Neuronal Network Models of Phase Separation Between Limb CPGs of Digging Sand Crabs

Ordinary differential equations are used to model a peculiar motor behaviour in the anomuran decapod crustacean Emerita analoga. Little is known about the neural circuitry that permits E. analoga to control the phase relationships between movements of the fourth legs and pair of uropods as it digs into sand, so mathematical models might aid in identifying features of the neural structures involved. The geometric arrangement of segmental ganglia controlling the movements of each limb provides an intuitive framework for modelling. Specifically, due to the rhythmic nature of movement, the network controlling the fourth legs and uropods is viewed as three coupled identical oscillators, one dedicated to the control of each fourth leg and one for the pair of uropods, which always move in bilateral synchrony. Systems of Morris–Lecar equations describe the voltage and ion channel dynamics of neurons. Each central pattern generator for a limb is first modelled as a single neuron and then, more realistically as a multi-neuron oscillator. This process results in high-dimensional systems of equations that are difficult to analyse. In either case, reduction to phase equations by averaging yields a two-dimensional system of equations where variables describe only each oscillator’s phase along its limit cycle. The behaviour observed in the reduced equations approximates that of the original system. Results suggest that the phase response function in the two dimensional system, together with minimal input from asymmetric bilateral coupling parameters, is sufficient to account for the observed behaviour.     

Predator-induced morphological changes in an amphibian: predation by dragonflies affects tadpole shape and color

Predator-induced defenses are well studied in plants and invertebrate animals, but have only recently been recognized in vertebrates. Gray treefrog (Hylachrysoscelis) tadpoles reared with predatory dragonfly (Aeshnaumbrosa) larvae differ in shape and color from tadpoles reared in the absence of dragonflies. By exposing tadpoles to tail damage and the non-lethal presence of starved and fed dragonflies, we determined that these phenotypic differences are induced by non-contact cues present when dragonflies prey on Hyla. The induced changes in shape are in the direction that tends to increase swimming speed; thus, the induced morphology may help tadpoles evade predators. Altering morphology in response to predators is likely to influence interactions with other species in the community as well. Received: 17 April 1996 / Accepted: 18 September 1996     

Muscle strength and soft tissue composition as measured by dual energy x-ray absorptiometry in women aged 18–87 years

Dual energy x-ray absorptiometry (DEXA) offers the possibility of assessing regional soft tissue composition, i.e. lean mass (LM) and fat mass : LM may be considered a measure of muscle mass. We examined age-related differences in LM, percentage fat (%fat) and muscle strength in 100 healthy non-athletic women aged 18–87 years. Relationships between muscle strength and leg LM in 20 elite female weight lifters and in 18 inactive women with previous hip fractures were also studied. The LM and %fat of the whole body, trunk, arms and legs were derived from a whole body DEXA scan. Isokinetic knee extensor strength (KES) and flexor strength (KFS) at 30° · s^–1 were assessed using an isokinetic dynamometer. The women aged 71–87 years had 35% lower KES and KFS than the women aged 18–40 years (P < 0.0001). Differences in LM were less pronounced. The LM of the legs, for instance, was 15% lower in the old than in the young women (P < 0.0001). In a multiple regression analysis with age, body mass, height and leg LM or KES as independent variables and KES or leg LM as the dependent variable, age was the most important predictor of KES (r _partial = −0.74, P < 0.0001). The same applied to KFS. Body mass, not age, was the most important predictor of leg LM (r _partial = 0.65, P < 0.0001) and of LM at all other measurement sites. The LM measured at different regions decreased equally with increasing age. The KES:leg LM ratio was negatively correlated with age (r = −0.70, P < 0.0001). The weight lifters had significantly higher KES:leg LM ratios than age-matched controls (+12%, P < 0.0001) and vice versa for the women with previous hip fractures (–36%, P < 0.0001). In conclusion, from our study it would seem that in healthy nonathletic women, age is a more important determinant of muscle strength than is LM as measured by DEXA. Muscle strengthening exercises and inactivity seem to have a considerably stronger influence on muscle strength than on LM. Accepted: 27 August 1996     

Heart rate and plasma lactate responses during submerged swimming and trained diving in California sea lions, Zalophus californianus

California sea lions, Zalophus californianus, were trained to elicit maximum voluntary breath holds during stationary underwater targeting, submerged swimming, and trained diving. Lowest heart rate during rest periods was 57 bpm. The heart rate profiles in all three protocols were dominated by a bradycardia of 20–50 bpm, and demonstrated that otariid diving heart rates were at or below resting heart rate. Venous blood samples were collected after submerged swimming periods of 1–3 min. Plasma lactate began to increase only after 2.3-min submersions. This rise in lactate and our inability to train sea lions to dive or swim submerged for periods longer than 3 min lead us to conclude that an aerobic limit had been reached. Due to the similarity of heart rate responses and swimming velocities recorded during submerged swimming and trained diving, this 2.3-min limit should approximate the aerobic dive limit in these 40-kg sea lions. Total body O₂ stores, based on measurements of blood and muscle O₂ stores in these animals, and prior lung O₂ store analyses, were 37–43 ml O₂ kg⁻¹. The aerobic dive limit, calculated with these O₂ stores and prior measurements of at-sea metabolic rates of sea lions, is 1.8–2 min, similar to that measured by the change in post-submersion lactate concentration. Accepted: 7 July 1996     

Effect of electrical stimulation of leg muscles on physiological responses during arm-cranking exercise in healthy men

The purpose of this study was to investigate the hypothesis that changes in physiological responses during arm-cranking exercise using electrical stimulation of the leg muscles (ACE-ES) compared to arm-cranking exercise alone (ACE) in able-bodied subjects (ABS) are based on an increase in active muscle mass rather than the enhancing effect of the leg muscle pump. In ABS the sympathetic nervous system induced vasoconstriction and activity of the leg muscle pump are intact, therefore, a normal redistribution of blood takes place during exercise. Consequently, ES should have no additional effect on the redistribution of blood in these ABS during exercise and, thus, changes in physiological responses will be based on an increase in active muscle mass. A group of 11 ABS performed three maximal arm-cranking tests. In the first test peak power output (PO _peak) was determined. The other tests were both submaximal and maximal ACE, once with ACE-ES and once with ACE. The PO _peak was not significantly different between ACE-ES and ACE. Oxygen uptake (O₂) increased significantly during ACE-ES compared to ACE. Cardiac output (), stroke volume (SV), heart rate and ventilation were not significantly different during ACE-ES compared to ACE. Respiratory exchange ratios were significantly lower during ACE-ES compared to ACE at 60% PO _peak and at maximal exercise. In conclusion, ACE-ES caused significant increases in O₂ with a lack of elevation in and SV during submaximal and maximal exercise in ABS. The results of this study suggest that changes in physiological responses during ACE-ES are based on an increase in the active muscle mass rather than stimulation of the leg muscle pump. Accepted: 6 August 1996     

Phlorotannins versus other factors affecting epiphyte abundance on the kelp Ecklonia radiata

We examined factors affecting the abundance and distribution of epiphytes (fouling) on the sublittoral kelp Ecklonia radiata. We first assessed the importance of phlorotannins as chemical defences against epiphytes by (a) correlating epiphyte loads on different parts of the thallus with the phlorotannin content of those tissues, and (b) experimentally testing the effects of variation in phlorotannin concentration against the settlement and growth of gametes of Ulva lactuca, a common epiphyte in the system. Tissue phlorotannin content was, at best, only weakly related to epiphyte loads, with r ² values typically <0.10. Inhibition of Ulva gametes only occurred at concentrations >10 mg l⁻¹, which is 5 orders of magnitude greater than levels of phlorotannins in the water column around beds of E. radiata, and 1–3 orders of magnitude greater than estimated levels in the boundary layer at the surface of the plant. We concluded that phlorotannins have a negligible impact on patterns of epiphytism on E. radiata, and proceeded to investigate other factors influencing the distribution and abundance of epiphytes. In our samples the relative age of different parts of the thallus was strongly correlated with epiphyte abundance, with epiphyte densities greatest on the oldest tissue and least on the youngest. Distal parts of the thalli also had greater epiphyte loads than basal parts. Field experiments in which kelp tissue was suspended at two heights in an E. radiata forest for varying lengths of time confirmed the importance of the length of time that the tissue was in the water, and its height in the water column, to the development of an epiphyte community. Comparison of epiphyte loads on tissue from primary (smooth) and secondary (rough) laminae in these experiments indicated that surface rugosity also affected fouling. Macroherbivores were rare on E. radiata, and abundances of mesofauna and epiphytes were positively related, suggesting that grazers were not important determinants of patterns of epiphyte abundance. Although phlorotannins have been previously suggested to play an important role as defences against epiphytes, we suggest that water-soluble compounds such as phlorotannins are less likely to be effective defences against epiphytes than non-polar metabolites, which can adhere to the surface of the producing organism.     

Sequential functioning of Sym-13 and Sym-31 , two genes affecting symbiosome development in root nodules of pea ( Pisum sativum L.)

Two Fix⁻ mutants of pea (Pisum sativum L.) which are unable to fix molecular nitrogen, E135f (sym-13) and Sprint-2Fix⁻ (sym-31), were crossed to create the doubly homozygous recessive line, named RBT (sym-13, sym-31). The ultrastructural organization of nodules of the RBT line was compared with that of each of the two parental mutant lines and with the original wild-type genotypes of the cultivars Sparkle and Sprint-2. It was shown that the RBT line is similar to the mutant line Sprint-2Fix⁻ in having abnormal symbiosome composition and bacteroids with relatively undifferentiated morphology. Because the phenotypic manifestation of the sym-31 mutant allele suppresses the phenotypic manifestation of the sym-13 mutant allele, it is concluded that the function of the gene Sym-31 (which is mutated in the Sprint-2Fix⁻ line) is necessary at an earlier stage of symbiosome development than the gene Sym-13 (which is mutant in the E135f line). Received: 28 October 1996 / Accepted: 22 January 1997     

Experimental manipulation of tail ornament size affects the hematocrit of male barn swallows (Hirundo rustica)

Ornamental tail feathers of male barn swallows (Hirundo rustica) confer an advantage in sexual selection because long-tailed males are preferred by females. However, the size of tail ornaments exceeds the natural selection optimum and males are predicted to pay an energetic cost for flying, directly related to tail length. An increase in hematocrit is an adaptive response to enhance oxygen uptake, for example during periods of intense locomotory activity. In this study, we analyzed the effect of experimental manipulation of tail length on the hematocrit of male barn swallows from an Italian and a Spanish population. We predicted that the natural decrease in hematocrit during the breeding season would be reduced by experimental elongation and enhanced by experimental shortening of tail ornaments. The results showed that the decrease in hematocrit was significantly different among tail treatments, and tail-elongated males had the smallest hematocrit reduction. In Italy, the hematocrit of tail-elongated males did not change after tail manipulation, while that of two control groups and tail-shortened males decreased. A comparatively high hematocrit in males with experimentally enlarged tail ornaments may be a response to increased energetic requirements and, hence, to oxygen demands for flying imposed by their tail morphology. Received: 22 June 1996 / Accepted: 23 October 1996     

The maxillary palp of Drosophila : ultrastructure and physiology depends on the lozenge gene

The ultrastructure and physiology of the maxillary palp of Drosophila melanogaster have been studied in wild-type and lozenge mutants. Olfactory physiology in the maxillary palp is shown to depend upon the lozenge(lz) gene. Reduced response amplitudes were recorded for all odorants tested, and the physiological defect was shown to map to the lz locus. The structure of the maxillary palp sensilla is described by scanning electron microscopy (SEM) at high magnification, initially in the wild-type. A linear arrangement of pores, connected by furrows, was found in one class of sensilla, the basiconic sensilla. In the lz ³ mutant, morphological alterations in the basiconic sensilla and duplications of sensilla are documented by SEM. The correlation of structural abnormalities in the lz sensilla and physiological abnormalities in odorant response are consistent with an olfactory role for the basiconic sensilla of the maxillary palp. Accepted: 10 September 1996     

Molecular cloning and characterization of Drosophila genes encoding small GTPases of the rab and rho families

We have isolated eight genes from Drosophila, small GTPases. They can be classified into three rab family genes (Drab2, Drab5, Drab11) and five rho family genes (Drac1a, Drac1b, Drac3, Dcdc42, DrhoA). While Drac3 is a novel type of rac gene, others are homologues of known mammalian genes for small GTPases. Northern blot analyses showed that all the genes are expressed throughout all developmental stages from embryo to adult. In situ hybridization to embryos revealed that Drab2, Drac1b, and Drac3 are highly expressed in the nervous system, in the trunk mesoderm, and in the cephalic mesoderm, respectively. Since hemocytes are derived from the cephalic mesoderm, we carried out double stainings using a hemocyte marker – anti-peroxidasin antibody – and Drac3 in situ hybridization. We found that Drac3 is expressed in hemocyte precursor cells. In the Drac3 deficiency embryos, the hemocyte precursor cells start to differentiate normally, but never develop into mature hemocytes, indicating that Drac3 is essential for their maturation. The DrhoA and Dcdc42 genes complemented S. cerevisiae rho1 and cdc42 mutations in the same manner as human rhoA and CDC42, respectively. These results suggest functional similarity between Drosophila and mammalian small GTPase genes. Received: 7 May 1996 / Accepted: 6 January 1997     

An endogenous motor program for sand crab uropods

A stereotyped pattern of spontaneous, rhythmic bursting in motoneurons of three prinicipal uropod muscles in the sand crab Emerita analoga has been recorded from a deafferented chain of the foru most-posterior abdominal ganglia. This endogenous motor program resembles the electromyogram pattern recorded from return-stroke and power-stroke muscles in swimming crabs in that (1) latencies of power-stroke bursts and burst periods are positively correlated with each other and (2) durations of power-stroke bursts are brief and mearly invarient. The endogenous program differs from the electromyogram pattern in having longer periods and return-stroke bursts which are brief and sporadic. The neural oscillator underlying the endogenous motor program, therefore, appears to drive the power stroke. Circumstantial evidence suggests that it may also inhibit return-stroke motoneurons concurrently with excitation of the power-stroke excitor.     

The effect of mermithid parasitism on predation of nymphal Baetis bicaudatus (Ephemeroptera) by invertebrates

We investigated how infection by the mermithid nematode Gasteromermis sp. affected predation on its nymphal mayfly host, Baetisbicaudatus, by two invertebrate predators – the stonefly nymphs of Kogotusmodestus and the caddisfly larvae of Rhyacophilahyalinata. Predation trials and behavioral observations were conducted in stream-side, flow-through experimental chambers. When parasitized and unparasitized prey were offered in equal numbers, K. modestus consumed significantly more parasitized than unparasitized nymphs. R. hyalinata consumed equal numbers of both prey types. Behavioral observations of foraging K.␣modestus on parasitized and unparasitized prey suggested that the increased consumption of parasitized nymphs was due to differences in the behavior of infected mayflies in response to the predator. Specifically, parasitized nymphs drifted less often to escape an approaching predator (non-contact encounters) compared to unparasitized nymphs, which increased the number of contact encounters and attacks that occurred between K.␣modestus and parasitized prey. Because all hosts are castrated, these behavioral alterations affect only the fitness of the parasite, which is killed along with its host by invertebrate predation. We present a number of hypotheses to explain why the parasite causes increased predation on its host. These include the large size of the parasite affecting the sensory abilities of the host, the larger energetic costs of escape behavior for parasitized individuals, and natural selection from fish predation against drifting behavior by parasitized individuals. Received: 27 May 1996 / Accepted: 30 September 1996     

Organisation of the bph gene cluster of transposon Tn 4371, encoding enzymes for the degradation of biphenyl and 4-chlorobiphenyl compounds

  Tn4371 is a 55 kb transposon which encodes enzymes for the degradation of biphenyl and 4-chlorobiphenyl compounds into benzoate and 4-chlorobenzo-ate derivatives. We constructed a cosmid library of Tn4371 DNA. The bph genes involved in biphenyl/4-chlorobiphenyl degradation were found to be clustered in the middle of the transposon. Sequencing revealed an organisation of the bph genes similar to that previously found in Pseudomonas sp. KKS102, i.e. the bphEGF genes are located upstream of bphA1A2A3 and bphA4 is separated from bphA1A2A3 by bphBCD. Consensus sequences for σ54-associated RNA polymerase were found upstream of bphA1 and bphEGF. Plasmid RP4::Tn4371 was transferred into a mutant of Alcaligenes eutrophus H16 lacking σ54. In contrast to wild-type H16 exconjugants, the σ54 mutant exconjugants could not grow on biphenyl, indicating the dependence of Tn4371bph gene expression on σ54. The Tn4371-encoded bph pathway was activated when biphenyl and various biphenyl-like compounds were present in the growth medium. Preliminary observations indicate the presence of a region outside the catabolic genes downstream of bphA4 which is involved in mediating at least the basal expression of BphC. Received: 13 May 1996 / Accepted: 16 September 1996     

Insulin and glucagon immunoreactivity during high-intensity exercise under opiate blockade

Eight fit men [maximum oxygen consumption (O_2max) 64.6 (1.9) ml · kg⁻¹ · min⁻¹, aged 28.3 (1.7) years (SE in parentheses) were studied during two treadmill exercise trials to determine the effect of endogenous opioids on insulin and glucagon immunoreactivity during intense exercise (80% O_2max). A double-blind experimental design was used with subjects undertaking the two exercise trials in counterbalanced order. Exercise trials were 20 min in duration and were conducted 7 days apart. One exercise trial was undertaken following administration of naloxone (N; 1.2 mg; 3 ml) and the other after receiving a placebo (P; 0.9% NaCl saline; 3 ml). Prior to each experimental trial a flexible catheter was placed into an antecubital vein and baseline blood samples were collected. Immediately after, each subject received either a N or P bolus injection. Blood samples were also collected after 20 min of continuous exercise (running). Glucagon was higher (P < 0.05), while insulin was lower (P < 0.05), during exercise compared with pre-exercise values in both trials. However, glucagon was higher (P < 0.05) in the P than in the N exercise trial [141.4 (8.3) ng · l⁻¹ vs 127.2 (7.6) ng · l⁻¹]. There were no differences in insulin during exercise between the P and N trials [50.2 (4.3) pmol · l⁻¹ vs 43.8 (5) pmol · l⁻¹]. These data suggest that endogenous opioids may augment the glucagon response during intense exercise. Accepted: 15 June 1996     

Homologous and heterologous expression of a ribosomal protein gene in Podospora anserina requires an intron

Although the role of introns in eucaryotic nuclear genes has been much debated, it remains underinvestigated in fungi. The AS1 gene of Podospora anserina contains three introns and encodes a ribosomal protein (S12) belonging to the well-conserved bacterial S19 family. We attempted to complement the highly pleiotropic mutation AS1-4 with a cDNA encoding the homologous human (S15) protein (rig gene) under the control of the AS1 promoter. In a control experiment, the AS1 ⁺ cDNA was unable to complement fully the AS1-4 mutation. It was assumed that the AS1 cDNA was not well expressed and that the AS1 gene needed intron(s) to be efficiently expressed. Addition of the first intron of the AS1 gene to the AS1 and rig cDNAs did indeed allow complementation of all the phenotypic defects of the AS1-4 mutation. These data lead to two main conclusions. First, the human S15 ribosomal protein is functional in Podospora. Second, full expression of the Podospora AS1 gene requires at least one intron. Received: 26 April 1996 / Accepted: 22 August 1996     

The relationship between plasma potassium concentration and muscle torque during recovery following intense exercise

The present study investigated the relationship between plasma potassium ion concentration ([K⁺]) and skeletal muscle torque during three different 15-min recovery periods after fatigue induced by four 30-s sprints. Four males and one female completed the multiple sprint exercise on three separate days; recovery was passive, i.e. no cycling exercise (PRec), active cycling at 30% peak oxygen consumption O_2peak (30% Rec) and active cycling at 60% O_2peak (60% Rec). Plasma [K⁺] was measured from blood sampled from an antecubital vein of subjects at rest and at 0, 3, 5, 10 and 15 min into each recovery. Isokinetic leg strength was measured at rest and at 1, 6, 11 and 16 min during each recovery. Following the exhaustive sprints, [K⁺] increased significantly from an average mean (SEM) resting value of 3.81 (0.07) mmol · l⁻¹ to 4.48 (0.19) mmol · l⁻¹ (P < 0.01). In all recovery conditions, plasma [K⁺] returned to resting levels within 3 min following the fourth sprint. However, in the two active recovery conditions plasma [K⁺] increased over the remainder of the recovery periods to 4.36 (0.12) mmol · l⁻¹ in the 30% Rec condition and 4.62 (0.12) mmol · l⁻¹ in the 60% Rec condition, the latter being significantly higher than the former (P < 0.01). The maximum torque measured following the sprints decreased significantly, on average, to 61.1 (8.36)% of peak levels (P < 0.01). After 15 min of recovery, maximum torque was highest in the 30% Rec condition at 92.13 (3.06)% of peak levels (P < 0.01), compared to 85.23 (3.64)% and 85.71 (0.82)% for the PRec and 60% Rec conditions, respectively. In contrast to the significant differences in plasma [K⁺] across all three recovery conditions, muscle torque recovery was significantly different in only the 30% Rec condition. In summary, recovery of peak levels of muscle torque following fatiguing exercise does not appear to follow changes in plasma [K⁺]. Accepted: 18 October 1996