Comparative analysis of the sea urchin genome has broad implications for the primitive state of deuterostome host defense and the genetic underpinnings of immunity in vertebrates. The sea urchin has an unprecedented complexity of innate immune recognition receptors relative to other animal species yet characterized. These receptor genes include a vast repertoire of 222 Toll-like receptors, a superfamily of more than 200 NACHT domain-leucine-rich repeat proteins (similar to nucleotide-binding and oligomerization domain (NOD) and NALP proteins of vertebrates), and a large family of scavenger receptor cysteine-rich proteins. More typical numbers of genes encode other immune recognition factors. Homologs of important immune and hematopoietic regulators, many of which have previously been identified only from chordates, as well as genes that are critical in adaptive immunity of jawed vertebrates, also are present. The findings serve to underscore the dynamic utilization of receptors and the complexity of immune recognition that may be basal for deuterostomes and predicts features of the ancestral bilaterian form.

Echinoderms occupy a critical and largely unexplored phylogenetic vantage point from which to infer both the early evolution of bilaterian immunity and the underpinnings of the vertebrate adaptive immune system. Here we present an initial survey of the purple sea urchin genome for genes associated with immunity. An elaborate repertoire of potential immune receptors, regulators and effectors is present, including unprecedented expansions of innate pathogen recognition genes. These include a diverse array of 222 Toll-like receptor (TLR) genes and a coordinate expansion of directly associated signaling adaptors. Notably, a subset of sea urchin TLR genes encodes receptors with structural characteristics previously identified only in protostomes. A similarly expanded set of 203 NOD/NALP-like cytoplasmic recognition proteins is present. These genes have previously been identified only in vertebrates where they are represented in much lower numbers. Genes that mediate the alternative and lectin complement pathways are described, while gene homologues of the terminal pathway are not present. We have also identified several homologues of genes that function in jawed vertebrate adaptive immunity. The most striking of these is a gene cluster with similarity to the jawed vertebrate Recombination Activating Genes 1 and 2 (RAG1/2). Sea urchins are long-lived, complex organisms and these findings reveal an innate immune system of unprecedented complexity. Whether the presumably intense selective processes that molded these gene families also gave rise to novel immune mechanisms akin to adaptive systems remains to be seen. The genome sequence provides immediate opportunities to apply the advantages of the sea urchin model toward problems in developmental and evolutionary immunobiology.

Animal movements result from a complex balance of many different forces. Muscles produce force to move the body; the body has inertial, elastic, and damping properties that may aid or oppose the muscle force; and the environment produces reaction forces back on the body. The actual motion is an emergent property of these interactions. To examine the roles of body stiffness, muscle activation, and fluid environment for swimming animals, a computational model of a lamprey was developed. The model uses an immersed boundary framework that fully couples the NavierStokes equations of fluid dynamics with an actuated, elastic body model. This is the first model at a Reynolds number appropriate for a swimming fish that captures the complete fluid-structure interaction, in which the body deforms according to both internal muscular forces and external fluid forces. Results indicate that identical muscle activation patterns can produce different kinematics depending on body stiffness, and the optimal value of stiffness for maximum acceleration is different from that for maximum steady swimming speed. Additionally, negative muscle work, observed in many fishes, emerges at higher tail beat frequencies without sensory input and may contribute to energy efficiency. Swimming fishes that can tune their body stiffness by appropriately timed muscle contractions may therefore be able to optimize the passive dynamics of their bodies to maximize peak acceleration or swimming speed.computational fluid dynamics | elasticity | locomotion

We present a theoretical model which is used to explain the intersegmental coordination of the neural networks responsible for generating locomotion in the isolated spinal cord of lamprey. A simplified mathematical model of a limit cycle oscillator is presented which consists of only a single dependent variable, the phase theta(t). By coupling N such oscillators together we are able to generate stable phase locked motions which correspond to traveling waves in the spinal cord, thus simulating "fictive swimming". We are also able to generate irregular "drifting" motions which are compared to the experimental data obtained from cords with selective surgical lesions.

The paper reports on a project to make a quadruped robot walk with medium forward speed on irregular terrain in an outdoor environment using a neural system model. The KEY WORDS-legged locomotion, quadruped, biologically inspired robot, adaptive walking on irregular terrain, neural system model, central pattern generator (CPG), reflex

We have been trying to induce a quadruped robot to walk with medium walking speed on irregular terrain based on biological concepts. We propose the necessary conditions for stable dynamic walking on irregular terrain in general, and we design the mechanical system and the neural system by comparing biological concepts with those necessary conditions described in physical terms. A PD controller at the joints can construct the virtual spring-damper system as the visco-elasticity model of a muscle. The neural system model consists of a central pattern generator (CPG) and reflexes. A CPG receives sensory input and changes the period of its own active phase. The desired angle and P-gain of each joint in the virtual springdamper system is switched based on the phase signal of the CPG. CPGs, the motion of the virtual spring-damper system of each leg and the rolling motion of the body are mutually entrained through the rolling motion feedback to CPGs, and can generate adaptive walking. We report on our experimental results of dynamic walking on terrains of medium degrees of irregularity in order to verify the effectiveness of the designed neuro-mechanical system. We point out the trade-off problem between the stability and the energy consumption in determining the cyclic period of walking on irregular terrain, and we show one example to solve this problem. MPEG footage of these experiments can be seen at http://www.kimura.is.uec.ac.jp.

We have been trying to induce a quadruped robot to walk with medium walking speed on irregular terrain based on biological concepts. We propose the necessary conditions for stable dynamic walking on irregular terrain in general, and we design the mechanical system and the neural system by comparing biological concepts with those necessary conditions described in physical terms. A PD controller at the joints can construct the virtual spring-damper system as the visco-elasticity model of a muscle. The neural system model consists of a central pattern generator (CPG) and reflexes. A CPG receives sensory input and changes the period of its own active phase. The desired angle and P-gain of each joint in the virtual springdamper system is switched based on the phase signal of the CPG. CPGs, the motion of the virtual spring-damper system of each leg and the rolling motion of the body are mutually entrained through the rolling motion feedback to CPGs, and can generate adaptive walking. We report on our experimental results of dynamic walking on terrains of medium degrees of irregularity in order to verify the effectiveness of the designed neuro-mechanical system. We point out the trade-off problem between the stability and the energy consumption in determining the cyclic period of walking on irregular terrain, and we show one example to solve this problem. MPEG footage of these experiments can be seen at http://www.kimura.is.uec.ac.jp.

1. Application of D-glutamate to the isolated spinal cord of the lamprey produces phasic activity in ventral roots, which is similar to that of the muscles of the intact swimming animal (5,18). Therefore, the isolated spinal cord may be used as a convenient model for the investigation of the generation of locomotor rhythms in a vertebrate. 2. Almost all slow muscle fibers exhibited excitatory junctional potentials (EJPs) during swimming activity. The number of EJPs per cycle increased with the intensity of ventral root (VR) bursting. Few twitch fibers were active, and these fired action potentials only during high intensities of VR bursts. 3. As was found by Russell and Wallén (25), myotomal motoneurons had oscillating membrane potentials during fictive swimming which, on the average, reached a peak depolarization in the middle of the VR burst (phi = 0.21 +/- 0.05; phi = 0 is defined as the onset of the VR burst, and the duration of the cycle is set equal to 1). Membrane potential oscillations in fin motoneurons were antiphasic to those of nearby myotomal motoneurons (peak depolarization phi = 0.68 +/- 0.05). 4. Lateral interneurons had oscillating membrane potentials in synchrony with those of myotomal motoneurons (peak depolarization phi = 0.21 +/- 0.10). Interneurons with axons projecting contralaterally and caudally (CC interneurons) had oscillating membrane potentials that peaked significantly earlier in the cycle (peak depolarization phi = 0.06 +/- 0.12). 5. Edge cells were only weakly modulated during fictive swimming. Their peak depolarizations occurred near the end of the VR burst (phi = 0.33 +/- 0.10). Most giant interneurons were not phasically modulated during fictive swimming. 6. Repetitive intracellular stimulation of Müller cells during fictive swimming generally evoked an increased burst intensity in ipsilateral VRs and a decreased burst intensity in contralateral VRs. The cells M3, B1, and B2 also produced increases or decreases in the frequency of VR bursts. Repetitive intracellular stimulation of sensory dorsal cells could also change the intensities and timing of VR bursts. 7. This study is an initial survey of lamprey spinal interneurons that participate in swimming activity. Lateral interneurons and CC interneurons are active during fictive swimming and probably help coordinate the undulations of the body, but their roles in pattern generation are not known. The central pattern generator is subject to modification by descending and sensory inputs.