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Visual pigments and oil droplets in diurnal lizards : a comparative study of Caribbean anoles

Ellis R. Loew^1,*, Leo J. Fleishman², Russell G. Foster^3, and Ignacio Provencio^3,

¹ Department of Biomedical Sciences, Cornell University, Ithaca, NY 14853, USA
² Department of Biology, Union College, Schenectady, NY 12308, USA
³ Department of Biology, University of Virginia, Charlottesville, VA 22903, USA
Present address: Department of Integrative and Molecular Neuroscience, Division of Neuroscience and Psychological Medicine, Imperial College School of Medicine, Charing Cross Hospital, Fulham Palace Road, London W6 8RF, UK
Present address: Department of Anatomy, Physiology, and Genetics, Uniformed Services University of the Health Sciences, 4301 Jones Bridge Road, Bethesda, MD 20814, USA

View larger version (24K): [in a new window]   Fig. 1. A cladogram showing the relationships between the species from this study based on Jackman et al. (1999) and T. R. Jackman (personal communication). The lengths of the arms of the cladogram have no quantitative meaning, although we have linked closely related species with short arms. There are other species between those listed in this cladogram: i.e. the closest taxa in this diagram are not necessarily the most closely related species known. Jackman et al. (1999) identified 17 major distinct clades within the genus Anolis. Five of these, plus the closely related outgroup genus Polychrus, are represented in our sample. *A. sagrei and A. equestris were collected from feral populations in Florida, but are Cuban in origin.

View larger version (38K): [in a new window]   Fig. 2. HPLC chromatograms (A-D) and absorption spectra (E,F) indicating chromophore type for six anoles. Vitamin A1 (A) and vitamin A2 (B) standard chromatograms. HPLC chromatograms of whole eye extracts from Anolis cristatellus (C) and A. carolinensis (D). The peaks corresponding to the 11-cis isomers are indicated by stars (filled star, vitamin A1 11-cis-retinaloxime; open star, vitamin A2 11-cis-retinaloxime). The elution time of the vitamin A1 11-cis-retinaloxime is 4.58 min while that of the vitamin A2 11-cis-retinaloxime is 5.16 min. (E) Absorption spectra of the 11-cis isomers from whole-eye extracts of A. cristatellus and four other Puerto Rican anoles (trace 1, A. gundlachi; trace 2, A. cristatellus; trace 3, A. evermanni; trace 4, A. pulchellus; trace 5, A. krugi). The bold trace is the spectrum of the vitamin A1 11-cis-retinaloxime standard. (F) Absorption spectrum of the 11-cis isomer from whole-eye extracts of A. carolinensis (trace 6) and the vitamin A2 11-cis-retinaloxime standard (bold trace). Vertical lines are drawn through the max of the spectra in E and F.

View larger version (31K): [in a new window]   Fig. 3. Typical normalized visual pigment absorbance spectra, in this case from Anolis cristatellus. The filled circles and smooth curves are for the best-fit visual pigments calculated from vitamin-A1-based template data. (A) Long-wavelength-sensitive pigment; (B) medium-wavelength-sensitive pigment; (C) short-wavelength-sensitive pigment; (D) ultraviolet-sensitive pigment.

View larger version (131K): [in a new window]   Fig. 4. Multispectral micrographs of pieces of flattened retina from Anolis cristatellus and Polychrus marmoratus. (A) Color images in white light. The diffuse yellow pigment present in the accessory member of the double cones is easily seen in the P. marmoratus images. (B) Black-and-white broad-band images of the same areas as in A. (C) The same areas imaged through a 500 nm low-pass interference filter. This is below the cut-off for the G droplets, which appear dark. (D) The same areas imaged through a 450 nm cut-off interference filter. In this case, both the Y and G classes of oil droplet appear dark. The droplets that remain light are the C class. Note, in particular, the rod-like outer segments seen in the P. marmoratus images (arrow) and their colorless oil droplets. Scale bars, 10 µm.

View larger version (21K): [in a new window]   Fig. 5. Typical absorbance spectra of oil droplets from the cones of Anolis cristatellus (A) and A. valencienni (B). The absorbances of the ellipsoid region of the accessory member of the double cones are also shown. All absorbances that saturated the microspectrophotometer output (optical density >2.5) were normalized to 1.0, while absorbances below saturation are shown relative to the normalized absorbances. Thus, the ellipsoid absorbance in A indicates a very pale ellipsoid and the absorbance of the C1 droplet in B indicates a pale droplet. Although the G and Y droplets always show very steep cut-offs, as seen here, the ellipsoid and C1 absorbances can be quite variable. The absorbance of the C2 droplet associated with the ultraviolet single cone is the dashed line in A and falls on the zero line in B.

View larger version (19K): [in a new window]   Fig. 6. Drawings of the photoreceptor cells typical of anoline lizards (see Crescitelli, 1972) showing the associations between cell type, visual pigment and oil droplet classes. The Polychrus marmoratus pattern is like that shown here for the anolids except for the presence of the rod-like cell containing a medium-wavelength-sensitive (MWS) pigment and a colorless oil droplet. LWS, long-wavelength-sensitive cell; SWS, short-wavelength-sensitive cell; UVS, ultraviolet-sensitive cell; C1, C2, G1, G2 and Y are classes of oil droplet.

View larger version (22K): [in a new window]   Fig. 7. Effects of oil droplet filtering on the theoretical capture area of Anolis cristatellus visual pigments. The solid curves were obtained by multiplying smoothed, normalized visual pigment spectra by the associated smoothed, normalized oil droplet transmission spectra. The dotted lines are the smoothed visual pigment spectra. The numbers are the calculated capture areas under the filtered visual pigment spectra obtained by integration and expressed as a percentage of the unfiltered pigment capture area. The effect of the oil droplets on the long-wavelength-sensitive and medium-wavelength-sensitive (MWS) cones is to reduce short-wavelength absorbance while reducing overall absorbance by 17% and 5%, respectively. In these cases, there is no change in the absorbance maximum. However, the position of the oil droplet cut-off of the MWS cone reduces the capture area to 29% of that of the unfiltered pigment and moves the absorbance peak from 443 to 525 nm. This also produces a much steeper short-wavelength cut-off, which could improve color discrimination in an opponent processing system (see text).