Immunocytochemical study of calretinin and calbindin D-28K expression in the retina of three cartilaginous fishes and a cladistian (Polypterus)

The distribution of two calcium-binding proteins, calbindin D-28K (CB) and calretinin (CR) was studied in the retina of a cladistian, Polypterus senegalus, and three cartilaginous fishes (Scyliorhinus canicula, Raja undulata and Torpedo marmorata). Western blot analysis of brain extracts revealed the lack of cross-reactivity of the used antibodies. In Polypterus, CB and CR immunoreactivities were observed in some amacrine and ganglion cells, but scarce cells showed CR/CB colocalization. Furthermore, CR immunoreactivity was present in a number of displaced bipolar cells and in some putative displaced ganglion cells, whereas CB immunoreactivity was found in some cones. No positive retinal structure was observed with the CB antibody used in cartilaginous fishes. Instead, CR was expressed in some amacrine, horizontal and ganglion cells of the dogfish and skate and, in some ganglion cells of the electric ray. The comparative analysis suggests, (1) the presence of CB-positive photoreceptor cells in the retina of cladistians seems to be apomorphic (in jawed fishes) in contrast with the plesiomorphic condition of this character in land vertebrates; (2) the presence of CR in amacrine and ganglion cells is a conserved feature along vertebrate phylogeny, whereas its variable expression in bipolar and horizontal cells represents a derived character; (3) the absence of CB in horizontal cells in cladistians could represent a derived character; and (4) the presence of CR displaced bipolar and putative displaced ganglion cells in Polypterus is shared with basal groups of actinopterygians.