Population dynamics of two small pelagic fish in the central-south area off Chile: delayed density-dependence and biological interaction

It is widely believed that environmental variability is the main cause for fluctuations in commercially exploited small pelagic fish populations around the world. Nevertheless, density-dependent factors also can drive population dynamics. In this paper, we analyzed thirteen years of a relative abundance index of two clupeoids fish populations coexisting in the central-south area off Chile, namely the common sardine, Strangomera bentincki, and anchovy, Engraulis ringens. We applied the classical diagnostic tools of time series analysis to the observed time-series. Also, the realized per capita population growth rate was studied with the aim of detecting the feedback structure that is characterizing the population dynamics of the two species. The analysis suggests that population fluctuations of the two species have an important density-dependent component, displaying first-order (direct density-dependent) and second-order (delayed density-dependent) simultaneously. The density-dependent component explained 70.5 and 55.6 % of the realized per capita population growth rate of common sardine and anchovy, respectively. The deterministic skeleton model showed an asymptotic convergence to equilibrium density. In presence of a stochastic environment, fluctuations were reproduced for the species showing a component of fluctuation with a period of 4 year. The intrinsic dynamics of each species is typical of interacting species resulting from trophic interactions. It is postulated that the second-order dynamics of S. bentincki and E. ringens in central-south Chile, may be the result from interactions with a specialist predator (the fishing fleet), interacting with exogenous environmental factors.     

Ethylene and IAA interactions in the inhibition of photoperiodic flower induction of <Emphasis Type="Italic">Pharbitis nil</Emphasis>

It has been shown that both IAA and ethylene application inhibit flower induction in the short-day plant Pharbitis nil. However application of IAA has elevated ethylene production in this plant, as well. Strong enhancement of ethylene production is also correlated with the night-break effect, which completely inhibits flowering. In order to determine what the role of IAA and ethylene is in the photoperiodic flower induction in Pharbitis nil, we measured changes in their levels during inductive and non-inductive photoperiods, and the effects of ethylene biosynthesis and action inhibitors on inhibition of flowering by IAA. Our results have shown that the inhibitory effect of IAA on Pharbitis nil flowering is not physiological but is connected with its effect on ethylene biosynthesis.     

Augmentation of wheat common root rot by Fusarium acuminatum

Wheat was inoculated with Bipolaris sorokiniana and Fusarium acuminatum individually and in combination under controlled conditions. Only B. sorokininana produced significant disease when used alone; however, F. acuminarum significantly augmented the effects of B. sorokiniana when plants are inoculated with both fungi.     

Saccharomyces cerevisiae mannoproteins form an external cell wall layer that determines wall porosity.

A beta-glucanase (Z-glucanase) from Zymolyase was freed from a protease (Z-protease) by affinity chromatography on alpha 2-macroglobulin-Sepharose columns and used to solubilize proteins from isolated cell walls of Saccharomyces cerevisiae. The cell wall proteins were labeled with 125I and subjected to sodium dodecyl sulfate-polyacrylamide gel electrophoresis and autoradiography. The bulk of the labeled material had very low mobility. Its mannoprotein nature was demonstrated by precipitation with specific antibodies and by conversion to a band with an average molecular weight of 94,000 after incubation with endo-beta-N-acetylglucosaminidase. The intact mannoproteins were hydrolyzed by Z-protease, but were resistant to the enzyme when the carbohydrate was first removed by endo-beta-N-acetylglucosaminidase. In intact cells, lysis of cell walls by Z-glucanase required a previous incubation with z-protease, which led to solubilization of most of the 125I-labeled proteins. Other proteases that did not attack the cell wall mannoproteins were unable to substitute for Z-protease. The specific effect of Z-protease is consistent with the notion that mannoproteins form a surface layer of the cell wall that penetrates the wall to some depth and shields glucans from attack by Z-glucanase. Mannoproteins, however, do not appear to cover the inner face of the cell wall, because isolated cell walls, in contrast to intact cells, were completely solubilized by Z-glucanase in the absence of protease. The function of mannoproteins in determining cell wall porosity was highlighted by the finding that horseradish peroxidase (Mr, 40,000) causes lysis of cells that had been treated with Z-protease. Depletion of mannoproteins by Z-protease also resulted in the disappearance of a darkly stained surface layer of the cell wall, as observed by electron microscopy. Other agents that facilitate cell lysis by Z-glucanase, such as 2-mercaptoethanol, digitonin, and high concentrations of salts, caused little or no solubilization of mannoprotein. We assume that they perturb and loosen the structure of the mannoprotein network, thereby increasing its porosity. The implications of our results for the construction of the yeast cell wall and the anchoring of mannoprotein to the cell are discussed.     

Factors influencing algal species assemblages on reef and cobble substrata off Ghana

Subtidal algal assemblages were studied on two substrata, rocky reefs and calcareous cobbles. Although the two occur together at comparable depths, their vegetation differs in species composition and richness, and in patterns of plant size, life form, and longevity. The reef bears a species-rich, patchy cover of small filamentous and crustose forms, with occasional clumps of more robust species. The cobbles support a sparse cover of large leafy and dendritic species in addition to many of the smaller species found on the reef. The floristic separation arises from differential establishment and survival of species under conditions of (1) grazing by fish and urchins (on the reef only), and (2) seasonal physical disturbance during storms leading to the removal of most algae (on the cobbles only). Both substrata show a seasonal floristic cycle, but the trend is more pronounced on cobbles. Species do not depart from randomness in their patterns of co-occurrence on individual cobbles or reef fragments. Interspecific competition appears comparatively unimportant in determining species composition on either substratum.     

Haemolysis of rabbit erythrocytes by a lectin from the seeds of<Emphasis Type="Italic">Croton tiglium</Emphasis>

The kinetics of haemolysis of rabbit erythrocytes byCroton tiglium lectin was studied as a function of concentration of the lectin and erythrocytes. The length of the prelytic period decreased with increasing lectin concentrations, indicating that the secondary events at the membrane which follow the binding of the lectin to cell surface carbohydrate receptors are accelerated at higher surface concentrations of the lectin. The rate or extent of haemolysis was not affected by the inclusion of ions like K⁺, Ca²⁺ and Mg²⁺ in the medium or by the substitution of ionic medium by a non-ionic medium. The inhibition of haemagglutination and haemolysis of rabbit red cells byCroton tiglium lectin by antilectin rabbit serum was observed. A possible mechanism of haemolysis by the lectin is discussed.