Biotic and abiotic determinants of the formation of ant mosaics in primary Neotropical rainforests

1. Ants are widespread in tropical rainforests, including in the canopy where territorially dominant arboreal species represent the main part of the arthropod biomass. 2. By mapping the territories of dominant arboreal ant species and using a null model analysis and a pairwise approach this study was able to show the presence of an ant mosaic on the upper canopy of a primary Neotropical rainforest (c. 1 ha sampled; 157 tall trees from 28 families). Although Neotropical rainforest canopies are frequently irregular, with tree crowns at different heights breaking the continuity of the territories of dominant ants, the latter are preserved via underground galleries or trails laid on the ground. 3. The distribution of the trees influences the structure of the ant mosaic, something related to the attractiveness of tree taxa for certain arboreal ant species rather than others. 4. Small‐scale natural disturbances, most likely strong winds in the area studied (presence of canopy gaps), play a role by favouring the presence of two ant species typical of secondary formations: Camponotus femoratus and Crematogaster levior, which live in parabiosis (i.e. share territories and nests but lodge in different cavities) and build conspicuous ant gardens. In addition, pioneer Cecropia myrmecophytic trees were recorded.     

Behavioral and Morphological Studies of Fetal Neural Transplants into SCN-Lesioned Rats

We have studied the effects of fetal neuronal grafts on the temporal pattern of drinking behavior of suprachiasmatic nuclei (SCN)-lesioned adult rats. Additionally, in an independent set of animals, the immunohistochemical staining for vasopressin, vasoactive intestinal polypeptide, and neuropeptide Y and the retinal connections to the hypothalamus were studied. The behavioral experiments indicate that anterior hypothalamic transplants induced reorganization of the temporal pattern of drinking behavior when placed in the third ventricle of adult hosts bearing complete SCN lesions, but not when placed in a cavity in the occipital cortex. Such rhythmicity persists only when the animals were recorded under constant darkness but not under constant light, indicating that the restored rhythmicity was generated endogenously but that the oscillator was extremely sensitive to light. Fetal occipital cortex induced reorganization of the temporal pattern of previously arrhythmic hosts, but it disappeared when the animals were recorded under constant light or constant darkness. It is clear that this rhythmicity was exogenous. In contrast to the cortical transplants, the hypothalamic transplants showed a morphological organization similar to that found in the normal hypothalamus regardless of their placement in the host brain. From these observations it is concluded that development of neocortex is more affected by environmental factors than that of the hypothalamus. Both hypothalamic and cortical transplants induced sprouting of retinal fibers into the anterior hypothalamus and the grafted tissue. It is possible that such fibers could be the neuroanatomical substrate by which rhythmicity is induced by cortical tissue.     

Studies on graft unions

Summary De novo formation of cytoplasmic cell connections are studied at the graft interface of 5 day old in vitro heterografts ofVicia faba onHelianthus annuus. Continuous and half plasmodesmata, both branched and unbranched, are described at various stages of development in non-division walls between unlike and like dedifferentiated callus cells. In apical portions of protruding callus cells and in the contact zone between opposing cells extremely thin wall parts with a striking ER/plasmalemma contact are observed. During subsequent thickening of the modified wall parts cytoplasmic strands enclosing constricted ER cisternae are entrapped within the newly deposited wall material. These cytoplasmic strands represent half plasmodesmata which—in case of fusion with corresponding structures of adjoining cells across the loosened wall matrix — form continuous cell connections. Golgi vesicles secreting wall material are involved in the process of forming half and continuous plasmodesmata, thus following the same mechanism of plasmodesmata development as described for isolated protoplasts in cell cultures. The findings suggest the existence of a unifying mechanism of secondary formation of plasmodesmata showing far-reaching similarities with the establishment of primary cell connections.     

Das visuelle System von Zonotrichia leucophrys gambelii

Zusammenfassung Der Verlauf der Sehbahn und die Lokalisation der optischen Zentren wurden bei Zonotrichia leucophrys gambelii (nordamerikanischer Ammernfink) nach einseitiger Augenexstirpation mit den Techniken von Nauta-Fink-Heimer, Bodian und Bielschowsky erforscht. Die Untersuchungen erstreckten sich über einen Zeitraum von 3 bis zu 120 Tagen nach der Operation. Zonotrichia leucophrys gambelii besitzt ein für Vögel typisches visuelles System. Die Hauptmasse der Optikusfasern endet im Stratum griseum et fibrosum superficiale des Tectum opticum. Weitere zentrale Endgebiete sind: Nucleus geniculatus lateralis, Nucleus lateralis anterior, Nucleus superficialis synencephali, Nucleus externus, tectales Grau und Nucleus ectomamillaris als Kern der basalen optischen Wurzel. Alle Fasern werden im Chiasma opticum total gekreuzt, auch der Tractus isthmo-opticus, ein efferentes Bündel, dessen Ursprung im Nucleus isthmo-opticus zu finden ist. Dieses efferente Fasersystem läßt sich im Stumpf des durchtrennten N. opticus noch 120 Tage nach der Operation gut versilbern. Eine direkte Verbindung von Retina und Hypothalamus war lichtmikroskopisch nicht nachweisbar. Neurosekretorisch aktive Zellen des Hypothalamus können zwar einen engen räumlichen Kontakt mit den optischen Fasern haben, Synapsen sind aber an diesen Stellen nicht zu erkennen. Es werden passagere Opticusfasern beschrieben, die auf dem Weg zum Nucleus lateralis anterior und Nucleus superficialis synencephali den Hypothalamus durchsetzen.

---

Neurohistological and experimental studies of the visual system in Zonotrichia leucophrys gambelii

Summary The course of the optic pathways and the positions of the optic centers have been investigated with unilaterally enucleated white-crowned sparrows, Zonotrichia leucophrys gambelii, using the techniques of Nauta-Fink-Heimer, Bodian, and Bielachowsky. The investigation involved birds examined 3–120 days after enucleation. The white-crowned sparrow has a typically avian visual system. The major bundles of optic fibers terminate in the stratum griseum et fibrosum superficiale of the tectum opticum. Further terminal areas are the nucleus geniculatus lateralis, nucleus lateralis anterior, nucleus superficialis synencephali, nucleus externus, the tectal gray, and the nucleus ectomamillaris of the basal optic root. There is a complete crossing of all fibers in the chiasma, including those of the tractus isthmo-opticus, an efferent bundle with its origin in the nucleus isthmo-opticus. This efferent fiber system can be well impregnated in the stump of the sectioned optic nerve up to 120 days after the operation. No direct connection between the retina and hypothalamus could be demonstrated by light microscopy. Hypothalamic neurosecretory cells can occur in close contact with optic fibers but no synapses have been recognized. Some optic fibers pass through the hypothalamus enroute to the nucleus lateralis anterior and the nucleus superficialis synencephali.

Mit Unterstützung durch die Deutsche Forschungsgemeinschaft. Herrn Prof. Dr. D.S. Farner, Department of Zoology, University of Washington, Seattle, Wash., danke ich für die Förderung dieser Studien (National Institutes of Health Research Grant No. 5 ROI NB 06187 to Professor D. S. Farner).     

Parallel processing of mechanosensory inputs from the locust ovipositor by intersegmental and local interneurones

The neural pathways underlying the processing of signals from locust (Schistocerca gregaria) ovipositor hairs by different classes of interneurones are investigated.Spikes in the sensory neurones from these hairs evoke chemically-mediated, unitary EPSPs with a short and constant latency in six identified non-giant projection interneurones with cell bodies in the terminal abdominal ganglion. Five of these interneurones receive direct inputs from the valves ipsilateral to their neuropilar branches, whereas the other receives direct inputs from valves on both sides. The sensory neurone from a single hair makes divergent connections with several interneurones and those from different hairs make convergent connections with a given interneurone. The amplitude of the EPSPs evoked depends on the position of a hair along the proximal-distal axis of the valve, with sensory neurones from more distal hairs generating larger amplitude EPSPs.Deflection of hairs also excites three of the four giant projection interneurones through polysynaptic pathways and some local interneurones in the terminal abdominal ganglion through monosynaptic connections. Branches of non-giant projection interneurones, local interneurones, but not those of the giant interneurones, overlap the axon terminals of the ovipositor hair afferents in the terminal abdominal ganglion.     

Synaptic connections of the common inhibitory motoneurone within the fifth thoracic ganglion of crayfish

The common inhibitor (CI) has been studied morphologically and electrophysiologically in the fifth thoracic ganglion of crayfish (Procambarus clarkii). It has a large soma and possesses two separate dendritic fields arising from distinct integrative segments.In vitro preparations display motor outputs ranging from tonic activity to fictive locomotion. The CI's tonic firing frequency increases as more excitors are recruited, and displays two peaks of frequency during fictive locomotion, one during stance, the other during swing.Paired intracellular recordings have been used to demonstrate the different central synaptic connections received or made by the CI. At least 27% of the proximal excitors receive monosynaptic connections from the CI corresponding to post-synaptic depolarizations of small amplitude mediated by GABA. However as they do not change the overall activities of the excitors which receive them, they may be used for local inhibition within the dendrites. Besides, electrical synapses between several proximal excitors and the CI may synchronize their activity.The CI receives synaptic connections arising from interneurones. Some are direct either by inhibitory monosynaptic connections or by electrical couplings whereas others arise through polysynaptic pathways. All these connections are functionally significant in the control of the CI firing activity and in its motor coordinations.     

Cross-connections coordinate postural motoneuron activity in the crayfish abdomen

Intracellular recordings and dye injections were used to examine mutual coupling among slow abdominal postural motoneurons in the 4th abdominal ganglion in crayfish (Procambarus clarkii). Intracellular current injection into one motoneuron altered the spike firing rate of some of its synergists. Depending on the polarity of the injected current, the premotor effect on the synergists was excitatory or inhibitory. The magnitude of the effect was intensity dependent. No dye coupling was found among the motoneurons following injection of Lucifer yellow. The morphological basis of the coupling was examined by differential filling of motoneuron pairs, one with horseradish peroxidase and the other with Lucifer yellow. The stained motoneurons were simultaneously visualized under light microscopy to determine the proximity of their differently colored dendrites. It was thus possible to locate the site of the presumed monosynaptic contacts between them. Combined physiological and morphological evidence suggests that these neurons are mutually coupled, forming part of an integrative system for abdominal posture control in crayfish.     

Afferent connections of physiologically identified neuronal complexes in the paraventricular nucleus of conscious Pekin ducks involved in regulation of salt- and water-balance

Summary The afferent connections of the paraventricular nucleus (PVN) of the domestic mallard (Pekin duck), Anas platyrhynchos, were demonstrated by means of microiontophoretic injection of horseradish peroxidase (HRP). To place the HRP injection exactly into the PVN, its location was identified prior to the injection by observing antidiuretic reactions to electrostimulations within the rostral hypothalamus of conscious, hydrated animals. Antidiuresis was induced only when electrostimulation was applied to a distinct hypothalamic area. Two different patterns of antidiuresis were observed: (i) an immediate reduction in rate of production of urine, and (ii) antidiuresis preceded by a period of increase in production of urine. Repeated stimulation of the same site with the same parameters resulted in decreasing antidiuretic effects. At the site where stimulation had elicited the most pronounced antidiuresis of either response type, HRP was injected microiontophoretically.Histological examination after 3–8 days of survival revealed delicate injection sites located exclusively in the periventricular portion of the PVN. Adjacent to the dorsal portion of the PVN retrogradely labeled tanycytes and intraependymal neurons were scattered in the ventricular wall. As demonstrated in neurohistological and electron-microscopic investigations, this ependymal region exhibits a particular arrangement of tanycytes and small neurons (10–15 m in diameter), some of which belong to the neurosecretory type.Additional HRP-labeled neuronal perikarya afferent to the PVN were demonstrated in the contralateral PVN, and on the ipsilateral side in the lateral septum, lateral hypothalamic area and locus coeruleus. Within the nuclei of the solitary tract, stained nerve cells were found ipsilateral as well as contralateral to the injection site.Several of the neurons demonstrated may be considered as candidates for the transmission of signals originating from various receptive structures relevant for the control of avian salt- and water-balance. The physiological results conform to the concept that neurons of the PVN influence urine formation by controlling the release of arginine-vasotocin (AVT). Evidence that suggests additional modes of control exerted by these neurons in salt- and water-balance is presented.Supported by grants from the Deutsche Forschungsgemeinschaft (Ko 758/1; Si 230/4-4)Portions of these results were presented on the occasion of the 54th Meeting of the Deutsche Physiologische Gesellschaft (Korf et al. 1981 a) and the 76th Meeting of the Anatomische Gesellschaft (Korf et al. 1981 b)     

Computational approaches from polymer physics to investigate chromatin folding

Microscopy and sequencing-based technologies are providing increasing insights into chromatin architecture. Nevertheless, a full comprehension of chromosome folding and its link with vital cell functions is far from accomplished at the molecular level. Recent theoretical and computational approaches are providing important support to experiments to dissect the three-dimensional structure of chromosomes and its organizational mechanisms. Here, we review, in particular, the String&Binders polymer model of chromatin that describes the textbook scenario where contacts between distal DNA sites are established by cognate binders. It has been shown to recapitulate key features of chromosome folding and to be able at predicting how phenotypes causing structural variants rewire the interactions between genes and regulators.