Sensory cues for the gradual frequency fall responses of the gymnotiform electric fish, Rhamphichthys rostratus

The sensory cues for a less known form of frequency shifting behavior, gradual frequency falls, of electric organ discharges (EODs) in a pulse-type gymnotiform electric fish, Rhamphichthys rostratus, were identified. We found that the gradual frequency fall occurs independently of more commonly observed momentary phase shifting behavior, and is due to perturbation of sensory feedback of the fish's own EODs by EODs of neighboring fish. The following components were identified as essential features in the signal mixture of the fish's own and the neighbor's EOD pulses: (1) the neighbor's pulses must be placed within a few millisecond of the fish's own pulses, (2) the neighbor's pulses, presented singly at low frequencies (0.2–4 Hz), were sufficient, (3) the frequency of individual pulse presentation must be below 4 Hz, (4) amplitude modulation of the sensory feedback of the fish's own pulses induced by such insertions of the neighbor's pulses must contain a high frequency component: sinusoidal amplitude modulation of the fish's own EOD feedback at these low frequencies does not induce gradual frequency falls. Differential stimulation across body surfaces, which is required for the jamming avoidance response (JAR) of wave-type gymnotiform electric fish, was not necessary for this behavior. We propose a cascade of high-pass and low-pass frequency filters within the amplitude processing pathway in the central nervous system as the mechanism of the gradual frequency fall response.Abbreviations EOD electric organ discharge - f frequency of EOD or pacemaker command signal - JAR jamming avoidance response - S ₁ stimulus mimicking fish's own EOD - f ₁ frequency of S₁ - S ₂ stimulus mimicking neighbor's EOD - f ₂ frequency of S₂     

Acoustic communication underground: vocalization characteristics in subterranean social mole-rats (Cryptomys sp., Bathyergidae)

In captive adult Zambian mole-rats 14 different sounds (13 true vocalizations) have been recorded during different behavioural contexts. The sound analysis revealed that all sounds occurred in a low and middle frequency range with main energy below 10 kHz. The majority of calls contained components of 1.6–2 kHz, 0.63–0.8 kHz, and/or 5–6.3 kHz. The vocalization range thus matched well the hearing range as established in other studies. The frequency content of courtship calls in two species of Zambian Cryptomys was compared with that in naked mole-rats (Heterocephalus glaber) and blind mole-rats (Spalax ehrenbergi) as described in the literature. The frequency range of maximum sound energy is negatively correlated with the body weight and coincides with the frequencies of best hearing in the respective species. In general, the vocalization range in subterranean mammals is shifted towards low frequencies which are best propagated in underground burrows. Accepted: 16 September 1996     

Single prenatal injections of melatonin or the D1-dopamine receptor agonist SKF 38393 to pregnant hamsters sets the offsprings' circadian rhythms to phases 180° apart

Pregnant Syrian hamsters with lesions of the suprachiasmatic nucleus (SCN) received single injections of melatonin or the D1-dopamine receptor agonist, SKF 38393 on day 15 of gestation (1 day before birth). Pups were weaned on postnatal day 20 and their freerunning activity rhythms recorded for 3–4 weeks. The pups' phases on the day of weaning were significantly clustered in both of the treatment groups, but the average phases differed by approximately 180°. The results demonstrate that a single prenatal stimulus is sufficient to set the phases of the hamsters' rhythms and that the phase established depends on the stimulus. Both c-fos mRNA and Fos protein were expressed in the fetal SCN after SKF 38393 injection but neither were expressed after melatonin injection. Simulations showed that a single stimulus could produce the observed synchrony from a population of uniformally distributed phases if the phase shifts were three to four times the magnitude of the adult hamster light phase response curve (PRC). A light pulse PRC mimicked the effect of an SKF 38393 injection and a dark-pulse PRC mimicked the effects of a melatonin injection. Together these results suggest that dopamine and melatonin either are, or mimic, maternal entraining signals that represent day and night. Accepted: 12 November 1996     

Non-consumptive effects of larval Salamandra on crustacean prey: can eggs detect predators?

Predators affect prey populations not only by prey consumption but also in nonconsumptive ways including modifying prey behavior. I tested the effects of fire salamander larvae (Salamandra infraimmaculata) on populations of co-occurring crustacean species in artificial outdoor pools. I also tested whether these effects were due entirely to prey consumption by Salamandra larvae or alternatively to some nonconsumptive effect. The soil (containing crustacean eggs) added to the artificial pools was collected from a dried-out temporary pool that is inhabited by Salamandra during the early part of the hydroperiod. I randomly assigned the pools to one of three treatments: control, free Salamandra, or caged Salamandra. Free salamander larvae could roam the entire pool and prey upon crustaceans. Caged salamander larvae were placed within a cage with having 250-μm mesh windows. They could not prey upon the crustaceans but could, for example, influence them by chemical cues. Densities of the three dominant crustacean species (Arctodiaptomus similis, Ceriodaphnia quadrangula and Cyzicus sp.) were drastically reduced in both salamander treatments compared to the control. Crustacean densities, however, were not significantly different between the two salamander treatments. One plausible explanation is that crustacean eggs can detect the presence of this predator via chemical cues and delay hatching. Received: 4 March 1996 / Accepted, 23 October 1996     

Functional dissection of a cell-division inhibitor, SulA, of Escherichia coli and its negative regulation by Lon

SulA is induced in Escherichia coli by the SOS response and inhibits cell division through interaction with FtsZ. To determine which region of SulA is essential for the inhibition of cell division, we constructed a series of N-terminal and C-terminal deletions of SulA and a series of alanine substitution mutants. Arginine at position 62, leucine at 67, tryptophan at 77 and lysine at 87, in the central region of SulA, were all essential for the inhibitory activity. Residues 3–27 and the C-terminal 21 residues were dispensable for the activity. The mutant protein lacking N-terminal residues 3–47 was inactive, as was that lacking the C-terminal 34 residues. C-terminal deletions of 8 and 21 residues increased the growth-inhibiting activity in lon ⁺ cells, but not in lon ⁻ cells. The wild-type and mutant SulA proteins were isolated in a form fused to E. coli maltose-binding protein, and tested in vitro for sensitivity to Lon protease. Lon degraded wild-type SulA and a deletion mutant lacking the N-terminal 93 amino acids, but did not degrade the derivative lacking 21 residues at the C-terminus. Futhermore, the wild-type SulA and the N-terminal deletion mutant formed a stable complex with Lon, while the C-terminal deletion did not. MBP fused to the C-terminal 20 residues of SulA formed a stable complex with, but was not degraded by Lon. When LacZ protein was fused at its C-terminus to 8 or 20 amino acid residues from the C-terminal region of SulA the protein was stable in lon ⁺ cells. These results indicate that the C-terminal 20 residues of SulA permit recognition by, and complex formation with, Lon, and are necessary, but not sufficient, for degradation by Lon. Received: 8 October 1996 / Accepted: 27 November 1996     

Changes in selected aspects of immune function in the leopard frog, Rana pipiens, associated with exposure to cold

The effect of exposure to low temperatures (5 °C) on lymphocyte proliferation, leukocyte populations, and serum complement levels was examined in the northern leopard frog, Rana pipiens. Proliferation of T lymphocytes in response to phytohemagglutinin stimulation was significantly decreased in frogs kept for 2, 3, and 5 months at 5 °C compared to that of animals kept at 22 °C. A significant increase in the average percentage of neutrophils and a decrease in the mean percentage of eosinophils was observed in the blood of frogs held for 5 months in the cold compared to animals held at 22 °C for the same length of time. Mean serum complement activity after 1 month at 5 °C was significantly reduced in comparison to animals held at 22 °C and was not detectable after 5 months in the cold. Recovery of complement levels at room temperature (22 °C) was also examined after cold exposure. Complement levels were significantly higher than controls (at 22 °C) in frogs returned to 22 °C for 7 and 14 days after 5 months in the cold. After frogs were held at 5 °C for 1 month, serum complement levels increased significantly within 2 days after returning to 22 °C and continued to rise 5 and 9 days after warming. Injections with Aeromonas hydrophila following a 5-week exposure to 5 °C failed to cause death or observable symptoms of disease in frogs that were returned to 22 °C. Accepted: 20 November 1996     

Wind-induced mechanical stimulation increases pest resistance in common bean

The activation of the phenylpropanoid pathway in plants by environmental stimuli is one of the most universal biochemical stress responses known. Induction of enzymes such as phenylalanine ammonia-lyase and peroxidase and the accumulation of such phenolics as lignin can occur in response to insect and pathogen attack, exposure to oxidizing pollutants, and mechanical stimulation, and are thought to function in the resistance of plants to damage by these stresses. I investigated whether induction of components of this generalized stress response by wind-induced mechanical stimulation could influence the resistance to pests of common bean. In greenhouse studies, exposure of 7- to 10-day-old bean seedlings to daily periods of fan-produced wind led to increased activities of peroxidase and cinnamyl alcohol-dehydrogenase and enhanced the accumulation of lignin in primary leaves of these plants. Egg production and population growth of two-spotted spider mites were reduced when offered leaves of mechanically-stimulated plants in leaf-disk and whole-plant bioassays. Infection by anthracnose after inoculation in a detached-leaf bioassay was also reduced in leaves of mechanically-stimulated plants. The consistent positive association between the enhanced activity of the lignin branch of the phenylpropanoid pathway and enhanced resistance to pests found in leaves of mechanically-stimulated plants illustrates one way in which exposure of plants to environmental stimuli that activate a generalized stress response (e.g., wind) can influence the interactions of those plants with other environmental stimuli (e.g., pests). Received: 1 October 1996 / Accepted: 23 January 1997     

Modulation of Drosophila heartbeat by neurotransmitters

The heart of Drosophila melanogaster is a simple muscular tube with a posterior pulsatile portion and a thoracic-cranial vessel. The pacemaker, located caudally, is myogenic. Its rate of firing is modulated by neurotransmitters. Serotonin, octopamine, norepineph-rine, dopamine, and acetylcholine accelerate the heart, in that order of potency. Dihydroxyphenylalanine, γ-aminobutyric acid, glutamate, and glycine have no effect. Generally, the regularity of the heartbeat is not adversely affected by treatment with any of these neurotransmitters. We show here that amnesiac, a neurological mutation, and Dihydroxyphenylalanine decarboxylase ^{temperature sensitive}, a mutation that interferes with synthesis of dopamine, norepinephrine, and serotonin, result in slower heart rate and reduced regularity across a normal range of temperatures for these flies. Dopamine-N-acetyltransferase, which is on the catabolic route to dopamine, serotonin, and octopamine, has no effect. hypoactiveC reduces the rate of the heart, but its mechanism of action is unknown. Accepted: 5 August 1996     

Light response characteristics of a morphologically diverse group of southern hemisphere conifers as measured by chlorophyll fluorescence

Unlike northern hemisphere conifer families, the southern family, Podocarpaceae, produces a great variety of foliage forms ranging from functionally broad-, to needle-leaved. The production of broad photosynthetic surfaces in podocarps has been linked qualitatively to low-light-environments, and we undertook to assess the validity of this assumption by measuring the light response of a morphologically diverse group of podocarps. The light response, as apparent photochemical electron transport rate (ETR), was measured by modulated fluorescence in ten species of this family and six associated species (including five Cupressaceae and one functionally needle-leaved angiosperm) all grown under identical glasshouse conditions. In all species, ETR was found to increase as light intensity increased, reaching a peak value (ETR_max) at saturating quantum flux (PPFD_sat), and decreasing thereafter. ETR_max ranged from 217 μmol electrons · m⁻² · s⁻¹ at a PPFD_sat of 1725 μmol photons · m⁻² · s⁻¹ in Actinostrobus acuminatus to an ETR of 60 μmol electrons · m⁻² · s⁻¹ at a PPFD_sat of 745 μmol electrons · m⁻² · s⁻¹ in Podocarpus dispermis. Good correlations were observed between ETR_max and both PPFD_sat and maximum assimilation rate measured by gas-exchange analysis. The effective quantum yield at light saturation remained constant in all species with an average value of 0.278 ± 0.0035 determined for all 16 species. Differences in the shapes of light response curves were related to differences in the response of non-photochemical quenching (q _n), with q _n saturating faster in species with low PPFD_sat. Amongst the species of Podocarpaceae, the log of average shoot width was well correlated with PPFD_sat, wider leaves saturating at lower light intensities. This suggests that broadly flattened shoots in the Podocarpaceae are an adaptation to low light intensity. Received: 15 April 1996 / Accepted: 30 September 1996     

Recognition of sex in the acoustic communication of the grasshopper Chorthippus biguttulus (Orthoptera, Acrididae)

Many gomphocerine grasshoppers communicate acoustically: a male's calling song is answered by a female which is approached phonotactically by the male. Signals and recognition mechanisms were investigated in Chorthippus biguttulus with regard to the cues which allow sex discrimination. (1) The stridulatory files on the hindfemur of both sexes are homologous in that they are derived from the same row of bristles, but convergent with respect to the “pegs”. In males the pegs are derived from the bristles, and in females from the wall of the bristle's cup. (2) Male and female songs are generated by similar, probably homologous motor programs, but differ in the duration, intensity, “gappyness” of syllables, risetime of pulses, and the frequency spectra. The hindleg co-ordination during stridulation and the resulting temporal song patterns are less variable in males than in females. (3) For both sexes, recognition of a mate's signal depends on species-specific syllable structure. For males it is essential that the female syllables consist of distinct short pulses, whereas females reject “gappy” syllables. Males strongly prefer “ramped” pulses, females respond to syllables irrespective of steeply or slowly rising ramps. Males react only to the low-frequency component, whereas females prefer spectra containing both, low and high frequency components. Accepted: 20 November 1996     

Gene cloning, sequencing and enzymatic properties of glutamate synthase from the hyperthermophilic archaeon Pyrococcus sp. KOD1

The gltA gene encoding a glutamate synthase (GOGAT) from the hyperthermophilic archaeon Pyrococcus sp. KOD1 was cloned as a 6.6 kb HindIII-BamHI fragment. Sequence analysis indicates that gltA encodes a 481- amino acid protein (53 269 Da). The deduced amino acid sequence of KOD1-GltA includes conserved regions that are found in the small subunits of bacterial GOGAT: two cysteine clusters, an adenylate-binding consensus sequence and an FAD-binding consensus sequence. However, no sequences homologous to the large subunit of bacterial GOGAT were found in the upstream or downstream regions. In order to examine whether GltA alone can act as a functional GOGAT, GltA was overexpressed in Escherichia coli BL21 (DE3) cells using an expression plasmid. GltA was purified to homogeneity and shown to be functional as a homotetramer of approximately 205 kDa, which is equivalent to the molecular weight of the native GOGAT from KOD1, thus indicating that KOD1-GOGAT is the smallest known active GOGAT. GltA is capable of both glutamine-dependent and ammonia-dependent synthesis of glutamate. Synthesis of glutamate by KOD1-GltA required NADPH, indicating that this enzyme is an NADPH-GOGAT (EC 1.4.1.13). The optimum pH for both activities was 6.5. However, GltA exhibited different optimum temperatures for activity depending on the reaction assayed (glutamine-dependent reaction, 80° C; ammonia-dependent reaction, 90° C). Received: 30 October 1996 / Accepted: 13 January 1997     

Effect of the removal of cleaner fish on the abundance and species composition of reef fish

The ecological significance of cleaner fish on coral reefs was investigated. I removed all cleaner fish, Labroides dimidiatus, from eight small reefs, measured the subsequent effect on the abundance and species composition of all reef fish after 3 and 6 months, and compared it with eight control reefs with cleaner fish. The removal of cleaner fish had no detectable effect on the total abundance of fish on reefs and the total number of fish species at both times. Multivariate analysis by non-metric multidimensional scaling and ANOSIM pairwise tests based on 191 fish species revealed no effect of cleaners on the community structure of fish. Similar results were obtained using principal components analysis on subsets of the data using the 33 most common fish species and the 15 most abundant species (≥5 individuals per reef ) with both log10 (x + 1) transformed data and with fish numbers standardized for abundance. This study demonstrates that the removal of cleaner fish for 6 months did not result in fish suffering increased mortality nor in fish leaving reefs to seek cleaning elsewhere. Received: 28 October 1996 / Accepted: 7 February 1997     

Oxygen consumption in the foraging honeybee depends on the reward rate at the food source

Oxygen consumption of the honeybee Apis mellifera ligustica was measured as a function of the flow rate supply of sucrose solution at an automatic feeder located inside a respirometric chamber. Trained bees freely entered the respirometric chamber and collected the sucrose solution supplied. The mean value of the O₂ consumption rate per visit increased with the sucrose flow rate, and for a given flow rate, with increasing locomotor activity. However, when no locomotor activity was displayed, O₂ consumption also increased with increasing nectar flow rate. Crop load attained at the end of the visit showed a positive relationship with the nectar flow rate; however, for a given flow rate, O₂ consumption showed either no correlation or a negative one with the final crop load attained. It is concluded that the energy expenditure of the foraging bee is controlled by a motivational drive whose intensity depends on the reward rate at the food source. Accepted: 30 July 1996     

Multisegmental motor activity in the segmentally restricted gin trap behavior in Manduca sexta pupae

Stimulation of sensory neurons innervating hairs in the gin traps on the abdomen of Manduca sexta pupae evokes a rapid bending of the abdomen that is restricted to one or more of the three articulating posterior segments. However, electrical stimulation of the gin trap sensory nerve in an isolated abdominal nerve cord evokes characteristic motor neuron activity in every abdominal segment. To determine if the segmentally distributed motor activity also occurred in intact animals and how it contributed to the segmentally restricted reflex movement, mechanical stimulation of the sensory hairs in intact animals was used to evoke reflex responses that were recorded as electromyograms synchronized with video recordings of the behavior. Motor activity was monitored during movements to determine if there was activity in many segments when the movement was restricted to one segment. Coordinated muscle activity was evoked throughout the abdomen in response to stimulation of any of the three gin traps, even when movement was restricted to one segment. Differences in the timing of ipsilateral and contralateral motor activity among segments allowed the closing of gin traps to be segmentally restricted. These findings suggest that the neural circuit underlying the gin trap reflex is distributed throughout the abdominal nerve cord. This network generates a complex, yet coordinated, motor pattern with muscular activity in many abdominal segments that produces a localized bending reflex. Accepted: 10 January 1997     

c-fos expression in the putative avian suprachiasmatic nucleus

c-fos induction was investigated as a potential component in the avian photic entrainment pathway and as a possible means of locating the central pacemaker in birds. In both quail (Coturnix coturnix japonica) and starlings (Sturnus vulgaris) exposure to 1 h of light induced Fos-lir in the visual suprachiasmatic nucleus but not in the medial suprachiasmatic nucleus. However, the degree of c-fos induction in the visual suprachiasmatic nucleus was similar at different circadian times despite the fact that the light pulses caused differential phase shifts in the locomotor rhythm. For golden hamsters the same experiment resulted in significantly different levels of Fos-lir in the suprachiasmatic nucleus, as well as different phase shifts. Starlings and hamsters were also entrained to T-cycles that caused a large daily phase shift (T = 21.5 h in starlings, T = 22.67 hours in hamsters), or no daily phase shift (T = free running period). No difference in the induced levels of Fos-lir in the visual suprachiasmatic nucleus region was observed between the two groups of starlings, but in hamsters there were significantly different levels of Fos-lir in the suprachiasmatic nucleus between the two groups. Accepted: 15 November 1996     

Evidence for NO-dependent vasodilation in the trout (Oncorhynchus mykiss ) coronary system

The effects of l-arginine, and its analogues N ^ω-nitro-l-arginine methyl ester and N ^ω-nitro-l-arginine on vascular resistance were investigated in the intact coronary system of an isolated non-working trout heart preparation. l-Arginine, at 10^–8 mol · l^–1induced a slight vasodilatory effect (max 10%). N ^ω-nitro-l-arginine methyl ester and N ^ω-Nitro-l-arginine in the range 10^–8–10^–4 mol · l^–1 caused dose-dependent increases in coronary resistance. The vasodilatory action of l-arginine was abolished when the preparation was pretreated with 10^–4 mol · l^–1 N ^ω-nitro-l-arginine or N ^ω-nitro-l-arginine methyl ester. Nitroprusside alone at 1 mmol · l^–1 induced a maximum vasodilation (30%) of the coronary system. Methylene blue a known inhibitor of guanylate cyclase, induced a strong vasoconstriction (already significant at 10^–5 mol · l^–1) and was able to overcome the vasodilative effect of nitroprusside. The endothelial nitric oxide agonists acetylcholine and serotonin, established in mammalian vessels, also mediate vasodilation in trout coronary system. In 50% of preparations, acetylcholine induced a biphasic response with vasodilation at low concentration (max 15% at 10^–8 mol · l^–1). Serotonin displayed a dose-response vasodilation in the range 10^–8–10^–4 mol · l^–1 (max 20%). These vasodilative effects were reduced or abolished by 10^–4 mol · l^–1 l-NA. These data support the existence of NO-mediated vasodilation mechanisms in the trout coronary system. Accepted: 1 July 1996     

Cold hardiness in Entomobrya nivalis (Collembola, Entomobryidae): annual cycle of polyols and antifreeze proteins, and antifreeze triggering by temperature and photoperiod

In the Swiss Prealps Entomobrya nivalis hibernates in an inactive state, hidden under bark flakes on spruce. For freeze avoidance it relies on thermal hysteresis proteins (THPs) and polyols (mainly ribitol, with small amounts arabitol and threitol). Polyols are present only during the inactive state, THPs additionally protect during the transition phase in spring and autumn, when animals are still active but frosts may occur. Peak values were recorded in February/March for THPs (3.5 °C hysteresis between melting and freezing point) and for polyols (26 μg mg⁻¹ FW; hemolymph osmolality 680 mosmol l⁻¹). E. nivalis is able to control its hemolymph osmolality independently of body water content. Mean osmolality in summer was 350– 440 mosmol l⁻¹, in winter it was elevated to 650 mosmol l⁻¹, due to a synthesis mainly of ribitol. Body water content varied between 1.8 and 3.3 mg H₂O mg⁻¹ DW, depending on humidity conditions. Experiments on triggering of antifreeze synthesis showed the action of temperature and photoperiod as cues, but there was also evidence for an endogenous rhythm. No clear correlation between antifreeze concentration and supercooling ability could be established, suggesting that gut content or other parameters also play an inportant role. Accepted: 18 November 1995