The minute brain of the copepod Tigriopus californicus supports a complex ancestral ground pattern of the tetraconate cerebral nervous systems.

Copepods are a diverse and ecologically crucial group of minute crustaceans that are relatively neglected in terms of studies on nervous system organization. Recently, morphological neural characters have helped clarify evolutionary relationships within Arthropoda, particularly among Tetraconata (i.e., crustaceans and hexapods), and indicate that copepods occupy an important phylogenetic position relating to both Malacostraca and Hexapoda. This taxon therefore provides the opportunity to evaluate those neural characters common to these two clades likely to be results of shared ancestry (homology) versus convergence (homoplasy). Here we present an anatomical characterization of the brain and central nervous system of the well-studied harpacticoid copepod species Tigriopus californicus. We show that this species is endowed with a complex brain possessing a central complex comprising a protocerebral bridge and central body. Deutocerebral glomeruli are supplied by the antennular nerves, and a lateral protocerebral olfactory neuropil corresponds to the malacostracan hemiellipsoid body. Glomeruli contain synaptic specializations comparable to the presynaptic "T-bars" typical of dipterous insects, including Drosophila melanogaster. Serotonin-like immunoreactivity pervades the brain and ventral nervous system, with distinctive deutocerebral distributions. The present observations suggest that a suite of morphological characters typifying the Tigriopus brain reflect a ground pattern organization of an ancestral Tetraconata, which possessed an elaborate and structurally differentiated nervous system.

Ground plan of the insect mushroom body: functional and evolutionary implications.

In most insects with olfactory glomeruli, each side of the brain possesses a mushroom body equipped with calyces supplied by olfactory projection neurons. Kenyon cells providing dendrites to the calyces supply a pedunculus and lobes divided into subdivisions supplying outputs to other brain areas. It is with reference to these components that most functional studies are interpreted. However, mushroom body structures are diverse, adapted to different ecologies, and likely to serve various functions. In insects whose derived life styles preclude the detection of airborne odorants, there is a loss of the antennal lobes and attenuation or loss of the calyces. Such taxa retain mushroom body lobes that are as elaborate as those of mushroom bodies equipped with calyces. Antennal lobe loss and calycal regression also typify taxa with short nonfeeding adults, in which olfaction is redundant. Examples are cicadas and mayflies, the latter representing the most basal lineage of winged insects. Mushroom bodies of another basal taxon, the Odonata, possess a remnant calyx that may reflect the visual ecology of this group. That mushroom bodies persist in brains of secondarily anosmic insects suggests that they play roles in higher functions other than olfaction. Mushroom bodies are not ubiquitous: the most basal living insects, the wingless Archaeognatha, possess glomerular antennal lobes but lack mushroom bodies, suggesting that the ability to process airborne odorants preceded the acquisition of mushroom bodies. Archaeognathan brains are like those of higher malacostracans, which lack mushroom bodies but have elaborate olfactory centers laterally in the brain.

Development-dependent and -independent ubiquitin expression in divisions of the cockroach mushroom body.

It has been proposed that the alpha and beta divisions of the mushroom bodies support intermediate and long-term memory whereas the gamma lobes support short-term memory. Here we investigate developmentally dependent versus developmentally independent alterations of mushroom body structure with special emphasis on its lobes. We show that in the cockroach mushroom bodies there are two types of plastic remodeling. One is developmental, in which episodic addition of new circuitry to the alpha and beta lobes is accomplished by newly born Kenyon cells. The second is revealed as a persistent alteration of structure within the gamma lobe. In the alpha/beta lobes, newly generated Kenyon cell axons extend glutamate-immunoreactive collaterals across layers of the axons of mature Kenyon cells. At specific times in each developmental episode (instar) these collaterals express ubiquitin, undergo localized degeneration, and are scavenged by glial cells. In contrast, the mature Kenyon cells that comprise the gamma lobe express detectable ubiquitin throughout each developmental episode. This pattern of ubiquitin expression suggests that the gamma lobe circuitry undergoes continuous modification independent of development.

Foraging experience, glomerulus volume, and synapse number: A stereological study of the honey bee antennal lobe.

The primary antennal sensory centers (antennal lobes) in the brain of the honeybee are highly compartmentalized into discrete spheres of synaptic neuropil called glomeruli. Many of the glomeruli can be identified according to their predictable size and location. This study examines T1-44, a prominent glomerulus on the dorsal surface of the antennal lobe. Previously, we have shown that the volume of T1-44 in 4-day-old workers performing tasks within the hive is significantly smaller than in foragers and that increases in volume are accompanied by an increase in total synapse number in this glomerulus. Here we examine whether foraging experience is essential for either changes in volume or for changes in synapse numbers in glomerulus T1-44. Five-day-old bees reared under normal colony conditions were compared with 5-day-old bees reared under isolated conditions, and also to 5-day-old bees that had been induced to forage precociously. A combination of light and electron microscopy was used to compare T1-44 volumes and synapse numbers in these three groups. Two groups of 11-day-old bees, precocious foragers and nonforagers, were also examined. The Cavalieri direct estimator of volume was applied to 1.5 microm sections of resin embedded brains. Selected sections were then re-embedded and prepared for transmission electron microscopy. Synapse densities were determined using the physical disector method on electron micrographs. Synapse density and glomerulus volume were combined to give an unbiased estimate of the total number of synapses. This study shows that while both volume and synapse numbers can be induced to increase prematurely in young (5-day-old) precocious foragers, foraging experience is not essential for these structural changes to occur in glomerulus T1-44.

Stereological analysis reveals striking differences in the structural plasticity of two readily identifiable glomeruli in the antennal lobes of the adult worker honeybee.

The primary antennal sensory centers (antennal lobes) in the brain of the honeybee are highly compartmentalized into discrete spheres of synaptic neuropil called glomeruli, many of which can be identified according to their predictable size and location. Glomeruli undergo significant changes in volume during the lifetime of the adult worker bee, at least some of which are activity dependent. This study tests the commonly expressed assumption that increases in neuropil volume are accompanied by an underlying increase in the number of synapses present in the tissue. A combination of light and electron microscopy was used to determine total synapse number within two glomeruli, T1-44 and T4-2(1). The Cavalieri direct estimator of volume was applied to 1.5 microm sections of resin-embedded brains. Selected sections were then re-embedded and prepared for transmission electron microscopy. Synapse densities were determined using the physical disector method on electron micrographs. Synapse density and glomerulus volume were combined to give an unbiased estimate of the total number of synapses. In glomerulus T1-44, a significant increase in volume was accompanied by a significant increase in the total number of synapses. In contrast, synapse counts in T4-2(1) remained unchanged, despite a significant increase in the volume of this glomerulus. These results demonstrate that synapse proliferation in antennal lobes of the adult worker bee is highly site specific. Although volumetric changes and changes in synapse number both contribute to the structural plasticity of the antennal lobes, these two components of plasticity appear to be independent processes.