nm to 572 nm [18]. Thyroxin T3 may mediate this shift [25].…”