Activation of human arm muscles during flexion/extension and supination/pronation tasks: A theory on muscle coordination

Generally the number of muscles acting across a joint exceeds the number of degrees of freedom available to the joint. This redundancy raises a problem regarding the ratio in which these muscles are activated during a particular motor task. In this paper we present a theory to explain the activation patterns of muscles used during voluntary and reflex induced contractions. The basic assumptions underlying the theory are that 1) coordination of muscles is based on synergistic muscle activities, 2) the synergisms involved satisfy certain transformations of muscle spindle signals to muscle activation signals and 3) muscle spindle output is proportional to the ratio of muscle stretch and muscle length in lengthening muscles, and is zero in shortening muscles. The theory is used to predict the recruitment threshold of motor units in six arm muscles during voluntary isometric contractions. All theoretical predictions are in reasonable agreement with the experimentally observed behavior of a large population of motor units within each muscle. However, within a single muscle sometimes motor-unit populations have been found to have different types of recruitment behavior. This deviating behavior is discussed in the light of the theory presented here.     

Size and fecundity hierarchies in an herbaceous perennial

Summary Inequalities in size in populations have potentially important effects on fitness but have rarely been examined in natural populations. I measured size (number of culms) and fecundity (number of spikelets) in five populations of the grass Danthonia spicata from 1981 to 1985. The populations were in sites in a Pinus-Quercus-Populus forest in northern lower Michigan, USA comprising a secondary succession sequence. The sites had been burned in 1980, 1954, 1948, 1936, and 1911, respectively. Mean sizes and fecundities and the amount of hierarchy in size and fecundity, measured by the Gini coefficient, were compared between the YOUNG population, 1980 burn site, and the OLD populations, 1954, 1948, 1936, and 1911 burn sites. I found large differences in mean size and fecundity between the YOUNG and OLD populations with much larger individuals in the YOUNG population. No differences in size hierarchies were found in either the first year of measurement or after five years. The fecundity hierarchies showed no significant difference among the populations in the first year but after five years the YOUNG population showed a significant decrease in amount of inequality. The longterm patterns of size and fecundity hierarchies differed because fecundity was a cumulative trait while size was not. Size inequalities may not always be a good measure of fecundity inequalities. Short-term measures of inequality in perennials may not be a good indicator of long-term values. In contrast to greenhouse studies, habitat light levels did not affect size hierarchies although they did affect fecundity hierarchies.     

Recruitment and local persistence of a freshwater bryozoan in stream riffles

Recruitment in clonal organisms often involves both sexual and asexual processes whereby new individuals are added to adult populations. During 1988–1990,1 examined local distribution and abundance patterns, substrate colonization, and recruitment in riffle populations of the freshwater bryozoan Plumatella emarginata. Abundance levels of P. emarginata at eight selected study sites were strongly dependent on substrate size but not on overall substrate availability or stream position. Colonization of large, P. emarginata-free substrates at one site resulted in an aggregated dispersion pattern of short-lived colonies and the production of locally persistent sessoblasts. Sessoblast production rates were dependent of colony size and number. However, the estimated single season rates of colonization and recruitment were insufficient to account for the high resident population on large substrates. I conclude that sessoblast persistence is essential for escaping mortality and promoting future recruitment and local population growth and that the population dynamics of P. emarginata are strongly nonequilibrial being influenced by unpredictable disturbances as well as by rates of recruitment which increase with local density.     

Multiobjective H2/H∞ synthetic gene network design based on promoter libraries

Some current promoter libraries have been developed for synthetic gene networks. But an efficient method to engineer a synthetic gene network with some desired behaviors by selecting adequate promoters from these promoter libraries has not been presented. Thus developing a systematic method to efficiently employ promoter libraries to improve the engineering of synthetic gene networks with desired behaviors is appealing for synthetic biologists.In this study, a synthetic gene network with intrinsic parameter fluctuations and environmental disturbances in vivo is modeled by a nonlinear stochastic system. In order to engineer a synthetic gene network with a desired behavior despite intrinsic parameter fluctuations and environmental disturbances in vivo, a multiobjective H₂/H_∞ reference tracking (H₂ optimal tracking and H_∞ noise filtering) design is introduced. The H₂ optimal tracking can make the tracking errors between the behaviors of a synthetic gene network and the desired behaviors as small as possible from the minimum mean square error point of view, and the H_∞ noise filtering can attenuate all possible noises, from the worst-case noise effect point of view, to achieve a desired noise filtering ability. If the multiobjective H₂/H_∞ reference tracking design is satisfied, the synthetic gene network can robustly and optimally track the desired behaviors, simultaneously.First, based on the dynamic gene regulation, the existing promoter libraries are redefined by their promoter activities so that they can be efficiently selected in the design procedure. Then a systematic method is developed to select an adequate promoter set from the redefined promoter libraries to synthesize a gene network satisfying these two design objectives. But the multiobjective H₂/H_∞ reference tracking design problem needs to solve a difficult Hamilton–Jacobi Inequality (HJI)-constrained optimization problem. Therefore, the fuzzy approximation method is employed to simplify the HJI-constrained optimization problem to an equivalent linear matrix inequality (LMI)-constrained optimization problem, which can be easily solved by selecting an adequate promoter set from the redefined promoter libraries using the LMI toolbox in Matlab.Based on the confirmation of in silico design examples, we can select an adequate promoter set from the redefined promoter libraries to achieve the multiobjective H₂/H_∞ reference tracking design. The proposed method can reduce the number of trial-and-error experiments in selecting an adequate promoter set for a synthetic gene network with desired behaviors. With the rapid increase of promoter libraries, this systematic method will accelerate progress of synthetic biology design.     

Structural correlates of stability and resilience in strong hierarchies

We investigated the structural correlates of stability and resilience in strong hierarchies, that is, systems that can be represented by a rooted tree. A simple exponential model that incorporates three variables (the total number of nodesN; the number of basal nodesn _B; and the number of single links among nodesN ₁) accounts for 95% of the observed variability in stability among trees in our sample population. For resilience the situation is even simpler, with about 89% of the population variation being accounted for by tree size (N). For strong hierarchies, size and shape are the principal correlates of stability, while size alone explains the major proportion of the variability in resilience among stable trees. These results suggest that reasonably accurate statistical predictions about the stability and resilience of strong hierarchies can be made from a small set of (relatively) easily measured variables, without detailed knowledge of system topology.     

Development of simple fitness landscapes for peptides by artificial neural filter systems

The applicability of artificial neural filter systems as fitness functions for sequence-oriented peptide design was evaluated. Two example applications were selected: classification of dipeptides according to their hydrophobicity and classification of proteolytic cleavage-sites of protein precursor sequences according to their mean hydrophobicities and mean side-chain volumes. The cleavage-sites covered 12 residues. In the dipeptide experiments the objective was to separate a selected set of molecules from all other possible dipeptide sequences. Perceptrons, feedforward networks with one hidden layer, and a hybrid network were applied. The filters were trained by a (1,) evolution strategy. Two types of network units employing either a sigmoidal or a unimodal transfer function were used in the feedforward filters, and their influence on classification was investigated. The two-layer hybrid network employed gaussian activation functions. To analyze classification of the different filter systems, their output was plotted in the two-dimensional sequence space. The diagrams were interpreted as fitness landscapes qualifying the markedness of a characteristic peptide feature which can be used as a guide through sequence space for rational peptide design. It is demonstrated that the applicability of neural filter systems as a heuristic method for sequence optimization depends on both the appropriate network architecture and selection of representative sequence data. The networks with unimodal activation functions and the hybrid networks both led to a number of local optima. However, the hybrid networks produced the best prediction results. In contrast, the filters with sigmoidal activation produced good reclassification results leading to fitness landscapes lacking unreasonable local optima. Similar results were obtained for classification of both dipeptides and cleavage-site sequences.     

Effects of microhabitat characteristics on the settlement and recruitment of a coral reef fish at two spatial scales

Populations of fishes on coral reefs are replenished by the settlement of pelagic larvae to demersal populations. Recruitment varies spatially and temporally and can exert strong effects on the dynamics of reef fish populations. This study examined the effect of microhabitat characteristics on small-scale and large-scale recruitment variation in the three-spot damselfish, Stegastes planifrons (Cuvier). Comparison of 0.25-m² quadrats occupied by three-spots with randomly sampled null quadrats showed that three-spots quadrats contained a higher percent cover of the coral Montastrea annularis than would be expected at random. Manipulative experiments on three types of 1.0-m² patch reefs (living M. annularis, dead Porites Porites and dead Acropora palmata) patch reefs on showed that this non-random distribution was established by microhabitat choice during settlement and not by differential post-settlement survival. The presence of conspecific juveniles did not affect settlement. Recruitment was monitored at nine sites on three islands over 3 years. Recruitment showed no consistent pattern in the relative levels of recruitment among sites. Similarly, no consistent relationship emerged between recruitment levels and microhabitat characteristics at the nine sites. For example, at this large scale, the percent cover of M. annularis explained variation in recruitment in only 1 out of 3 years. These results suggest that small-scale recruitment patterns are influenced by microhabitat choice during settlement, but that these habitat effects do not scale up to influence large-scale variation in recruitment.     

The scale of demographic heterogeneity in a population of Peromyscus leucopus

Summary The patterns of spatial heterogeneity in density and demography were studied in a population of Peromyscus leucopus inhabiting a deciduous forest in west-central Indiana. A series of 9 live-trapping grids sampled densities from 3 spatial scales: 3 ha, 80 ha and 350 km². We found high levels of variation within all three spatial scales. There was as much variation within a single, large grid as within the entire Sugar Creek Valley (350 km²). The patterns of density variation were not temporally stable and thus represent variation in demographic processes rather than simply differences in habitat quality. We suggest that the Sugar Creek Valley represents a metapopulation of Peromyscus leucopus composed of a number of temporally unstable subpopulations. This structure provides considerable demographic variation that dispersing animals can exploit.     

Size distribution,growth and production of Sargassum thunbergii in an intertidal zone of Padori,west coast of Korea

This paper considers an estimation of population parameters by mathematical equations on the basis of size distribution and biomass obtained from a field survey. Sargassum thunbergii, a dominant alga in t he surf zone of Korean coast, was investigated from June 1990 to August 1991. We divided the population into interval groups according to the time of recruitment. A separation of the population by age or size was impossible because of difficulties arising from continuous recruitment over the growing season. The basic idea for the solution is based on the assumption that the estimated values of the growth rate, number of births and deaths reflect real values, if the biomass and coefficient of variation obtained from the field investigation are coincident with those calculated by equations. The predicted behavior of the population is as follows: the proportion of numbers of recruits is large, but the contribution of recruits to biomass and production is insignificant, and the growth curve shows a sigmoid pattern.     

Inter- and intraspecific alarm response in the termiteMacrotermes subhyalinus (Rambur)

Summary In laboratory studies we analysed the reaction of aMacrotermes subhyalinus (Rambur) colony in contact with a small group of test termites placed at the head of a gallery system. As test termites we usedMacrotermes subhyalinus of the same nest,Macrotermes subhyalinus of a foreign nest and the related sympatric speciesMacrotermes bellicosus (Smeathman).Contact with an introduced group of foragers evokes a significant recruitment of workers and minor soldiers. The intensity of the response depends on the test groups we introduced. The highest recruitment results withM. bellicosus andM. subhyalinus from a foreign nest. During the recruitment the increase in the number of minor soldiers is greater than the increase in the number of workers, and as a result, the relative proportion of the soldiers in the population leaving the nest is significantly higher than before.Direct observations at the place of contact show thatM. bellicosus individuals are immediately attacked and killed. In the case ofM. subhyalinus from a foreign nest, not all individuals are killed; some are only intensively examined with the antennae or seized with the mandibles.M. subhyalinus from the same nest are rarely attacked. Mostly they are antennally inspected or enter the gallery system unhindered.The recruitment of termites from the nest is released by major workers which, after contact with the test termites, run back into the nest and provoke an alarm by tactile stimuli. On their way back into the nest the major workers, in all probability, lay a pheromone trail which leads the outgoing alarmed termites towards the place of contact.     

Sprouting traits and population structure of co-occurring Castanopsis species in an evergreen broad-leaved forest in southern China

The role of sprouting in regeneration was compared betweenfour co-occurring Castanopsis species in an evergreen broad-leavedforest in southern China. We considered the largest stem of an individualto be the main stem, while all of the other stems were consideredto be sprouts. To evaluate the degree of dependence on sproutingin the regeneration of each species, we measured the ratio of thenumber of all sprouts to the number of main stems in a population. The ratio was 4.3 in Castanopsis fordii and > 1in Castanopsis fabri and Castanopsis carlesii, althoughit was < 1 in Castanopsis fargesii. The ratioof the number of all sprouts to the number of main stems in a populationwas represented by the product of two factors of sprouting ability:the proportion of sprouting individuals among all individuals andthe number of sprouts per sprouting individual. The proportion ofsprouting individuals among all individuals differed significantlyamong the four species, while the number of sprouts per sproutingindividual did not. Specific variations in sprouting traits werein agreement with the population structure of each species. Specieswith fewer small individuals in the size distribution had a higherratio than species with L-shaped distribution. Furthermore, specieswith a clumped distribution had a larger ratio than species thatwere randomly distributed. These results suggested that specieswhose recruitment by seedlings was temporally and spatially restrictedwere relatively more dependent on sprouting in the regeneration. The specific gradient of sprouting ability and the relative dependencyon sprouts to maintain a population can enhance the coexistenceof these four congeneric species.     

Patterns of Solidago altissima ramet growth and mortality: the role of below-ground ramet connections

Summary For the rhizomatous perennial, Solidago altissima, I identified clonal fragments in the field, mapped ramet spatial locations, and documented patterns of ramet recruitment, growth, and mortality. Parent ramet size influenced the size and number of daughter ramets produced, and small ramets had lower survivorship and fecundity than large ramets. Similarly, small rhizomes tended to develop into small ramets, and ramets that survived to produce daughter ramets had longer parent-daughter rhizome connections than ramets that did not survive. In addition, most ramets that died during the growing season were connected to (genetically identical) ramets that persisted. There were large size inequalities among rhizomes, ramets, and clonal fragments. Inequalities in the size of ramets increased during the early part of the growing season, then decreased at the end of the season; similar patterns were observed for the growth of clonal fragments. In both instances, the decrease in size inequality could be attributed to the mortality of small individuals (ramets or clonal fragments). I found little evidence that ramet size hierarchies were structured by intraspecific competition. For example, path analyses and randomization tests indicated that size variation among S. altissima ramets was influenced little by the size of their near neighbors (but was influenced by parent size and rhizome size). In addition, within-season variation for the relative size and growth rate of individual ramets led to poor correlations between early and final ramet size; this result suggests that there was no stable hierarchy of dominant and suppressed ramets. I discuss implications of my results for contrasting interpretations of clonal plant population dynamics.     

Effect of grass litter on seedling recruitment of the critically endangered Cistus heterophyllus in Spain

Cistus heterophyllus Desf. is an Iberian-north African endemic plant species with only one wild population left in Spain. A sowing experiment under controlled conditions was designed to assess both the seedling emergence rate and the growth fitness of C. heterophyllus on a litter layer of Brachypodium retusum (Pers.) Beau., which forms dense carpets around the remnant population of C. heterophyllus. In order to test the physical effects of B. retusum litter on Cistus seedling recruitment, six sowing treatments in pots were compared that combined three doses of litter (0, 0.6 and 1.1 kg m⁻²) and different physical locations of the seeds (above, below the litter layer or within a litter–peat mixture). B. retusum litter interfered with the seedling recruitment of C. heterophyllus, negatively affecting both seedling emergence and their early growth. Such an effect was more severe when the seeds were located above a 0.6 kg m⁻² litter layer, although seedlings from seeds sown under the litter showed a growth reduction ranging from 58% to 100% compared to the no-litter treatment. A significant negative effect of the litter–peat mixture on the number of leaves, leaf size and number of branches of seedlings was also inferred. These data provide experimental evidence on the serious threat that B. retusum litter represents for the conservation and regeneration of the last population of C. heterophyllus in Spain.     

Recruitment and year-to-year variability in a population of Macoma balthica (L.)

Because of methodological problems, macrobenthic studies usually neglect the juvenile stages of invertebrate communities, due to the fact that appearance of recruits in samples is only detected some weeks or even months after their true recruitment. During this period, the temporary meiobenthos undergoes high rates of mortality. From year to year, juvenile survival rate is thus responsible for temporal patterns observed in adult population densities.The results presented here relate to the population dynamics of the tellinid bivalve Macoma balthica (L.). A study of temporary meiobenthos was conducted over two consecutive years in an intertidal Macoma-community located at the mouth of the Gironde Estuary in southwest France. Sampling of juvenile stages required short intervals (2 weeks) between successive samplings and a fine sieving mesh size (63 µm). Other population parameters, such as temporal patterns in density, reproductive cycle, and individual growth, were recorded.Recruitment processes showed a year-to-year variability, with regard to settlement density, settlement period, and survival rate. In 1983, recruitment was moderate and protracted over several months. Only one main recruitment period was detected in 1984, resulting in a high juvenile density. In a previous study (1977), by contrast, recruitment was almost non-existent.This variability is discussed as a function of climatic and sedimentological conditions which prevailed in the estuary throughout the study period. However, none of these physical factors appeared to underlie the recruitment fluctuation in Macoma balthica. It is suggested that biological interactions are of prime importance in regulating population densities in this community.     

Populations with Explicit Borders in Space and Time: Concept,Terminology, and Estimation of Characteristic Parameters

Biologists studying short-lived organisms have become aware of the need to recognize an explicit temporal extend of a population over a considerable time. In this article we outline the concept and the realm of populations with explicit spatial and temporary boundaries. We call such populations “temporally bounded populations”. In the concept, time is of the same importance as space in terms of a dimension to which a population is restricted. Two parameters not available for populations that are only spatially defined characterise temporally bounded populations: total population size, which is the total number of individuals present within the temporal borders, and total residence time, which is the sum of the residence times of all individuals. We briefly review methods to estimate these parameters. We illustrate the concept for the large blue butterfly (Maculinea nausithous) and outline insights into ecological and conservation-relevant processes that cannot be gained without the use of the concept.     

Recruitment patterns of Daphnia: a key for understanding midsummer declines?

The population dynamics of D. galeata was studied during the period May–July of 5 successive years in Bautzen Reservoir (Germany). Sampling intervals of 3–4 days and knowledge of the egg–age distribution made it possible to calculate recruitment of the population for each sampling interval. In every year, high Daphnia densities and high Secchi depth readings (clear-water stage) were recorded in spring. In two of the 5 years, Daphnia abundance remained on a high level throughout summer, whereas in the remaining 3 years a midsummer decline of Daphnia was observed. Birth rates declined while the abundance increased in spring in every year but the timing of this decline in relation to abundance and the onset of the clear-water stage revealed no obvious pattern, distinguishing between years with or without a midsummer decline. However, cumulative recruitment did reveal a clear pattern: in years without a midsummer decline, cumulative recruitment increased slowly, but continually, starting from a low level in early May. By contrast, in years with a midsummer decline, recruitment increased strongly during one week in May, starting from a relatively high density level (>30 ind. l^–1). This quick increase was followed by extended periods with low recruitment. Consequently, in these years the population was dominated by a strong peak-cohort of about the same age during the clear-water stage. If all members of this cohort, while approaching their maximum life-span, die off during a short time interval, this may have a pronounced effect on the initiation of a midsummer decline. From an extensive literature search, evidence is provided that the findings of this study represent a demographic mechanism which is of general importance for the initiation of a midsummer decline in temperate waters.     

Temporal and spatial patterns in the recruitment of Gibbula umbilicalis

Size/frequency samples have been taken from populations of the trochid gastropod Gibbula umbilicalis at a number of sites around the British Isles each year since 1978. In the North of Scotland, close to the limit of the species distribution, recruitment was generally poor and populations were sparse and dominated by large old individuals. While such poor recruitment may reflect the low density of adults and the shortage of nursery areas, a distinct temporal pattern of recruitment was also evident. At the start of our study, the majority of sites had population structures biased towards young animals implying that conditions for the settlement of larvae or their subsequent survival had been favourable over a wide area. In the years that followed only the enclosed Loch Eriboll regularly received substantial recruitment. Further to the south, in Wales and S.W. England, recruitment was usually more regular, populations were more dense and individuals smaller.     

Conditional gene knockout using cre recombinase

Cre recombinase has become an important instrument for achieving precise genetic manipulation in mice. Many of these desired genetic manipulations rely on Cre’s ability to direct spatially and temporally specified excision of a predesignated DNA sequence that has been flanked by directly repeated copies of the loxP recombination site. Success in achieving such conditional mutagenesis in mice depends both on the careful design of conditional alleles and on reliable detection of cre gene expression. These procedures include PCR, immunohistochemistry and the use of a recombination-proficient GFP-tagged Cre protein.     

Patterns of resource use within a population of xanthid crabs occupying bryozoan colonies

Summary Heads, i.e. colonies, of the bryozoan Schizoporella pungens are discrete resource units which protect the xanthid crab Pilumnus sayi from predation. The heads were spatially clumped, temporally stable, and differed both qualitatively and quantitatively. The number of adult P. sayi was equal to the number of heads with preferred characteristics, yet a relative shortage of this resource is suggested with concomitant intraspecific competition. Adult crabs occupied heads individually except for infrequent cases of double occupancy by a male and female. Further spatial pattern among crabs was consistent with a hypothesis that males defend clumps of heads occupied by females.