Organization and evolutionary trends of primary olfactory brain centers in Tetraconata (Crustacea+Hexapoda)

The phylogenetic relationships within the Arthropoda have been controversial for more than a century. Today, comparative studies on the structure and development of the nervous system contribute important arguments to this discussion, so that the term "neurophylogeny" was coined for this discipline. The large number of recent studies on the nervous system in various nonmodel arthropods indicates that we are far advanced in the process of analyzing the cellular architecture of the arthropod nervous system in a depth that will ultimately provide characters at a level of resolution equal or even superior to that of characters traditionally used in morphological phylogenetic studies. This article sets out to summarize the current state of the discussion on arthropod phylogeny and briefly evaluates the morphological characters that have been used as arguments in favor of the traditional Tracheata hypothesis. Then, a thorough overview is given of characters derived from structure and development of the arthropod brain and the ventral nerve cord from the cellular level to the level of larger neuropil systems. These characters support the new Tetraconata hypothesis suggested by Dohle and provide evidence for a clade that unites malacostracan and remipede crustaceans with the Hexapoda.

/pdf/neurophylogeny-architecture-of-the-nervous-system-and-a-p8d4299ik2.pdf

Neurophylogeny: Architecture of the nervous system and a fresh view on arthropod phyologeny.

In their natural environment, many insects need to identify and evaluate behaviorally relevant odorants on a rich and dynamic olfactory background. Behavioral studies have demonstrated that bees recognize learned odors within less than 200ms, indicating a rapid processing of olfactory input in the sensory pathway. We studied the role of the honeybee antennal lobe network in constructing a fast and reliable code of odor identity using in vivo intracellular recordings of individual projection neurons (PNs) and local interneurons (LNs). We found a complementary ensemble code where odor identity is encoded in the spatio-temporal pattern of response latencies as well as in the pattern of activated and inactivated PN firing. This coding scheme rapidly reaches a stable representation within 50-150ms after stimulus onset. Testing an odor mixture versus its individual compounds revealed different representations in the two morphologically distinct types of lateral and median (l- and m-) PNs. Individual m-PNs mixture responses were dominated by the most effective compound (elemental representation) whereas l-PNs showed suppressed responses to the mixture but not to its individual compounds (synthetic representation). The onset of inhibition in the membrane potential of l-PNs coincided with the responses of putative inhibitory interneurons that responded significantly faster than PNs. Taken together, our results suggest that processing within the local interneuron network of the AL is an essential component of constructing the antennal lobe population code.

/pdf/rapid-odor-processing-in-the-honeybee-antennal-lobe-network-2i5a682yii.pdf

Rapid odor processing in the honeybee antennal lobe network.

In their natural environment, many insects need to identify and evaluate behaviorally relevant odorants on a rich and dynamic olfactory background. Behavioral studies have demonstrated that bees recognize learned odors within <200 ms, indicating a rapid processing of olfactory input in the sensory pathway. We studied the role of the honeybee antennal lobe network in constructing a fast and reliable code of odor identity using in vivo intracellular recordings of individual projection neurons (PNs) and local interneurons (LNs). We found a complementary ensemble code where odor identity is encoded in the spatio-temporal pattern of response latencies as well as in the pattern of activated and inactivated PN firing. This coding scheme rapidly reaches a stable representation within 50–150 ms after stimulus onset. Testing an odor mixture versus its individual compounds revealed different representations in the two morphologically distinct types of lateral- and median PNs (l- and m-PNs). Individual m-PNs mixture responses were dominated by the most effective compound (elemental representation) whereas l-PNs showed suppressed responses to the mixture but not to its individual compounds (synthetic representation). The onset of inhibition in the membrane potential of l-PNs coincided with the responses of putative inhibitory interneurons that responded significantly faster than PNs. Taken together, our results suggest that processing within the LN network of the AL is an essential component of constructing the antennal lobe population code.

Rapid Odor Processing in the Honeybee Antennal Lobe Network

Background
During the evolutionary radiation of Crustacea, several lineages in this taxon convergently succeeded in meeting the physiological challenges connected to establishing a fully terrestrial life style. These physiological adaptations include the need for sensory organs of terrestrial species to function in air rather than in water. Previous behavioral and neuroethological studies have provided solid evidence that the land hermit crabs (Coenobitidae, Anomura) are a group of crustaceans that have evolved a good sense of aerial olfaction during the conquest of land. We wanted to study the central olfactory processing areas in the brains of these organisms and to that end analyzed the brain of Coenobita clypeatus (Herbst, 1791; Anomura, Coenobitidae), a fully terrestrial tropical hermit crab, by immunohistochemistry against synaptic proteins, serotonin, FMRFamide-related peptides, and glutamine synthetase.

/pdf/brain-architecture-in-the-terrestrial-hermit-crab-coenobita-122nn0wd2n.pdf

Brain architecture in the terrestrial hermit crab Coenobita clypeatus (Anomura, Coenobitidae), a crustacean with a good aerial sense of smell

Neurons in the olfactory deutocerebrum of the spiny lobster, Panulirus argus, were recorded intracellularly and filled with biocytin. Recorded neurons arborized in the olfactory lobe (OL), a glomerular neuropil innervated by olfactory and some presumptive mechanosensory antennular afferents. The neurons responded to chemosensory input from the lateral antennular flagellum bearing the olfactory sensilla but not the medial flagellum bearing many non-olfactory chemosensory sensilla. Many neurons received additional mechanosensory input. Thus the OL integrates specifically olfactory with mechanosensory input. OL neurons had multiglomerular arborizations restricted to one or two of the three horizontal layers of the columnar glomeruli. OL local interneurons comprised “core” neurons with tree-like neurites and terminals in the base of the glomeruli and “rim” neurons with neurites surrounding the OL and terminals in the cap/subcap. The somata of OL local interneurons lay in the medial soma cluster (100000 somata). OL projection neurons arborized in the base of the glomeruli and ascended via the olfactory glomerular tract to the lateral protocerebrum. A parallel projection pathway is constituted by projection neurons of the accessory lobe, a glomerular neuropil without afferent innervation but intimate links to the OL. The projection neuron somata constituted the lateral soma cluster (200000 somata).

Processing of antennular input in the brain of the spiny lobster, panulirus argus. ii. the olfactory pathway

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Processing of antennular input in the brain of the spiny lobster, panulirus argus. ii. the olfactory pathway