Light from down under

Coral-algae symbiosis is a key feature of tropical corals and is highly dependent on the efficiency with which solar energy is attenuated by the coral. Scleractinian corals are among the most efficient light collectors in nature because of the modulation of the internal light field in the coral skeleton. Interestingly, coral skeleton particles composing the sandy bottoms in reef margins sustain these optical characteristics. In the present study, we examined two free-living coral species - Heterocyathus aequicostatus (Caryophyllidae) and Heteropsammia cochlea (Dendrophylliidae) - common on biogenic coarse carbonate sand of the Great Barrier Reef but absent from fine sand at the same depth. In coarse carbonate sand, light penetrates a few millimeters below the surface and propagates along horizontal distances of a few centimeters. In fine sand, almost all of the light is reflected back to the water column. For photosynthetic sand-dwelling organisms such as the studied species, with over one-third of their surface area facing the substrate, light flux to their underside may be beneficial. A correlation was found between the diameter of these corals and the distance that light may travel in the sand under the coral. Laboratory and field measurements show that the symbiotic algae on the underside of the corallites are photosynthetically active even when the coral is partially buried, implying sufficient light penetration. Other organisms in the study site, such as fungid corals and foraminiferans, with different morphologies, have different light-trapping strategies but are also photosynthesizing on their underside. The importance of the substrate type to the performance of the three main partners of the symbiosis (coral, endosymbiotic algae and a sipunculan worm) is highlighted, and is a striking example of co-evolution

A retinal code for motion along the gravitational and body axes

Self-motion triggers complementary visual and vestibular reflexes supporting image-stabilization and balance. Translation through space produces one global pattern of retinal image motion (optic flow), rotation another. We examined the direction preferences of direction-sensitive ganglion cells (DSGCs) in flattened mouse retinas in vitro. Here we show that for each subtype of DSGC, direction preference varies topographically so as to align with specific translatory optic flow fields, creating a neural ensemble tuned for a specific direction of motion through space. Four cardinal translatory directions are represented, aligned with two axes of high adaptive relevance: the body and gravitational axes. One subtype maximizes its output when the mouse advances, others when it retreats, rises or falls. Two classes of DSGCs, namely, ON-DSGCs and ON-OFF-DSGCs, share the same spatial geometry but weight the four channels differently. Each subtype ensemble is also tuned for rotation. The relative activation of DSGC channels uniquely encodes every translation and rotation. Although retinal and vestibular systems both encode translatory and rotatory self-motion, their coordinate systems differ

SNP-SNP interactions in breast cancer susceptibility

BACKGROUND: Breast cancer predisposition genes identified to date (e.g., BRCA1 and BRCA2) are responsible for less than 5% of all breast cancer cases. Many studies have shown that the cancer risks associated with individual commonly occurring single nucleotide polymorphisms (SNPs) are incremental. However, polygenic models suggest that multiple commonly occurring low to modestly penetrant SNPs of cancer related genes might have a greater effect on a disease when considered in combination. METHODS: In an attempt to identify the breast cancer risk conferred by SNP interactions, we have studied 19 SNPs from genes involved in major cancer related pathways. All SNPs were genotyped by TaqMan 5'nuclease assay. The association between the case-control status and each individual SNP, measured by the odds ratio and its corresponding 95% confidence interval, was estimated using unconditional logistic regression models. At the second stage, two-way interactions were investigated using multivariate logistic models. The robustness of the interactions, which were observed among SNPs with stronger functional evidence, was assessed using a bootstrap approach, and correction for multiple testing based on the false discovery rate (FDR) principle. RESULTS: None of these SNPs contributed to breast cancer risk individually. However, we have demonstrated evidence for gene-gene (SNP-SNP) interaction among these SNPs, which were associated with increased breast cancer risk. Our study suggests cross talk between the SNPs of the DNA repair and immune system (XPD-[Lys751Gln] and IL10-[G(-1082)A]), cell cycle and estrogen metabolism (CCND1-[Pro241Pro] and COMT-[Met108/158Val]), cell cycle and DNA repair (BARD1-[Pro24Ser] and XPD-[Lys751Gln]), and within carcinogen metabolism (GSTP1-[Ile105Val] and COMT-[Met108/158Val]) pathways. CONCLUSION: The importance of these pathways and their communication in breast cancer predisposition has been emphasized previously, but their biological interactions through SNPs have not been described. The strategy used here has the potential to identify complex biological links among breast cancer genes and processes. This will provide novel biological information, which will ultimately improve breast cancer risk management

Functional diversity in the color vision of cichlid fishes

Abstract Background Color vision plays a critical role in visual behavior. An animal's capacity for color vision rests on the presence of differentially sensitive cone photoreceptors. Spectral sensitivity is a measure of the visual responsiveness of these cones at different light wavelengths. Four classes of cone pigments have been identified in vertebrates, but in teleost fishes, opsin genes have undergone gene duplication events and thus can produce a larger number of spectrally distinct cone pigments. In this study, we examine the question of large-scale variation in color vision with respect to individual, sex and species that may result from differential expression of cone pigments. Cichlid fishes are an excellent model system for examining variation in spectral sensitivity because they have seven distinct cone opsin genes that are differentially expressed. Results To examine the variation in the number of cones that participate in cichlid spectral sensitivity, we used whole organism electrophysiology, opsin gene expression and empirical modeling. Examination of over 100 spectral sensitivity curves from 34 individuals of three species revealed that (1) spectral sensitivity of individual cichlids was based on different subsets of four or five cone pigments, (2) spectral sensitivity was shaped by multiple cone interactions and (3) spectral sensitivity differed between species and correlated with foraging mode and the spectral reflectance of conspecifics. Our data also suggest that there may be significant differences in opsin gene expression between the sexes. Conclusions Our study describes complex opponent and nonopponent cone interactions that represent the requisite neural processing for color vision. We present the first comprehensive evidence for pentachromatic color vision in vertebrates, which offers the potential for extraordinary spectral discrimination capabilities. We show that opsin gene expression in cichlids, and possibly also spectral sensitivity, may be sex-dependent. We argue that females and males sample their visual environment differently, providing a neural basis for sexually dimorphic visual behaviour. The diversification of spectral sensitivity likely contributes to sensory adaptations that enhance the contrast of transparent prey and the detection of optical signals from conspecifics, suggesting a role for both natural and sexual selection in tuning color vision.</p

Feedback from horizontal cells to cones mediates color induction and may facilitate color constancy in rainbow trout.

Color vision is most beneficial when the visual system is color constant and can correct the excitations of photoreceptors for differences in environmental irradiance. A phenomenon related to color constancy is color induction, where the color of an object shifts away from the color of its surroundings. These two phenomena depend on chromatic spatial integration, which was suggested to originate at the feedback synapse from horizontal cells (HC) to cones. However, the exact retinal site was never determined. Using the electroretinogram and compound action potential recordings, we estimated the spectral sensitivity of the photoresponse of cones, the output of cones, and the optic nerve in rainbow trout. Recordings were performed before and following pharmacological inhibition of HC-cone feedback, and were repeated under two colored backgrounds to estimate the efficiency of color induction. No color induction could be detected in the photoresponse of cones. However, the efficiency of color induction in the cone output and optic nerve was substantial, with the efficiency in the optic nerve being significantly higher than in the cone output. We found that the efficiency of color induction in the cone output and optic nerve decreased significantly with the inhibition of HC-cone feedback. Therefore, our findings suggest not only that color induction originates as a result of HC-cone feedback, but also that this effect of HC-cone feedback is further amplified at downstream retinal elements, possibly through feedback mechanisms at the inner plexiform layer. This study provides evidence for an important role of HC-cone feedback in mediating color induction, and therefore, likely also in mediating color constancy

Retinal region of polarization sensitivity switches during ontogeny of rainbow trout

Polarization sensitivity (PS) in vertebrate vision is controversial, perhaps because its underlying mechanism has remained obscure. An issue that might have added to the controversy is that rainbow trout (Oncorhynchus mykiss), which have served as the primary model system for polarization-based orientation, lose their ability to orient relative to celestial polarized-light patterns when parr (fry) transform into migratory smolts (juveniles), which would benefit most from polarization-based orientation. Here we addressed two key questions: (1) what is the mechanism underling PS?, and (2) how can the paradoxical loss of PS in trout smolts be reconciled? We assessed PS from optic nerve recordings in parr and smolts and found that the retinal region with enhanced PS shifted from the ventral retina in parr to the dorsal retina in smolts. This adaptation may allow fish to use the most reliable polarization field encountered at each life stage, the celestial polarization field in the shallow-swimming parr and the depth-insensitive underwater polarization field in the deep-swimming smolts. In addition, we assessed spectral sensitivity across the retina and during ontogeny and fit a cascade retinal model to PS data. We found that differential contribution of two cone detectors with orthogonal PS could drive the variation in PS and that feedback from horizontal cells to cones could explain the differential amplification of PS. This elegant arrangement, in which weak PS of cones is amplified and tuned by retinal networks, allows for PS without interfering with sampling of other visual information and illustrates how sensory systems may simultaneously process disparate aspects of physical environment