Fig. 4. Six models that predict the expected shift in visual pigment
max
(
max=A2max–A1max)
when one opsin is combined with vitamins A1 and A2
chromophores as a function of the max of the vitamin
A1 member. These models are compared with the observed values for
rods, middle-wavelength-sensitive (MWS) and long-wavelength-sensitive (LWS)
cones in coho salmon (Oncorhynchus kisutch, Walbaum). Lines
representing each model are from Bridges
(Bridges, 1965) (gray dotted),
from Dartnall and Lythgoe (Dartnall and
Lythgoe, 1965) (gray dashed), from Tsin et al.
(Tsin et al., 1981) (gray
dotted and dashed), from Whitmore and Bowmaker
(Whitmore and Bowmaker, 1989)
(black dotted and dashed), from Harosi
(Harosi, 1994) (solid black),
and from Parry and Bowmaker (Parry and
Bowmaker, 2000) (black dashed). The observed range for rods, MWS
and LWS cones based on the mean per fish are plotted as filled circles. The
spectral shift observed in rods and LWS cones falls within the predicted range
of the models, indicating that the observed variance in max
values can be explained by a change in chromophore ratio. The spectral shift
of MWS cones lies outside the range predicted by all six models and therefore
the variance in max of MWS cones cannot be explained by a
shift in chromophore ratio alone: a second RH2 opsin subtype is implicated in
this shift.