Abstract: Despite significant progress in understanding the evolution of the mammalian brain, relatively little is known of the patterns of evolutionary change in the avian brain. In particular, statements regarding which avian taxa have relatively larger brains and brain regions are based on small sample sizes and statistical analyses are generally lacking. We tested whether psittaciforms (parrots, cockatoos and lorikeets) have larger brains and forebrains than other birds using both conventional and phylogenetically based methods. In addition, we compared the psittaciforms to primates to determine if cognitive similarities between the two groups were reflected by similarities in brain and telencephalic volumes. Overall, psittaciforms have relatively larger brains and telencephala than most other non-passerine orders. No significant difference in relative brain or telencephalic volume was detected between psittaciforms and passerines. Comparisons of other brain region sizes between psittaciforms and other birds, however, exhibited conflicting results depending upon whether body mass or a brain volume remainder (total brain volume - brain region volume) was used as a scaling variable. When compared to primates, psittaciforms possessed similar relative brain and telencephalic volumes. The only exception to this was that in some analyses psittaciforms had significantly larger telencephala than primates of similar brain volume. The results therefore provide empirical evidence for previous claims that psittaciforms possess relatively large brains and telencephala. Despite the variability in the results, it is clear that psittaciforms tend to possess large brains and telencephala relative to non-passerines and are similar to primates in this regard. Although it could be suggested that this reflects the advanced cognitive abilities of psittaciforms, similar studies performed in corvids and other avian taxa will be required before this claim can be made with any certainty.

Abstract: Within a particular animal taxon, larger bodied species generally have larger brains. Increased brain size usually correlates with increased behavioral repertoires and often with superior cognitive abilities. Bumblebees are eusocial insects that show pronounced size polymorphism among workers, whereas in honey bees size variation is much less pronounced. Recent studies suggest that within a given colony, large bumblebee workers are more efficient foragers and are better learners than their smaller sisters. Here we examine the allometric relationship between brain and body size of worker bumblebees and honey bees. We find that larger bees have larger brains and that most brain components show a similar size increase as the overall brain. One particular brain structure, the central body, is relatively smaller in large bumblebees compared to small bees. The same is true for the mushroom body lobes, whereas the mushroom body calyces, which receive sensory input, are not reduced in larger bumblebees or honey bees. Honey bees have relatively smaller brains, as well as smaller mushroom bodies, than bumblebee workers. We discuss why brain or mushroom body size does not necessarily correlate with the degree of a species' social organization.

Abstract: For both humans and other animals, the abilities to integrate separate sound elements over time into coherent perceptual representations, or ‘auditory streams’, and to segregate these auditory streams

Abstract: Via the accumulation of data from across the neuroanatomy literature, we estimate the manner in which (i) the number of neocortical areas varies with neocortex size, and (ii) the number of area-area c

Abstract: Animals coordinate their physiological state with external cues to appropriately time reproduction. These external cues exert effects through influences on the gonadotropin-releasing hormone neurons (GnRH), at the apex of the hypothalamus-pituitary-gonad (HPG) axis. In green treefrogs, mating calls are important regulators of reproductive behavior and physiology. Reception of mating calls causes an increase in androgen levels, and androgens promote the production of mating calls, demonstrating a mutual influence between the communication and endocrine systems. In order to investigate the central nervous system correlates of social regulation of the HPG axis in green treefrogs, we exposed males to a mating chorus or a control stimulus (tones), counted the resulting number of septo-preoptic GnRH-immunoreactive cells (GnRH-ir), and measured changes in plasma androgens. We found that reception of the mating chorus caused an increase in the number of GnRH-ir cells. As previously shown, we also found that the reception of the mating chorus resulted in higher androgen levels, suggesting that the higher GnRH-ir cell number represents increased GnRH production and release. We suggest that mating calls are an important supplementary cue that promotes GnRH production and release within the context of GnRH regulation by seasonal cues. Previous studies have proposed a neuroanatomical link between the anuran auditory system and GnRH neurons. Our results demonstrate a functional role for this proposed sensory-endocrine circuit, and show for the first time an influence of acoustic signals on GnRH neurons.

Abstract: In fish the terminal nerve is comprised of a group of cells with somata adjacent to the olfactory bulb and processes that extend both anteriorly to the olfactory mucosa and posteriorly to the telencephalon. In teleost fish an additional group of axons extends along the optic tract and delivers putative neuromodulators to the retina. One peptide – gonadotropin-releasing hormone (GnRH) – has been implicated as a prime candidate neuromodulator based on electrophysiological evidence that exogenous application influences neural activity. Here we describe the expression patterns of two GnRH receptor subtypes in the retina of a teleost fish, Astatotilapia (Haplochromis) burtoni. The type 1 GnRH receptor (GnRH-R1) was expressed in cells of the amacrine cell layer – where lateral inputs affect the flow of visual information from photoreceptors to the brain – and in a distribution and location pattern similar to dopaminergic interplexiform cells. Immunohistochemical labeling of GnRH fibers revealed varicosities along terminal nerve axons near the amacrine cell layer and near cells immunoreactive for tyrosine hydroxylase, a dopaminergic cell marker. This finding supports an existing model that the terminal nerve forms synapses with dopaminergic interplexiform cells. Surprisingly, the type 2 GnRH receptor (GnRH-R2) was abundantly expressed in ganglion cells, which lie along the direct pathway of visual information to the brain. These data suggest that GnRH from the TN could broadly influence processing of retinal signals both in lateral processing circuits through GnRH-R1 and in the vertical throughput pathway through GnRH-R2.

Abstract: Craniofacial evolution is considered fundamental to the origin of vertebrates and central to this process was the formation of a migratory, multipotent cell population known as the neural crest. The number of cell types that arise from the neural crest is truly astonishing as is the number of tissues and organs to which the neural crest contributes. In addition to forming melanocytes as well as many neurons and glia in the peripheral nervous system, neural crest cells also contribute much of the cartilage, bone and connective tissue of the face. These multipotent migrating cells are capable of self renewing decisions and based upon these criteria are often considered stem cells or stem cell-like. Rapid advances in our understanding of neural crest cell patterning continue to shape our appreciation of the evolution of neural crest cells and their impact on vertebrate craniofacial morphogenesis.

Abstract: We postulated that the retinas of bird species that are the earlier singers are more sensitive to low light conditions than species that sing closer to sunrise. The selected species were the American

Abstract: Changes in response to photoperiod are of fundamental importance to seasonal cycles in avian reproductive development. In this study we present data on photo-induced changes in gonadotropin-releasing

Abstract: The mode of visual information processing during visuospatial tasks differs across species and is supposed to depend on evolutionary and ecological factors. Humans show reaction times that increase wi

Abstract: Mental rotation is a widely accepted concept indicating an image-like mental representation of visual information and an analogue mode of information processing in certain visuospatial tasks. In the task of discriminating between image and mirror-image of rotated figures, human reaction times increase with the angular disparity between the figures. In animals, tests of this kind yield inconsistent results. Pigeons were found to use a time-independent rotational invariance, possibly indicating a non-analogue information processing system that evolved in response to the horizontal plane of reference birds perceive during flight. Despite similar ecological demands concerning the visual reference plane, a sea lion was found to use mental rotation in similar tasks, but its processing speed while rotating three-dimensional stimuli seemed to depend on the axis of rotation in a different way than found for humans in similar tasks. If ecological demands influence the way information processing systems evolve, hominids might have secondarily lost the ability of rotational invariance while retreating from arboreal living and evolving an upright gait in which the vertical reference plane is more important. We therefore conducted mental rotation experiments with an arboreal living primate species, the lion-tailed macaque. Performing a two-alternative matching-to-sample procedure, the animal had to decide between rotated figures representing image and mirror-image of a previously shown upright sample. Although non-rotated stimuli were recognized faster than rotated ones, the animal's mean reaction times did not clearly increase with the angle of rotation. These results are inconsistent with the mental rotation concept but also cannot be explained assuming a mere rotational invariance. Our study thus seems to support the idea of information processing systems evolving gradually in response to specific ecological demands.

Abstract: The distribution of the neurotransmitter gamma-aminobutyric acid (GABA) is not well understood for non-mammalian vertebrates. We thus used immunocytochemistry to locate putative GABAergic cells in the brains of male bullfrogs (Rana catesbeiana) and South African clawed frogs (Xenopus laevis). GABA-immunoreactive cell bodies were broadly distributed throughout the brains of both species with similar general patterns. In both, the greatest numbers of GABA-positive cells were found in the olfactory bulb, thalamus, and optic tectum, but virtually no major brain region lacked GABAergic cells. Species differences were also apparent. The density of GABA-immunoreactive cells was substantially higher in many areas of the bullfrog brain, compared to Xenopus. Bullfrogs possessed extensive cell populations in the medial pallium, preoptic area, optic tectum, torus semicircularis and tegmentum but cells were fewer in these locations in Xenopus. In the bullfrog hindbrain, GABA-immunoreactive cell bodies were restricted to very narrow and distinct populations. In Xenopus, however, cells in a similar position were fewer and spread more extensively. The distribution of GABA cells in the brain of these two species supports the hypotheses that GABA is involved in control of olfaction, audition, vision and vocalization. However, differences in the distribution of GABA between the bullfrog and Xenopus suggest that the extent of the GABAergic influence might vary between species.

Abstract: The distribution of lamprey gonadotropin-releasing hormone (GnRH)-I and -III has been extensively characterized by immunocytochemistry in the forebrain of the sea lamprey, Petromyzon marinus. However, the cellular location of lamprey GnRH-III mRNA expression by in situ hybridization in the lamprey brain has not been determined. We show for the first time the location of expression of lamprey GnRH-III, as well as provide a more comprehensive in situ study of lamprey GnRH-I and glutamic acid decarboxylase (GAD; GABA-synthesizing enzyme) mRNA expression in the brain of the lamprey in different reproductive life stages. Colorimetric and dual-label fluorescent amplification methods of in situ hybridization were used on brain tissue sections of adult, juvenile, and larval sea lamprey. In each life stage of the lamprey, expression of lamprey GnRH-I was shown in the preoptic area (POA) and the hypothalamus forming the characteristic arc-like cell population extending from the preoptic nucleus (NPO) to the neurohypophysis. Lamprey GnRH-III expression was also seen in the POA of each life stage in close proximity to lamprey GnRH-I mRNA containing neurons. GAD expression was shown in distinct cell clusters in and around the POA, in the olfactory bulb, in the dorsal thalamus beneath the habenular region, and also in the ventral-medial hypothalamus stretching from the periventricular region to the anterior portion of the rhombencephalon. Using dual-label in situ hybridization, we have shown that lamprey GnRH-I and -III mRNA are colocalized in the same cells in the POA in adult lampreys. Dual-label in situ hybridization also showed close proximity of GAD mRNA containing neurons and GnRH containing neurons in the POA. These data suggest that gamma-aminobutyric acid (GABA) may directly affect GnRH release in the brain of the sea lamprey.

Abstract: The orbicularis oris and buccinator muscles of mammals form an important subset of the facial musculature, the perioral muscles. In many taxa, these muscles form a robust muscular hydrostat capable of

Abstract: Apart from the pioneering studies of Ramon y Cajal [1893] and Rochon-Duvigneaud [1943], few studies have been devoted to the detailed study of the cytological and biochemical structure of the chameleon retina. In the present study we analyzed the expression of calbindin (CB), calretinin (CR) and parvalbumin (PV) immunoreactivities in the chameleon retina, and compared their distribution with those found in the retinas of other vertebrate species. CB immunoreactivity is dense in photoreceptors, horizontal and some lower amacrine cells. The most intense immunoreactivity was observed for calretinin; CR-ir amacrine cells are distributed throughout the inner nuclear, inner plexiform, and ganglion cell layers of the retina. Horizontal cells also display immunoreactivity to CR. A few retinal interneurons are weakly PV-ir. Double-labeling shows that all PV-ir or CB-ir cells, except the photoreceptors, are also strongly CR-ir. The distributions of these calcium-binding proteins in the chameleon retina share similarities with those observed in mammalian and avian retinas. In addition, the widespread distribution and co-localization of CB and CR reinforces the idea that these proteins play a general role in buffering the intracellular calcium levels in retinal cells. Furthermore, CB- and CR-immunoreactivities have enabled us to identify for the first time axon-bearing horizontal cells in the peripheral retina of the chameleon, very similar to those described in mammals.

Abstract: Bimodal cells in the torus semicircularis of the toadfish respond to both directional acoustic stimuli and hydrodynamic stimuli. Our previous physiological work indicated that bimodal cells may be distributed throughout the torus semicircularis. In this study, neurobiotin was used to compare the distribution of auditory-only and bimodal sites and to assess the inputs to those sites. A brief neurobiotin injection with short survival time was used to document the recording location. In other fish, a longer injection and survival time was used at an auditory-only or a bimodal site to fill the axons of the medullary inputs. Auditory-only sites were located in the most dorsal and medial sites in nucleus centralis. Bimodal sites were identified within both nucleus centralis and nucleus ventrolateralis. The greatest number of retrogradely filled cell bodies was found in the descending octaval nucleus following injection at auditory-only recording sites in nucleus centralis. In contrast, retrogradely filled cell bodies were found in both the descending octaval nucleus and the lateral line nucleus medialis following injection at bimodal sites in nucleus centralis or nucleus ventrolateralis. Retrogradely filled cell bodies were located in the dorsal and ventral divisions of the secondary octaval population from injections at either bimodal or auditory-only sites. The secondary octaval population has been implicated in auditory processing based on previous studies of both auditory specialist and auditory generalist fishes; however, this study is the first to reveal the potential role of the secondary octaval population in directional hearing in a fish. Relatively large numbers of retrogradely filled cells around the lateral lemniscus at consistent locations in the medulla indicate that a perilemniscal cell group also might be a component of the directional hearing circuit.

Abstract: In interactions, many tropical stomatopod species display conspicuous colored body spots that can communicate information about the sender’s state (eg, sex, aggressiveness, etc) Species inhabiting

Abstract: The size of the brain and its macro-anatomical parts in 206 birds representing 19 anseriform species and 4 tribes (Anserini, Anatini, Aythyini and Mergini) was the subject of a comparative analysis T

Abstract: The pure rod retina of a deep-sea eel species was used as a model system for the study of the differentiation of horizontal, bipolar, amacrine and ganglion cells. We wanted to test the hypothesis that the functional organization of the inner retina is less complex than in species with duplex, rod- and cone-containing retinae. We used immunocytochemistry, backfilling ganglion cells with fluorescent dextranes and microinjection of Lucifer Yellow, to visualize the micromorphology of the various cell types in a confocal microscope. The pure rod retina contains a single type of horizontal cell. The inner plexiform layer is 10–15 µm thick and shows three main sublayers. Bipolar terminals are found in all sublayers, but the majority are found in the inner sublamina b (PKC-immunoreactive cells, that in fish with duplex retinae receive a mixed rod-cone input). The neurochemical diversity of amacrine cells in terms of immunoreactivity does not differ from other teleosts; this similarity includes the pattern of dendritic stratification and ramification as revealed by microinjection. Ten different types of ganglion cells are distinguished based on the sizes of their perikaryon and dendritic field, and the stratification pattern in the inner plexiform layer. This is similar to the situation in catfish with retinae containing a single type of cone in addition to a majority of rods. In this respect, the differences between pure rod retinae and duplex retinae containing a single cone type were less obvious than hypothesized. In the deep-sea eel, the density of dendritic ramification in amacrine and ganglion cells was strongly reduced. This may be functionally related to the fact that vision in the deep sea environment relies exclusively on bioluminescence and is represented by burst-like emissions of point sources. This requires a mode of retinal signal processing that is less complex than in duplex retinae and involves a lower density of dendritic branching and synapses.

Abstract: Segmentation is crucial to the development of the vertebrate body plan Underlying segmentation in the head is further revealed when cranial neural crest cells emerge from even numbered rhombomeres in the hindbrain to form three stereotypical migratory streams that lead to the peripheral branchial arches To test the role of intrinsic versus extrinsic cues in influencing an individual cell’s trajectory, we implanted physical barriers in the chick mesoderm, distal to emerging neural crest cell stream fronts We analyzed the spatio-temporal dynamics as individual neural crest cells encountered and responded to the barriers, using time-lapse confocal imaging We find the majority of neural crest cells reach the branchial arch destinations following a repeatable series of events by which the cells overcome the barriers Even though the lead cells become temporarily blocked by a barrier, cells that follow from behind find a novel pathway around a barrier and become de novo leaders of a new stream Surprisingly, quantitative analyses of cell trajectories show that cells that encounter an r3 barrier migrate significantly faster but less directly than cells that encounter an r4 barrier, which migrate normally Interestingly, we also find that cells temporarily blocked by the barrier migrate slightly faster and change direction more often In addition, we show that cells can be forced to migrate into normally repulsive territory These results suggest that cranial neural crest cell trajectories are not intrinsically determined, that cells can respond to minor alterations in the environment and re-target a peripheral destination, and that both intrinsic and extrinsic cues are important in patterning

Abstract: The results presented herein report quantitative data relative to the distribution and morphological characteristics of both types of neuromasts encountered on the trunk lateral line of the sea bass (Dicentrarchus labrax, L.). These data were obtained from scanning electron micrographs. They indicate that, as expected, each modified scale of the sea bass possessed a single canal neuromast with long axis oriented parallel to the fish's long axis. In contrast to several fish species, two thirds of superficial neuromasts observed herein were oriented perpendicular to the fish's long axis. However, whatever the main orientation of superficial neuromasts, two thirds of their hair bundles were oriented parallel to the long axis of the animal with approximately half of them in the direction of the head. Similar ratios were observed for canal neuromasts whatever the area of the maculae: central or peripheral. For both types of neuromasts it was not possible to clearly distinguish a paired organization of hair bundles with opposing polarities. Superficial neuromasts on each trunk canal scale were located on either the dorsal or ventral side of the canal and appeared to be distributed along the trunk lateral line with a higher probability to be encountered closer to the operculum. The frequency of presence and the average number of superficial neuromasts per scale increased with fish size. We observed a size gradient for canal neuromasts between the operculum and caudal peduncle. This gradation was correlated with a reduction of the width of the central area of the canal segment. Canal neuromasts were always localized in the larger portions of the canal segments. Taken together, these results point out some specific features associated with the sea bass trunk lateral line. With the previous report, they establish the first full description of the trunk lateral line of sea bass and will be useful for upcoming experiments regarding the function of the two types of neuromasts.

Abstract: The present study is the first attempt to make comparisons of the visual response properties between tectal and thalamic neurons with spatially overlapping receptive fields by using extracellular recording and computer mapping techniques. The results show that in neuronal pairs about 70% of thalamic cells have excitatory receptive field alone, whereas 85% of tectal cells possess an excitatory receptive field surrounded by an inhibitory receptive field. In 70% of pairs the tectal cells are selective for direction of motion different from that which the thalamic cells prefer. Most thalamic cells prefer high speeds (80-160 degrees/s), whereas tectal cells prefer intermediate (40 degrees/s) or low (10-20 degrees/s) speeds. Photergic and scotergic cells exist in the thalamus but not in the tectum. These results provide evidence that tectal and thalamic cells extract different visual information from the same region of the visual field. The functional significance of these differences is discussed.

Abstract: Male Cassin’s finches (Carpodacus cassinii) sing long, complex songs that incorporate many elements mimicked from other species. Although one-year-old males (males in their first br

Abstract: Using immunohistochemistry in light microscopy, the myelin basic protein and proteolipid protein were localized on sections of the spinal cord enlargements of opossums, Monodelphis domestica, to determine the timecourse of myelinogenesis therein and compare it with other events of motor systems development. Additional tissue not processed for immunohistochemistry was prepared for transmission electron microscopy. No immunolabeling for either protein occurred on spinal sections from the newborn opossum, but in electron microscopy occasional fibers surrounded by loose, irregular membranous rings were seen on the outskirts of the ventral horn. Immunolabeling was detected first in the brachial enlargement during the second week, presumably on motoneuronal, vestibular and reticular axons. The areas of the dorsal columns, other spino-encephalic, reticulospinal and propriospinal projections became labeled in the third week, and the area of rubrospinal axons at 4 weeks. In the brachial gray matter, immunolabeling appeared along ventrodorsal and lateromedial gradients from the fourth to seventh weeks. Labeling developed similarly in the white and gray matter of the lumbosacral enlargement, but 3-5 days later than at brachial levels. Labeling intensity in the white and gray matter increased until at least 4 months, but remained light in laminae I-III. Thus, myelinogenesis in the spinal cord enlargements of the opossum is protracted and follows general rostrocaudal, ventrodorsal and lateromedial sequences. It occurs later than synaptogenesis at comparable levels of the cord, but earlier than myelinogenesis in the corresponding ventral and dorsal roots. Spinal myelinogenesis correlates with the development of sensorimotor reflexes, weight support and quadrupedal locomotion.

Abstract: Weakly electric fish produce electric organ discharges (EODs) used for electrolocation and communication. In the brown ghost knifefish, Apteronotus leptorhynchus , several neuron typ

Abstract: Wallerian degeneration is a very well described phenomenon in the vertebrate nervous system. In arthropods, and especially in crustaceans, nerve fiber degeneration has not been described extensively. In addition, literature shows that the events do not follow the same patterns as in vertebrates. In this study we report, by qualitative and quantitative ultrastructural analyses, the features and time course of the protocerebral tract degeneration following extirpation of the optic stalk. No remarkable changes were observed seven days after lesion. After 28 days the protocerebral tracts presented apparently preserved small and large diameter axons and some degenerating medium axons, with irregular contours and empty-looking aspect of the axoplasm. Forty days after the ablation of the optic stalks, both small (type I) and medium (type II and III) axons revealed signs of partial or total degeneration, but large nerve fibers (type IV) were still intact. After 45 days, the tract showed signs of advanced stage of degeneration and, apart from large axons, normal-looking fibers were almost absent. At these 3 last time points, degenerating axons displayed different electron densities and aspects, probably correlating to different onset times of the process. In addition, cells with granules in their cytoplasm, possibly hemocytes, were quite distinct, especially at 40 and 45 days after axotomy. These cells might share with glial cells the function of phagocytosis of cellular debris during the protocerebral tract degeneration. Quantitative analysis showed that the number of degenerating fibers increased significantly from 28 to 40 days after lesion, whereas the number of normal fibers decreased accordingly. Measurements of cross-sectional areas of normal and degenerating axons showed that types II and III (medium) start to degenerate before type I (small). Type IV (large) axons do not degenerate, even after 40 days. Therefore, we can conclude that degeneration in these afferent fibers starts late after axotomy, but proceeds at a faster rate afterwards until the complete degeneration of small and medium axons.

Abstract: The central posterior nucleus of teleost fish is a cluster of neurons in the dorsal thalamus that plays an important role in controlling social behaviors. In the weakly electric gymnotiform fish, <

Abstract: Fish in the family Mormyridae produce weak electric organ discharges that are used in orientation and communication. The peripheral and central anatomy of the electrosensory system has been well studied in the species Gnathonemus petersii, but comparative studies in other species are scarce. Here we report on one genus of mormyrid that displays a remarkable change in the electrosensory lateral line lobe (ELL), the hypertrophied rhombencephalic structure that receives primary electroreceptor input. Although all other mormyrids studied have three distinct zones on each side of the ELL, fish of the genus Stomatorhinus exhibit only two. Therefore, the two-zone ELL is a unique derived characteristic shared by Stomatorhinus. We examined the cutaneous electroreceptors that project to the ELL in Stomatorhinus. All three types of electroreceptors previously described for G. petersii were present, but there was a significant change in one type, the mormyromast. Both mormyromast sensory cell types (A- and B-cells) are present, but the B-cell is not innervated in Stomatorhinus. We conclude that, although all cutaneous sensory cells are present, the missing B-cell afferents account for the loss of the dorsolateral zone of the ELL, and therefore the loss of an entire sensory map. Because mormyromasts are involved in electrolocation behavior, this anatomical difference is probably related to differences in electrolocation abilities. Stomatorhinus could prove to be an excellent system for linking evolutionary changes in behavior with modifications in their neural substrates.

Abstract: Sadler, 2004]. Key regulatory input for the mechanisms that specify regional identity in the hindbrain and spinal cord are derived from HOX genes. HOX genes encode transcription factors containing a homeodomain and via their interaction with co-factors represent a family of master regulatory genes [for review see Fritzsch, 1998; Chandrasekhar, 2004; Sadler, 2004; Carroll et al., 2005; Murakami et al., 2005]. HOX genes play fundamental roles in organization of the basic rostro-caudal body plan of animal embryos and participate in axial patterning in many tissues [for review see Sadler, 2004; Carroll et al., 2005]. An intriguing property of HOX genes is that they are organized in clusters and their expression along the rostro-caudal axis refl ects their relative positions along the chromosome [for review see Sadler, 2004; Carroll et al., 2005]. Hence, they appear to represent a molecular means for specifying different regional characters and have been co-opted to patterning of the CNS, cranial and axial skeleton, gut, limbs, and mesodermal organs [for review see Sadler, 2004; Carroll et al., 2005]. Gainand loss-of-function experiments in several vertebrate taxa corroborate the role of HOX genes in hindbrain segmentation and patterning that sometimes lead to homeotic transformations or defects in the identity of specifi c rhombomeres [for review see Fritzsch, 1998; Chandrasekhar, 2004; Sadler, 2004; Carroll et al., 2005; Murakami et al., 2005]. With the intent of integrating developmental and evolutionary concepts, the 16th Annual Karger Workshop entitled ‘Hindbrain Evolution, Development, and Organization Revisited’ brought together in San Diego, Calif., developmental, evolutionary and comparative neurobiologists who have investigated early hindbrain patterning and organization from a variety of perspectives and vertebrate systems. The development of the hindbrain is an evolutionarily important event in the organization of the central nervous system [for review see Nieuwenhuys, 1998; Fritzsch, 1999; Sadler, 2004; Murakami et al., 2005]. The vertebrate hindbrain develops into cerebellum, pons, and medulla, and it is responsible for controlling essential functions such as respiration and heart function. In addition, the cranial nerves projecting from the hindbrain control muscles in the face, mandible, and eyes [for review see Nieuwenhuys, 1998; Barlow, 2002; Sadler, 2004]. To date, in all vertebrates that have been studied the segmented period of the hindbrain occurs shortly after the neural tube forms and results in a series of seven to eight rhombomeres that form along the anterior-posterior extent of the neural tube [for review see Nieuwenhuys, 1998; Sadler, 2004]. The rhombomeres are marked by distinct morphologies and patterns of gene expression that formulate the basic plan for generation of specifi c sets of neurons along the anterior-posterior extent of the rostral neural tube [for review see Nieuwenhuys, 1998; Published online: October 25, 2005