Denis Shcherbakov

Flow on the right side of the gastrocoel roof plate is dispensable for symmetry breakage in the frog Xenopus laevis.

Leftward flow of extracellular fluid breaks the bilateral symmetry of most vertebrate embryos, manifested by the ensuing asymmetric induction of Nodal signaling in the left lateral plate mesoderm (LPM). Flow is generated by rotational beating of polarized monocilia at the posterior notochord (PNC; mammals), Kupffers vesicle (KV; teleost fish) and the gastrocoel roof plate (GRP; amphibians). To manipulate flow in a defined way we cloned dynein heavy chain genes dnah5, 9 and 11 in Xenopus. dnah9 expression was closely related to motile cilia from neurulation onwards. Morphant tadpoles showed impaired epidermal ciliary beating. Leftward flow at the GRP was absent, resulting in embryos with loss of asymmetric marker gene expression. Remarkably, unilateral knockdown on the right side of the GRP did not affect laterality, while left-sided ablation of flow abolished marker gene expression. Thus, flow was required exclusively on the left side of the GRP to break symmetry in the frog. Our data suggest that the substrate of flow is generated within the GRP and not at its margin, disqualifying Nodal as a candidate morphogen.

Magnetosensation in zebrafish

Abstract Many species, from bacteria to vertebrates, have been reported to use the geomagnetic field as a major cue for oriented short and long range migration [1–10], but the molecular nature of the underlying receptor has remained elusive. One of the main reasons may be that past attempts to train animals to respond to magnetic stimuli proved surprisingly difficult [11]. We present a novel approach to magnetic conditioning, using a fast, fully automated assay system relying on negative reinforcement. Weak electric impulses were applied to punish fish that failed to escape upon magnetic field alterations (avoidance behaviour). Using this assay we first demonstrate magnetosensation in Mozambique tilapia, a fish migrating regularly between freshwater and the sea. Next we wondered whether non-migratory fish have a magnetic sense, such as zebrafish, the genetic fish model organism. Zebrafish were trained in groups of 4 individuals, and statistically highly significant reactions to magnetic field changes were recorded. The demonstration of magnetosensation in zebrafish opens a possibility to genetically identify the magnetoreceptor and its downstream signalling cascade.

Sensitivity Differences in Fish Offer Near-Infrared Vision as an Adaptable Evolutionary Trait

Near-infrared (NIR) light constitutes an integrated part of solar radiation. The principal ability to sense NIR under laboratory conditions has previously been demonstrated in fish. The availability of NIR in aquatic habitats, and thus its potential use as a cue for distinct behaviors such as orientation and detection of prey, however, depends on physical and environmental parameters. In clear water, blue and green light represents the dominating part of the illumination. In turbid waters, in contrast, the relative content of red and NIR radiation is enhanced, due to increased scattering and absorption of short and middle range wavelengths by suspended particles and dissolved colored materials. We have studied NIR detection thresholds using a phototactic swimming assay in five fish species, which are exposed to different NIR conditions in their natural habitats. Nile and Mozambique tilapia, which inhabit waters with increased turbidity, displayed the highest spectral sensitivity, with thresholds at wavelengths above 930 nm. Zebrafish, guppy and green swordtail, which prefer clearer waters, revealed significantly lower thresholds of spectral sensitivity with 825–845 nm for green swordtail and 845–910 nm for zebrafish and guppy. The present study revealed a clear correlation between NIR sensation thresholds and availability of NIR in the natural habitats, suggesting that NIR vision, as an integral part of the whole spectrum of visual abilities, can serve as an evolutionarily adaptable trait in fish.

Near-infrared orientation of Mozambique tilapia Oreochromis mossambicus.

Light plays a pivotal role in animal orientation. Aquatic animals face the problem that penetration of light in water is restricted through high attenuation which limits the use of visual cues. In pure water, blue and green light penetrates considerably deeper than red and infrared spectral components. Submicroscopic particles and coloured dissolved organic matter, however, may cause increased scattering and absorption of short-wave components of the solar spectrum, resulting in a relative increase of red and infrared illumination. Here we investigated the potential of near-infrared (NIR) light as a cue for swimming orientation of the African cichlid fish (Cichlidae) Oreochromis mossambicus. A high-throughput semi-automated video tracking assay was used to analyse innate behavioural NIR-sensitivity. Fish revealed a strong preference to swim in the direction of NIR light of a spectral range of 850-950nm at an irradiance similar to values typical of natural surface waters. Our study demonstrates the ability of teleost fish to sense NIR and use it for phototactic swimming orientation.

Behaviour of the plathelminth Symsagittifera roscoffensis under different light conditions and the consequences for the symbiotic algae Tetraselmis convolutae

ABSTRACT Symsagittifera roscoffensis is a plathelminth living in symbiosis with the green algae Tetraselmis convolutae. Host and symbiont are a model system for the study of endosymbiosis, which has so far mainly focused on their biochemical interactions. Symsagittifera roscoffensis is well known for its positive phototaxis that is hypothesized to optimize the symbionts light perception for photosynthesis. In this study, we conducted a detailed analysis of phototaxis using light sources of different wavelength and brightness by videotracking. Furthermore, we compared the behavioural data with the electron transfer rate of the photosystem from cultured symbiotic cells. The symbiotic algae is adapted to low light conditions, showing a positive electron transfer rate at a photosynthetically active radiation of 0.112 µmol photons m−2 s−1, and S. roscoffensis showed a positive phototactic behaviour for light intensities up to 459.17 µmol photons m−2 s−1, which is not optimal regarding the needs of the symbiotic cells and may even harm host and symbiont. Red light cannot be detected by the animals and therefore their eyes seem not to be suitable for measuring the exact photosynthetically active radiation to the benefit of the photosymbionts. Summary: Symsagittifera roscoffensis lives in a photoautotroph symbiosis with the green algae Tetraselmis convolutae. The two partners have different preferences in light exposure.

Author Correction: Zebrafish and medaka offer insights into the neurobehavioral correlates of vertebrate magnetoreception

Text for Correction