Topographic analysis of the ganglion cell layer in the retina of the four-eyed fish Anableps anableps

Fish of the genus Anableps (Anablepidae, Cyprinodontiformes) have eyes that are adapted for simultaneous aerial and aquatic vision. In this study we investigate some of the corresponding retinal specializations of the adult Anableps anableps eye using retinal transverse sections and wholemounts. The linear dimensions of the retina were found to be asymmetric with a greater representation of the dorsal compared to the ventral visual field. The total number of neurons in the ganglion cell layer of the ventral hemiretina was on average 3.6 times greater than the values obtained in the dorsal hemiretina. Isodensity contour maps revealed a prominent horizontal visual streak in the ventral hemiretina with an average peak cell density of 18,286 cells/mm2. A second less-well-developed horizontal visual streak was also observed in the dorsal hemiretina. A sub-population of large cells with soma areas between 74 and 188 μm2 was identified and found to be distributed evenly across both hemiretinas. Together, these results show that the sampling gain of the ventral retina is significantly greater than the dorsal segment, that retinal specializations important for mediating acute vision are present in the parts of the visual field immediately above and below the surface of the water, and that visual functions related with the large ganglion cells require more even sampling across the visual field. The relevance of these retinal specializations to the feeding and other behavioral strategies adopted by Anableps is discussed.

Variants of Smooth Salmonella entericaSerovar Enteritidis That Grow to Higher Cell Density Than the Wild Type Are More Virulent

ABSTRACT Salmonella enterica serovar Enteritidis that grows to a higher cell density (SE-HCD) than wild type while retaining O-chain lipopolysaccharide was isolated by transforming wild type serovar Enteritidis with the cell density sensor plasmid pSB402 and selecting for bioluminescence. A luminescent strain, SE-HCD, that emitted light in proportion with cell density and opacity through stationary phase was isolated. After a peak cell density of 1.5 × 1011CFU/ml was observed, luminescence decreased, although opacity continued to increase. Scanning electron microscopy revealed that changes in luminescence and opacity past peak cell density were associated with lysis of a swarming hyperflagellated coccobacillary cell type and emergence of a 10-to-30-fold-elongated rod cell type that lacked cell surface structures. Vigorous aeration was required to induce this dramatic cellular differentiation. The virulence of two isogenic variants with different patterns of light emission at an opacity of 0.2 after the culture was diluted 10-fold (1/10 OD) was assessed in animal models. Whereas SE-HCD1 killed 70% of 6-day-old chicks challenged subcutaneously, the same dose of SE-HCD2 did not kill any chicks. Conversely, subcutaneous challenge of hens with SE-HCD2 contaminated eggs five and seven times more often, respectively, than did SE-HCD1 or wild type serovar Enteritidis. Intravenous challenge with SE-HCD2 contaminated 22% of eggs versus 0.5% with wild type, depressed egg production for 4 weeks, and caused clinical signs of Gallinarum Disease (Fowl Typhoid) in hens. SE-HCD2 produced no contaminated eggs following oral infection, whereas wild type contaminated 1.3% of eggs. Thus, SE-HCD2 is better at contaminating eggs than wild type, but only by parenteral challenge. These results suggest that it may be possible to separate luminescent serovar Enteritidis into groups that infect different age groups and organs and contaminate eggs.