Age-dependent changes in the transcription levels of 5-day-old Euglena gracilis cells, which showed positive gravitaxis, 6-day-old cells without gravitactic orientation, and older cells (9- and 11-day-old, which displayed a precise negative gravitaxis) were determined through microarray analysis. Hierarchical clustering of four independent cell cultures revealed pronounced similarities in transcription levels at the same culture age, which proves the reproducibility of the cultivation method. Employing the non-oriented cells from the 6-day-old culture as a reference, about 2779 transcripts were found to be differentially expressed. While positively gravitactic cells (5-day-old culture) showed only minor differences in gene expression compared to the 6-day reference, pronounced changes of mRNAs (mainly an increase) were found in older cells compared to the reference culture. Among others, genes coding for adenylyl cyclases, photosynthesis, and metabolic enzymes were identified to be differentially expressed. The investigated cells were grown in batch cultures, so variations in transcription levels most likely account for factors such as nutrient depletion in the medium and self-shading. Based on these findings, a particular transcript (e.g., transcript 19556) was downregulated using the RNA interference technique. Gravitaxis and phototaxis were impaired in the transformants, indicating the role of this transcript in signal transduction. Results of the experiment are discussed regarding the increasing importance of E. gracilis in biotechnology as a source of valuable products and the possible application of E. gracilis in life-support systems.

The way species use their habitat dictates their intra- and interspecific interactions. We studied the effects of the microhabitat type and slope on the movement behaviour of the Saharan horned viper (Cerastes cerastes) in its natural habitat. This viper occurs in sand dunes and moves mostly by sidewinding. Additionally, we studied the microhabitat preference of desert rodents-the vipers\' main prey. We placed the vipers on different natural dune slopes and recorded their behaviour. We found a strong anti-gravitactic response: vipers moved more frequently towards the top of the dune than in any other direction, despite a decrease in stride length with increasing slope. The foraging-related behaviour of the vipers was concentrated in the dune semi-stable areas rather than its stable or shifting sand areas. We measured rodent activity by placing seed trays in the dune allowing the rodents to collect seeds. Rodent activity was the highest in the shifting sands, closely followed by the semi-stable microhabitat. These results suggest the vipers use the semi-stable microhabitat mainly for foraging and may use the shifting sand areas as commuting routes between such areas. This study may be of use for conservation efforts of psammophilic species in desert dunes.

Vertical migration as well as horizontal dispersion is important in the ecological strategy of planktonic larvae of sedentary corals. We report in this paper unique vertical swimming behavior of planulae of the reef-building coral Acropora tenuis. Several days after fertilization, most of the planulae stayed exclusively at either the top or the bottom of the rearing tank. A good proportion of the planulae migrated almost vertically between top and bottom with fairly straight trajectories. Planulae sometimes switched their swimming direction via a sharp turn between the opposite directions. Quantitative analyses demonstrated that planulae kept constant speed while swimming either upward or downward, in contrast to frequent changes of direction and speed in horizontal swimming. Statistical comparison of propulsive speeds, estimated from swimming speeds and passive sedimentation, revealed gravikinesis of planulae, where the propulsive speed was significantly greater in downward swimming than upward swimming. The larval density hydrodynamically estimated was 0.25% lower than sea water density, which might be explained by the large quantity of lipids in planulae. Also, the deciliated larvae tended to orient oral end-up during floatation, presumably due to asymmetrical distribution of the endogenous light lipids. Plasticity of the larval tissue geometry could easily cause relocation of the center of forces which work together to generate gravitactic-orientation torque and, therefore, abrupt changing of the gravitactic swimming direction. The bimodal gravitactic behavior may give a new insight into dispersal and recruitment of coral larvae.

Human exploration of space and other celestial bodies bears a multitude of challenges. The Earth-bound supply of material and food is restricted, and in situ resource utilisation (ISRU) is a prerequisite. Excellent candidates for delivering several services are unicellular algae, such as the space-approved flagellate Euglena gracilis. This review summarizes the main characteristics of this unicellular organism. Euglena has been exposed on various platforms that alter the impact of gravity to analyse its corresponding gravity-dependent physiological and molecular genetic responses. The sensory transduction chain of gravitaxis in E. gracilis has been identified. The molecular gravi-(mechano-)receptors are mechanosensory calcium channels (TRP channels). The inward gated calcium binds specifically to one of several calmodulins (CaM.2), which, in turn, activates an adenylyl cyclase. This enzyme uses ATP to produce cAMP, which induces protein kinase A, followed by the phosphorylation of a motor protein in the flagellum, initiating a course correction, and, finally, resulting in gravitaxis. During long space missions, a considerable amount of food, oxygen, and water has to be carried, and the exhaled carbon dioxide has to be removed. In this context, E. gracilis is an excellent candidate for biological life support systems, since it produces oxygen by photosynthesis, takes up carbon dioxide, and is even edible. Various species and mutants of Euglena are utilized as a producer of commercial food items, as well as a source of medicines, as it produces a number of vitamins, contains numerous trace elements, and synthesizes dietary proteins, lipids, and the reserve molecule paramylon. Euglena has anti-inflammatory, -oxidant, and -obesity properties.

Arthropods and in particular insects show a great variety of different exoskeletal sensors. For most arthropods, spatial orientation and gravity perception is not fully understood. In particular, the interaction of the different sensors is still a subject of ongoing research. A disadvantage of most of the experimental methods used to date to study the spatial orientation of arthropods in behavioral experiments is that the body or individual body parts are fixed partly in a non-natural manner. Therefore, often only the movement of individual body segments can be used to evaluate the experiments. We here present a novel experimental method to easily study 3D-escape movements in insects and analyze whole-body reaction. The animals are placed in a transparent container, filled with a lightweight substrate and rotating around two axes. To verify our setup, house crickets (Acheta domesticus) with selectively manipulated gravity-perceiving structures were analyzed. The spatial orientation behavior was quantified by measuring the time individuals took to escape toward the surface and the angular deviation toward the gravitational vector. These experiments confirm earlier results and therefore validated our experimental setup. Our new approach thus allows to investigate several comprehensive questions regarding the spatial orientation of insects and other animals.

Euglena gracilis is a photosynthetic flagellate. To acquire a suitable position in its surrounding aquatic environment, it exploits light and gravity primarily as environmental cues. Several physiological studies have indicated a fine-tuned relationship between gravity sensing (gravitaxis) and light sensing in E. gracilis. However, the underlying molecular mechanism is largely unknown. The photoreceptor photoactivated adenylyl cyclase (PAC) has been studied for over a decade. Nevertheless, no direct/indirect interaction partner (upstream/downstream) has been reported for PAC. It has been shown that a specific protein, kinase A (PKA), showed to be involved in phototaxis and gravitaxis. The current study reports the localization of the specific PKA and its relationship with PAC.

Gravity plays an important role in most life forms on Earth. Yet, a complete molecular understanding of sensing and responding to gravity is lacking. While there are anatomical differences among animals, there is a remarkable conservation across phylogeny at the molecular level. Caenorhabditis elegans is suitable for gene discovery approaches that may help identify molecular mechanisms of gravity sensing. It is unknown whether C. elegans can sense the direction of gravity.
In aqueous solutions, motile C. elegans nematodes align their swimming direction with the gravity vector direction while immobile worms do not. The worms orient downward regardless of whether they are suspended in a solution less dense (downward sedimentation) or denser (upward sedimentation) than themselves. Gravitaxis is minimally affected by the animals\' gait but requires sensory cilia and dopamine neurotransmission, as well as motility; it does not require genes that function in the body touch response.
Gravitaxis is not mediated by passive forces such as non-uniform mass distribution or hydrodynamic effects. Rather, it is mediated by active neural processes that involve sensory cilia and dopamine. C. elegans provides a genetically tractable system to study molecular and neural mechanisms of gravity sensing.

The placozoan Trichoplax adhaerens has been bridging gaps between research disciplines like no other animal. As outlined in part 1, placozoans have been subject of hot evolutionary debates and placozoans have challenged some fundamental evolutionary concepts. Here in part 2 we discuss the exceptional genetics of the phylum Placozoa and point out some challenging model system applications for the best known species, Trichoplax adhaerens.

Larvae of the tunicate Ciona intestinalis possess a central nervous system of 177 neurons. This simplicity has facilitated the generation of a complete synaptic connectome. As chordates and the closest relatives of vertebrates, tunicates promise insight into the organization and evolution of vertebrate nervous systems. Ciona larvae have several sensory systems, including the ocellus and otolith, which are sensitive to light and gravity, respectively. Here, we describe circuitry by which these two are integrated into a complex behavior: the rapid reorientation of the body followed by upward swimming in response to dimming. Significantly, the gravity response causes an orienting behavior consisting of curved swims in downward-facing larvae but only when triggered by dimming. In contrast, the majority of larvae facing upward do not respond to dimming with orientation swims-but instead swim directly upward. Under constant light conditions, the gravity circuit appears to be inoperable, and both upward and downward swims were observed. Using connectomic and neurotransmitter data, we propose a circuit model that can account for these behaviors. The otolith consists of a statocyst cell and projecting excitatory sensory neurons (antenna cells). Postsynaptic to the antenna cells are a group of inhibitory primary interneurons, the antenna relay neurons (antRNs), which then project asymmetrically to the right and left motor units, thereby mediating curved orientation swims. Also projecting to the antRNs are inhibitory photoreceptor relay interneurons. These interneurons appear to antagonize the otolith circuit until they themselves are inhibited by photoreceptors in response to dimming, thus providing a triggering circuit.

The way the unicellular, biflagellated, green alga Chlamydomonas orients upward has long been discussed in terms of both mechanics and physiology. In this study, we focus on the mechanics, i.e. the \'passive\' mechanisms, of gravitaxis. To rotate the body upwards, cellular asymmetry is critical. Chlamydomonas can be depicted as a nearly spherical cell body with two anterior, symmetric flagella. The present study looks at the question of whether the existence of the flagella significantly affects torque generation in upward reorientation. The \'density asymmetry model\' assumes that the cell is spherical and bottom-heavy and that the shape and weight of the flagella are negligible, while the \'shape asymmetry model\' considers the shape of the flagella. Both our experimental and simulation results revealed a considerable contribution from shape asymmetry to the upward orientation of Chlamydomonas reinhardtii, which was several times larger than that of density asymmetry. From the experimental results, we also quantified the extent of bottom-heaviness, i.e. the distance between the centers of gravity and the figure when the cell body is assumed to be spherical. Our estimation was approximately 30 nm, only one-third of previous assumptions. These findings indicate the importance of the viscous drag of the flagella to the upward orientation, and thus negative gravitaxis, in Chlamydomonas.