In comparative studies using model organisms, extant taxa are often referred to as basal. The term suggests that such taxa are descendants of lineages that diverged early in the history of some larger taxon. By this usage, the basal metazoans comprise just four phyla (Placozoa, Porifera, Cnidaria, and Ctenophora) and the large clade Bilateria. We advise against this practice because basal refers to a region at the base or root of a phylogenetic tree. Thus, referring to an extant taxon or species as basal, or as more basal than another, can be misleading. While much progress has been made toward understanding some of the phylogenetic relationships within these groups, the relationships among them are still largely not known with certainty. Thus, sound inferences from comparative studies of model organisms demand continued illumination of phylogeny. Hypotheses about the mechanisms underlying metazoan evolution can be drawn from the study of model organisms in Cnidaria, Ctenophora, Placozoa, and Porifera, but it is clear that these model organisms are likely to be derived in many respects. Therefore, testing these hypotheses requires the study of yet additional model organisms. The most effective tests are those that investigate model organisms with phylogenetic positions among two sister groups comprising a larger clade of interest.

The symbiotic life style involves mutual ecological, physiological, structural, and molecular adaptations between the partners. In the symbiotic association between anthozoans and photosynthetic dinoflagellates (Symbiodinium spp., also called zooxanthellae), the presence of the endosymbiont in the animal cells has constrained the host in several ways. It adopts behaviors that optimize photosynthesis of the zooxanthellae. The animal partner has had to evolve the ability to absorb and concentrate dissolved inorganic carbon from seawater in order to supply the symbiont's photosynthesis. Exposing itself to sunlight to illuminate its symbionts sufficiently also subjects the host to damaging solar ultraviolet radiation. Protection against this is provided by biochemical sunscreens, including mycosporine-like amino acids, themselves produced by the symbiont and translocated to the host. Moreover, to protect itself against oxygen produced during algal photosynthesis, the cnidarian host has developed certain antioxidant defenses that are unique among animals. Finally, living in nutrient-poor waters, the animal partner has developed several mechanisms for nitrogen assimilation and conservation such as the ability to absorb inorganic nitrogen, highly unusual for a metazoan. These facts suggest a parallel evolution of symbiotic cnidarians and plants, in which the animal host has adopted characteristics usually associated with phototrophic organisms.

Studies of environmental signaling in animals have focused primarily on organisms with relatively constrained responses, both temporally and phenotypically. In this regard, existing model animals (e.g., “worms and flies”) are particularly extreme. Such animals have relatively little capacity to alter their morphology in response to environmental signals. Hence, they exhibit little phenotypic plasticity. On the other hand, basal metazoans exhibit relatively unconstrained responses to environmental signals and may thus provide more general insight, insofar as these constraints are likely traits derived during animal evolution. Such enhanced phenotypic plasticity may result from greater sensitivity to environmental signals, or greater abundance of suitable target cells, or both. Examination of what is known of the components of environmental signaling pathways in cnidarians reveals many similarities to well-studied model animals. In addition to these elements, however, macroscopic basal metazoans (e.g., sponges and cnidarians) typically exhibit a system-level capability for integrating environmental information. In cnidarians, the gastrovascular system acts in this fashion, generating local patterns of signaling (e.g., pressure, shear, and reactive oxygen species) via its organism-wide functioning. Contractile regions of tissue containing concentrations of mitochondrion-rich, epitheliomuscular cells may be particularly important in this regard, serving in both a functional and a signaling context. While the evolution of animal circulatory systems is usually considered in terms of alleviating surface-to-volume constraints, such systems also have the advantage of enhancing the capacity of larger organisms to respond quickly and efficiently to environmental signals. More general features of animals that correlate with relatively unconstrained responses to environmental signals (e.g., active stem cells at all stages of the life cycle) are also enumerated and discussed.

Sea anemones (Phylum Cnidaria; Class Anthozoa, Order Actiniaria) exhibit a diversity of developmental patterns that include cloning by fission. Because natural histories of clonal and aclonal sea anemones are quite different, the gain and loss of fission is an important feature of actiniarian lineages. We have used mitochondrial DNA and nuclear intron DNA phylogenies to investigate the evolution of longitudinal fission in sixteen species in the genus Anthopleura, and reconstructed an aclonal ancestor that has given rise at least four times to clonal descendents. For A. elegantissima from the northeastern Pacific Ocean, a transition to clonality by fission was associated with an up-shore habitat shift, supporting prior hypotheses that clonal growth is an adaptation to the upper shore. Fission in Actiniaria likely precedes its advent in Anthopleura, and its repeated loss and gain is perplexing. Field studies of the acontiate sea anemone Aiptasia californica provided insight to the mechanisms that regulate fission: subtidal Aiptasia responded to experimentally destabilized substrata by increasing rates of pedal laceration. We put forth a general hypothesis for actiniarian fission in which sustained tissue stretch (a consequence of substratum instability or intrinsic behavior) induces tissue degradation, which in turn induces regeneration. The gain and loss of fission in Anthopleura lineages may only require the gain and loss of some form of stretching behavior. In this view, tissue stretch initiates a cascade of developmental events without requiring complex gene regulatory linkages.

Colonial basal metazoans often encounter members of their own species as they grow on hard substrata, with the encounters typically resulting in either fusion of close relatives or rejection between unrelated colonies. These allorecognition responses play a critical role in maintaining the genetic and physiological integrity of the colony. Allorecognition responses in basal metazoans are controlled by highly variable genetic systems. The molecular nature of such systems, however, remains to be determined. Current efforts to identify the genes and molecules controlling allorecognition in basal metazoans have followed two pathways: identification of molecules differentially expressed in incompatible interactions, and positional or map-based cloning of allorecognition genes. Most studies following the first approach have been performed with marine demosponges, while those following the second approach have centered on the cnidarian of the genus Hydractinia. Here, I discuss the latter, focusing primarily on the genetic control of allorecognition responses.

Programmed cell death occurs in most, if not all life forms. It is used to sculpt tissue during embryogenesis, to remove damaged cells, to protect against pathogen infection and to regulate cell numbers and tissue homeostasis. In animals cell death often occurs by a morphologically and biochemically conserved process called apoptosis. A novel group of cysteine proteases, referred to as caspases, constitute the central component of this process. Caspases are activated following the induction of apoptosis and cleave a variety of cellular substrates, thus giving rise to the characteristic morphological events of apoptosis. Apoptosis is rapid and cell corpses are removed by phagocytosis. Recent work has shown that apoptosis also occurs in Cnidaria and Porifera, thus extending the origin of this evolutionary innovation down to the first metazoan animal phyla. Here, we review several examples of the role of apoptosis in cnidarians and then summarize new results on the subcellular localization of caspases and the control of apoptosis in Hydra. We show by immuncytochemistry that caspases in Hydra are localized in mitochondria. Following induction of apoptosis caspases are released from mitochondria as proenzymes and then activated by proteolytic cleavage in the cytoplasm. We also present evidence that apoptosis in Hydra is dramatically stimulated by inhibitors of PI3-kinase. Since PI3-kinase is a central component of growth factor signaling cascades in higher metazoans, this result suggests that control of apoptosis by growth factors is also evolutionarily conserved. We speculate on the role of growth factors in the evolution of apoptosis.

An in-depth understanding of the biology of animals will require the generation of genomics resources from organisms from all phyla in the metazoan phylogenetic tree. Such resources will ideally include complete genome sequences and comprehensive EST (expressed sequence tag) datasets for each species of interest. Of particular interest in this regard are animals in the early diverging non-bilaterian phyla Porifera, Placozoa, Cnidaria, and Ctenophora. Publications describing the results from the use of genomics approaches in these phyla have only recently begun to appear (Kortschak et al., 2003; Yang et al., 2003; Steele et al., 2004). Issues to be considered here include choosing the basal metazoan species to examine with genomics approaches, the relative advantages and disadvantages of genome sequencing versus EST projects, and the resources and infrastructure required to carry out such projects successfully.

Animals commonly modify their behavior in the presence of a conspecific or in response to signals. This is particularly true in the context of aggressive exchanges, which animals use to form networks of social relationships and to communicate social status associated with those relationships. Although hierarchical structures are a widespread phenomenon that has been studied extensively, the dynamic communication processes, specifically chemical communication in this review, is relatively overlooked. In particular, it is the exchange of information during agonistic interactions that mediates hierarchies and/or alters the outcomes of agonistic interactions. Given the theoretical appeal of these interactions, and the evolutionary importance and taxonomic diversity associated with social hierarchies, it is not surprising that the sensory mechanisms involved in the formation and maintenance of hierarchical structures have received recent attention. In crayfish, dominance is thought to be largely determined by physical superiority, where encounters are largely dyadic and fighting behavior is highly stereotyped. However, recent evidence has shown that the outcome of dyadic encounters are dependent upon a number of factors other than physical size, that include the exchange of chemical information during encounters, previous social history, and the intrinsic neurochemical state of opponents. We have attempted to provide a comprehensive analysis of the extrinsic chemical processes (previous history, sensory communication, etc.) and intrinsic chemical processes (neurochemical state) that produce and maintain dominance relations and social hierarchies in crayfish. We hope that this review will bring together a global picture of the processes that determine a crayfish's social standing and how intrinsic and extrinsic chemicals have substantial effects on aggressive states and agonistic bouts.

Amacrine cells are interneurons that have diverse functions in retinal signal processing. In order to study signaling and modulation in retinal amacrine cells, we employ a simplified culture system containing identifiable GABAergic amacrine cells. Immunocytochemistry experiments indicate that GABAergic amacrine cells express metabotropic glutamate receptor 5 (mGluR5), a group I mGluR usually linked to the IP3 signaling pathway. Ca² imaging experiments using an mGluR5-specific agonist indicate that these receptors are functional and when activated, can stimulate temporally diverse Ca² elevations. To begin to establish the role of these receptors in modulating amacrine cell function, we have used electrophysiological methods to ask whether ion channels are the targets of mGluR5-dependent modulation. Here we discuss our results indicating that activation of mGluR5 leads to enhancement of currents through GABA_A receptors. This enhancement is dependent upon elevations in cytosolic Ca² and activation of protein kinase C (PKC). To explore the consequences of Ca² elevations in another context, we have used nitric oxide (NO) donors to mimic the effects of activating the Ca² -dependent synthetic enzyme for NO, neuronal nitric oxide synthase. We find that exposure to NO donors also enhances the amplitude of currents through GABA_A receptors. Together, these results indicate that glutamate from presynaptic bipolar cells has the potential to work through multiple mechanisms to regulate the function of amacrine-to-amacrine cell GABAergic synapses.

Glucocorticoid secretion occurs in a circadian pattern and in response to stress. Among the broad array of glucocorticoid actions are multiple effects in the brain, including negative feedback regulation of hypothalamic hormone secretion. The negative feedback of glucocorticoids occurs on both rapid and delayed time scales, reflecting different regulatory mechanisms. While the slow glucocorticoid effects are widely held to involve regulation of gene transcription, the rapid effects are too fast to invoke genomic mechanisms. We provide a brief overview of multiple lines of evidence for membrane-associated glucocorticoid receptors in the brain, with an emphasis on our recent findings of a rapid, G protein-dependent glucocorticoid action in the rat hypothalamus. We have observed a novel mechanism of rapid glucocorticoid inhibition of parvocellular neuroendocrine cells of the hypothalamic paraventricular nucleus (PVN) mediated by the retrograde release of endocannabinoids and suppression of synaptic glutamate release. This acute glucocorticoid action may underlie the rapid inhibitory effect of glucocorticoids on hypothalamic neuroendocrine function, and provides a potential model for the rapid glucocorticoid effects that occur in several areas of the brain.

Vertebrates and arthropods share the common problem of controlling a rigid, articulated skeleton using neurally-controlled, striated muscle. Since this condition has arisen independently in the two groups, there is no reason to assume, a priori, that the control mechanisms used by the two groups will be the same. Indeed, there appear to be fundamental differences in the tactics used by the two groups. Insects and crustaceans use small numbers of heterogeneous motoneurons, while vertebrates (mammals especially) use many, more homogeneous, motor axons. In particular, arthropods make extensive use of peripheral neuromodulation to alter the properties of both neuromuscular junctions and muscle fibers. There has been little consideration of the functional consequences of these differences. I suggest that, faced with a size constraint on the number of motor units available, arthropods use peripheral modulation of muscle properties to achieve the flexibility and dynamic range that vertebrates achieve through recruitment of motor units.

This paper is a mini review of kinetic and kinematic evidence on the control of the hand with emphasis on grasping. It is not meant to be an exhaustive review, rather it summarizes current research examining the mechanisms through which specific patterns of coordination are elicited and observed during reach to grasp movements and static grasping. These coordination patterns include the spatial and temporal covariation of the rotation at multiple joints during reach to grasp movements. A basic coordination between grip forces produced by multiple digits also occurs during whole hand grasping such that normal forces tend to be produced in a synchronous fashion across pairs of digits. Finally, we address current research that suggests that motor unit synchrony across hand muscles and muscle compartments might be one of the neural mechanisms underlying the control of grasping.