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     

Highly independent olfactory receptor sites for naturally occurring bile acids in the sea lamprey, Petromyzon marinus

(1) Electro-olfactogram recording was used to determine whether the olfactory epithelium of adult sea lamprey is specifically sensitive to bile acids, some of which have been hypothesized to function as pheromones. Ten bile acids were selected from 38 which had already been pre-screened for olfactory activity. These compounds were first tested on their own, then as adapting stimuli, and finally as components of mixtures (2) The lamprey-specific bile acids, petromyzonol sulfate and allocholic acid, were the most potent compounds tested. Five other bile acids were also detectable at picomolar concentrations. Petromyzonol sulfate had a distinctive dose-response curve. (3) Cross-adaptation demonstrated that sensitivity to bile acids is attributable to at least four independent classes of olfactory receptor sites and that both the nature and position of conjugating group(s) are critical to receptor specificity. Notably, petromyzonol sulfate has its own highly specific and independent receptor site. The situation for unconjugated bile acids was more complex and there appeared to be several sub-classes of receptor sites for these compounds. (4) Mixture studies largely confirmed the cross-adaptation results, describing receptor site independence for the same four sets of odorants. Mixture enhancement was also seen when expected and there was no evidence of mixture suppression. (5) Together, these data demonstrate that conspecific bile acids are discriminated by the olfactory epithelium of the sea lamprey, supporting the possibility that these compounds may function as migratory pheromones. Accepted: 23 November 1996     

Multiple excitatory receptor types on individual olfactory neurons: implications for coding of mixtures in the spiny lobster

The aim of our paper was to investigate whether single olfactory receptor neurons (ORNs) of the spiny lobster Panulirus argus functionally express more than one type of receptor, examine the consequences of this on coding of mixtures, and compare principles of odorant mixture coding by spiny lobsters with that by the channel catfish, which has been studied extensively using the same experimental and analytical procedures (Caprio et al. 1989; Kang and Caprio 1991). We examined responses of individual taurine-sensitive ORNs to binary mixtures of excitatory compounds, either competitive agonists (taurine, β-alanine, hypotaurine) or non-competitive agonists (taurine, l-glutamate, ammonium chloride, adenosine-5′-monophosphate). Responses to mixtures were compared to two indices: mixture discrimination index (MDI) and independent component index (ICI). Binary mixtures of competitive agonists had MDI values close to 1.0, as expected for competitors. Mixtures of non-competitive agonists had ICI values averaging 0.83, indicating the effects of the components are not independent. We conclude that individual olfactory cells of spiny lobsters can express more than one type of receptor mediating excitation, one of which typically has a much higher density or affinity, and that spiny lobster and catfish olfactory cells encode mixtures of two excitatory agonists using similar rules. Accepted: 20 December 1996     

Genetic evidence of a coupling role for the TraG protein family in bacterial conjugation

The ability of conjugative plasmids from six different incompatibility groups to mobilize a set of mobilizable plasmids was examined. The mobilization frequencies of plasmids RSF1010, ColE1, ColE3, and CloDF13 varied over seven orders of magnitude, depending on the helper conjugative plasmid used. Mobilization of CloDF13 was unique in that it did not require TrwB, TraG or TraD (all members of the TraG family) for mobilization by R388, RP4 or F, respectively. CloDF13 itself codes for an essential mobilization protein (MobB) which is also a TraG homolog, only requiring a source of the genes for pilus formation. Besides, CloDF13 was mobilized efficiently by all conjugative plasmids, suggesting that TraG homologs are the primary determinants of the mobilization efficiency of a plasmid, interacting differentialy with the various relaxosomes. Previous results indicated that TraG and TrwB were interchangeable for mobilization of RSF1010 and ColE1 by PIL_W (the pilus system of IncW plasmids) but TraG could not complement conjugation of trwB mutants, suggesting that additional interactions were taking place between TrwB and oriT(R388) that were not essential for mobilization. To further test this hypothesis, we analyzed the mobilization frequencies of ColE1 and RSF1010 by the P, W, and F pili in the presence of alternative TraG homologs. The results obtained indicated that the frequency of mobilization was determined both by the particular TraG-like protein used and by the pilus system. Thus, TraG-like proteins are not generally interchangeable for mobilization. Therefore we suggest that the factors that determine the frequencies of transfer of different MOB regions are the differential interactions of TrwB with pilus and relaxosome. Received: 9 September 1996 / Accepted: 17 December 1996     

Temporal selectivity for complex signals by single neurons in the torus semicircularis of Pleurodema thaul (Amphibia: Leptodactylidae)

Responses of auditory neurons in the torus semicircularis (TS) of Pleurodema thaul, a leptodactylid from Chile, to synthetic stimuli having diverse temporal patterns and to digitized advertisement calls of P. thaul and three sympatric species, were recorded to investigate their temporal response selectivities. The advertisement call of this species consists of a long sequence of sound pulses (a pulse-amplitude-modulated, or PAM, signal) having a dominant frequency of about 2000 Hz. Each of the sound pulses contains intra-pulse sinusoidal-amplitude-modulations (SAMs). Synthetic stimuli consisted of six series in which the following acoustic parameters were systematically modified, one at a time: PAM rate, pulse duration, number of pulses, and intra-pulse SAM rate. The carrier frequency of these stimuli was set at the characteristic frequency (CF) of the isolated units (n = 47). Response patterns of TS units to synthetic call variants reveal different degrees of selectivities for each of the temporal variables, with populations of neurons responding maximally to specific values found in the advertisement call of this species. These selectivities are mainly shaped by neuronal responsiveness to the overall sound energy of the stimulus and by the inability of neurons to discharge to short inter-pulse gaps. Accepted: 30 October 1996     

Effects of aerobic exercise on the torque-velocity relationship in cycling

The kinetics of the torque-velocity (T-ω) relationship after aerobic exercise was studied to assess the effect of fatigue on the contractile properties of muscle. A group of 13 subjects exercised until fatigued on a cycle ergometer, at an intensity which corresponded to 60% of their maximal aerobic power for 50 min (MAP60%); ten subjects exercised until fatigued at 80% of their maximal aerobic power for 15 min (MAP80%). Of the subjects 7 exercised at both intensities with at least a 1-week interval between sessions. Pedalling rate was set at 60 rpm. The T-ω relationship was determined from the velocity data collected during all-out sprints against a 19 N · m braking torque on the same ergometer, according to a method proposed previously. Maximal theoretical velocity (ω₀) and maximal theoretical torque (T ₀) were estimated by extrapolation of the linear T-ω relationship. Maximal power (P _max) was calculated from the values of T ₀ and ω₀ (P _max = 0.25 ω₀ T ₀). The T-ω relationships were determined before, immediately after and 5 and 10 min after the aerobic exercise. The kinetics of ω₀, T ₀ and P _max was assumed to express the effects of fatigue on the muscle contractile properties (maximal shortening velocity, maximal muscle strength and maximal power). Immediately after exercise at MAP60% a 7.8% decrease in T ₀ and 8.8% decrease in P _max was seen while the decrease in ω₀ was nonsignificant, which suggested that P _max decreased in the main because of a loss in maximal muscle strength. In contrast, MAP80% induced a 8.1% decrease in ω₀ and 12.8% decrease in P _max while the decrease in T ₀ was nonsignificant, which suggested that the main cause of the decrease in P _max was probably a slowing of maximal shortening velocity. The short recovery time of the T-ω relationship suggests that the causes of the decrease of torque and velocity are processes which recover rapidly. Accepted: 25 November 1996     

Effects of changes in atmospheric carbon dioxide on the location of hosts by the moth, Cactoblastis cactorum

Sensory organs that detect CO₂ are common in herbivorous moths and butterflies, but their function has been unclear until now. As the CO₂ gradients in the vicinity of a host plant depend on its physiological condition, CO₂ could provide a sensory cue for the suitability of the plant as a larval food source. This study investigated whether changing the atmospheric CO₂ concentration affected oviposition by Cactoblastis cactorum on its host, the cactus Opuntia stricta. On host plants exposed to rapid fluctuations in CO₂ concentration, the frequency of oviposition was reduced by a factor of 3.2 compared to the control. As the fluctuations mask the much smaller CO₂ signals generated by the plants, this suggests that those signals constitute an important component of the host identification process. On host plants exposed to a constant background of doubled CO₂, oviposition was also reduced, by a factor of 1.8. An increased background reduces host signal detectability, partially as a consequence of a general principle of sensory physiology (Weber-Fechner's law), and partially due to other factors specific to CO₂-receptor neurons. Received: 4 October 1996 / Accepted: 16 January 1997     

Effects of plant hybridization on herbivore-parasitoid interactions

We studied the effects of host plant hybridization on the survival and mortality of the leaf-mining moth Phyllonorycter salicifoliella on hybrid and parental willow plants in the field and in a common garden experiment. P. salicifoliella survival differed significantly among three willow taxa in the field in 1994 but not in the field in 1995 or in the common garden. Parasitism by eulophid wasps differed significantly among taxa in 1994 and appeared to account for the variation in their survival. In the field in 1995, host feeding predation varied significant among taxa. The theory of tritrophic interactions predicts that plant genotype can affect natural enemy impact, and this study supports this prediction. Significant variation in survival and eulophid parasitism was also found among genotypes within taxa in the field in both years and in the common garden experiment. The common garden results show that genetic differences in plants affect the herbivore-parasitoid interaction. Variation among years in the patterns of survival and causes of mortality among field plants suggest that genotype by environment interactions may be important. Received: 1 March 1996 / Accepted: 4 November 1996     

Post-dispersal predation on Pinus sylvestris seeds by Fringilla spp: ground substrate affects selection for seed color

Background matching might lower the risk of seeds being eaten by seed predators that search visually. In aviary experiments, we analyzed the selection of diff erent-colored seeds by ground-feeding finches (Fringillacoelebs and F.montifringilla) against four naturally occurring forest soil substrates. The substrates were fresh burn (black), 6-year-old burn (brown), mineral soil (pale yellow) and Pleuroziumschreberi feather moss (green). We used color-sorted seeds of Pinussylvestris, a species with a large natural variation in seed color, ranging from pale yellow to black. Although seeds were scattered on the substrates at a density of only 91 seeds m⁻², birds removed seeds effectively. Both bird species found more pale than dark seeds on the fresh burn substrate. F. montifringilla also recovered more pale than dark seeds on the old burn, and more dark than pale seeds on mineral soil. In moss, the birds found very few seeds compared to the other substrates, and there was no color discrimination. P.sylvestris is frequently regenerating after fire, suggesting that dark seeds would be favored under selection from visually searching predators. Fire-adapted conifers with serotinous cones, e.g., Pinuscontorta ssp. latifolia, which spread their seeds primarily on freshly burnt surfaces, produce uniformly black or dark brown seeds. However, regeneration of the non-serotinous P.sylvestris is often extended for several years after a fire, during which substrate color and structure change. This may have helped to maintain variation in seed color. When regeneration of a plant species occurs on a substrate of uniform color, we believe that selection by visually searching seed predators will result in the evolution of cryptic seed color. Received: 16 August 1996 / Accepted: 30 November 1996     

The effect of a thiamin derivative on exercise performance

The purpose of this study was to investigate the effect of a thiamin derivative, thiamin tetrahydrofurfuryl disulfide (TTFD), on oxygen uptake (˙VO₂), lactate accumulation and cycling performance during exercise to exhaustion. Using a randomized, double-blind, cross-over design with a 10-day washout between trials, 14 subjects ingested either 1 g · day⁻¹ of TTFD or a placebo (PL) for 4 days. On day 3, subjects performed a progressive exercise test to exhaustion on a cycle ergometer for the determination of ˙VO_2submax, ˙VO_2peak, lactate concentration ([La⁻ ]), lactate threshold (Th_La) and heart rate ( f _c). On day 4, subjects performed a maximal 2000-m time trial on a cycle ergometer. A one-way analysis of variance (ANOVA) with repeated measures was used to determine significant differences between trials. There were no significant differences detected between trials for serial measures of ˙VO_2submax, [La⁻] or f _c. Likewise, ˙VO_2peak [PL 4.06 (0.19) TTFD 4.12 (0.19) l · min⁻¹, P = 0.83], Th_La [PL 2.47 (0.17), TTFD 2.43 (0.16) l · min⁻¹, P = 0.86] and 2000-m performance time [PL 204.5 (5.5), TTFD 200.9 (4.3) s, P = 0.61] were not significantly different between trials. The results of this study suggest that thiamin derivative supplementation does not influence high-intensity exercise performance. Accepted: 19 December 1996     

Spongivory on Caribbean reefs releases corals from competition with sponges

Competition for space is an important process on tropical coral reefs. Few studies have examined the role sponges play in community structure despite the fact that many sponges are competitively superior to reef-building corals in space acquisition. Surveys conducted throughout the Florida Keys indicated that Chondrilla nucula was involved in about 30% of all coral-sponge interactions; this sponge has also been observed in 40–50% of coral-sponge interactions on other Caribbean reefs. C. nucula is also the top prey item of the Hawksbill turtle, and among the preferred prey of several spongivorous fish. I examined how predation influenced sponge competitive abilities (particularly those of C. nucula), and whether this type of indirect effect had important consequences for community dynamics in the Florida Keys. Exclusion of sponge predators (primarily angelfish) resulted in increased sponge overgrowth, with a subsequent greater loss of coral cover, compared to uncaged pairwise interactions. When caged, the corals Dichocoenia stokesii and Siderastrea sideraea lost significantly greater surface area and number of polyps to the sponge C. nucula compared to uncaged interactions. For caged interactions involving the sponge Ectyoplasia ferox, there was a trend for greater loss of  S. sideraea surface area and polyps compared to uncaged interactions. Predation had a greater affect on C. nucula than on any of the other sponges examined. Predator exclusion experiments performed with naturally occurring coral-sponge interactions demonstrated a significant decrease in total coral cover compared to uncaged controls. It is proposed that indirect effects arising from spongivory (especially consumption of  C. nucula) may have large community consequences. Species diversity on Caribbean reefs may be maintained, at least in part, by spongivores. Received: 28 January 1998 / Accepted: 22 June 1998     

Homologous recombination involving cox2 is responsible for a mutation in the CMS-specific mitochondrial locus of Petunia

We have characterized the only mutation detected so far in S-Pcf, the mitochondrial cytoplasmic male sterility (CMS)-specific locus of petunia. This locus consists of three open reading frames (ORFs): the first contains part of atp9, an intron-less cox2 pseudogene (which does not contain the original cox2 ATG) and the unidentified reading frame urf-s; the second and third ORFs correspond to the only copies of nad3 and rps12 genes in the genome, respectively. In the cell line R13-138, which was generated from a male-sterile somatic hybrid (line SH13-138), a change in the first ORF of the S-Pcf locus has been characterized: the atp9 sequence has been lost, while exon1 of the normal copy of the cox2 gene (including the original ATG sequence) and the adjacent 5′ sequence of the petunia recombination repeat, have been introduced. The data suggest that this reorganization of mtDNA is the consequence of a homologous recombination event involving part of the cox2 coding region, and that the cox2 coding region may serve as an active site for inter- or intra-mtDNA homologous recombination. The results further suggest that in line SH13-138 (or during its maintenance in tissue culture), segregation of the S-Pcf-containing mtDNA molecules has occurred, and the mutant mtDNA is now predominant in the population. Received: 9 September 1996 / Accepted: 27 January 1997     

Altitudinal patterns in host suitability for forest insects

Conspecific trees growing at high and low-elevations encounter different growing conditions and may vary in their suitability as hosts for herbivorous insects. Mountain tree populations may be more resistant to herbivory if low temperatures constrain growth more than they constrain photosynthesis, resulting in increased secondary metabolism (temperature hypothesis). Alternatively, mountain trees may be fertilized by atmospheric nitrogen deposition and become more palatable to insects (atmospheric deposition hypothesis). We evaluated these two hypotheses by comparing high- and low-elevation trees with insect bioassays and analyses of foliar nitrogen and condensed tannin. Contrary to the temperature hypothesis, high-elevation foliage had higher leaf nitrogen (six of six tree species) and allowed higher growth rates of Lymantria dispar larvae (five of six tree species). The nitrogen deposition hypothesis was broadly supported by measurements from two mountains showing that high-elevation trees tended to have higher leaf nitrogen, lower leaf tannins, and support higher insect growth performance than conspecific trees from lower elevations. The deposition hypothesis was further supported by fertilization studies showing that simulated atmospheric nitrogen deposition changed the foliar chemistry of valley trees to resemble that of high-elevation trees. Predictions that the altitudinal gradient in foliar chemistry and host suitability should be steepest on mountains receiving more deposition were largely not supported, but interpretations are complicated by lack of replication among mountains. In the northeastern United States, increased host suitability of high-elevation trees seems sufficient to influence the population dynamics and community composition of herbivores. Atmospheric nitrogen deposition offers a promising hypothesis to explain and predict some important spatial patterns in herbivory. Received: 21 September 1997 / Accepted: 12 June 1998     

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     

Wind-sensitive interneurones in the spider CNS (Cupiennius salei ): directional information processing of sensory inputs from trichobothria on the walking legs

Spiders can use air particle movements to localize moving prey. We studied the responses of 32 wind-sensitive interneurones in the hunting spider Cupiennius salei to prey stimuli. Stimulation with a tethered flying fly or with artificial air pulses activated plurisegmental interneurones that responded to changes in air movement velocity and were thus well suited to represent the highly fluctuating air stream typical of prey stimuli. In most interneurones (n = 18) the responses to the stimulation of different legs were not significantly different from each other. Different interneurones had different response characteristics and their latencies largely overlapped suggesting that there is parallel processing of the signals by populations of interneurones with different response characteristics. In two interneurones the number of spikes and the spiking pattern elicited by stimulation of each of the eight legs markedly differed depending on the leg stimulated. These neurones may play an important role in directional information processing. Stimulation of the adjacent legs from front to back or from back to front revealed two interneurones sensitive to the direction of successive stimulation of the legs. These neurones may be able to detect the motion of an air movement source in a preferred direction and thus act as nearfield motion detectors to localize a moving prey item. Accepted: 28 September 1996     

The impact of vertebrate and invertebrate predators on a stream benthic community

I assessed the impact of both vertebrate and invertebrate predators on a lotic benthic community in a 1-month-long experiment, using enclosures containing cobble/gravel bottoms, with large-mesh netting that allowed invertebrates to drift freely. Brown trout (Salmo trutta) and leeches (Erpobdella octoculata) were used as predators and four treatments were tested: a predator-free control, leeches only, trout only, and leeches and trout together. A density of 26.7 leeches/m² (20 leeches/enclosure) and 1.3 trout/m² (one trout per enclosure) was stocked into the enclosures. The total biomass of invertebrate prey was significantly lower in the trout and trout plus leech treatments than in the leech and control treatments, which were due to strong negative effects of trout on Gammarus. On the individual prey taxon level, both trout and leeches affected the abundance of Asellus , Baetis and Ephemerella, whereas the abundance of Gammarus was only affected by trout, and the abundance of Orthocladiinae and Limnephilidae was only affected by leeches. In the treatment with trout and leeches together, the abundance of Ephemerella and Baetis was higher than when trout or leeches were alone, which was probably due to predator interactions. Leeches and trout had no effects on prey immigration but did affect per capita emigration rates. Both trout and leeches indirectly increased periphyton biomass in enclosures, probably due to their strong effects on grazers. Both trout and leeches were size-selective predators, with trout selecting large prey, and leeches selecting small prey. Size-selective predation by trout and leeches affected the size structure of five commonly consumed prey taxa. Trout produced prey populations of small sizes owing to consumption of large prey as well as increased emigration out of enclosures by these large prey. Leech predation produced prey assemblages of larger size owing to consumption and increased emigration of small prey. These results suggest that in lotic habits, predatory invertebrates can be as strong interactors as vertebrate predators. Received: 23 June 1997 / Accepted: 4 May 1998     

Neural correlates of habituation of the proleg withdrawal reflex in larvae of the hawk moth, Manduca sexta

The larval proleg withdrawal reflex of the hawk moth, Manduca sexta, exhibits robust habituation. This reflex is evoked by deflecting one or more mechanosensory planta hairs on a proleg tip. We examined neural correlates of habituation in an isolated proleg preparation consisting of one proleg and its segmental ganglion. Repeated deflection of a single planta hair caused a significant decrease in the number of action potentials evoked in the proleg motor nerve (which carries the axons of proleg retractor motor neurons). Significant response decrement was seen for interstimulus intervals of 10 s, 60 s and 5 min. Response decrement failed to occur in the absence of repetitive stimulation, the decremented response recovered spontaneously following a rest, and electrical stimulation of a body wall nerve facilitated the decremented response (a neural correlate of dishabituation). Adaptation of sensory neuron responses occurred during repeated hair deflections. However, when adaptation was eliminated by direct electrical stimulation of sensory neurons, the response in the proleg motor nerve still decreased significantly. Muscle recordings indicated that the response of an identified proleg retractor motor neuron decreased significantly during habituation training. Thus, habituation of the proleg withdrawal reflex includes a central component that is apparent at the level of a single motor neuron. Accepted: 20 December 1996