Ultrastructural and cytochemical studies on the germarium of Dicrocoelium dendriticum (Plathelminthes,Digenea)

Summary The unpaired germarium of Dicrocoelium dendriticum contains many female germ cells at different stages of maturation and is enveloped by a fibrous basal lamina-like structure and a multilayered cytoplasmic sheath whose origins and functions are discussed. The maturation process of primary oocytes occurs completely within the prophase of the first meiotic division. It has been divided into three stages, as previously suggested for monogeneans. Stage I corresponds to oogonia and early oocytes which are located in the distal germinative area of the gonad. These cells are characterized by a high nucleo/cytoplasmic ratio and a poorly differentiated cytoplasm. Stage II corresponds to maturing oocytes grouped in the central area of the gonad and exhibiting long synaptonemal complexes and a prominent nucleolus. The main feature of cytoplasmic differentiation is the increase in the number of RER and Golgi complex which are involved in the production of small electron-dense granules. Stage III corresponds to mature oocytes located in the proximal area of the germarium near the origin of the oviduct. In this stage, the granules become regularly distributed in a monolayer in the peripheral ooplasm and make contact with the oolemma. They show a distinctive complex structure, are composed of proteins and glycoproteins and do not contain polyphenols. Their possible role as cortical granules is discussed in relation to chemical composition and previous studies on other Plathelminthes. Neither yolk globules nor glycogen are present in the oocytes.Abbreviations I oogonium and early oocyte - II growing oocyte - III mature oocyte - cg cortical granule - cs cytoplasmic sheath - db dense body - ecm extra cellular matrix - ER endoplasmic reticulum - fl fibrous extracellular layer - gc Golgi complex - m mitochondria - N nucleus - nu nucleolus - RER rough endoplasmic reticulum - sc synaptonemal complex     

Detection of Aeromonas hydrophila in food with an enzyme-linked immunosorbent assay

A microtitration plate, antibody capture, enzyme-linked immunosorbent assay was developed for the detection of Aeromonas hydrophila serotype O : 11 (highly virulent strains). The assay utilizes a detector antibody which shows no cross-reactions with Aeromonas strains other than serotype O : 11 or non- Aeromonas competing organisms. The detector antibody is mixed with the sample and incubated for 1 h, microcentrifuged and the supernatant fluid (unadsorbed antibody) titred in a microtitre plate coated with A. hydrophila cells from serotype O : 11. All the A. hydrophila strains from serotype O : 11 tested reacted strongly with the detector antibody. Also by culturing and performing the immunoassay with the detector antibody we established and quantified the presence of A. hydrophila O : 11 in different foods.     

Mechanisms of group-hunting in vertebrates

Group-hunting is ubiquitous across animal taxa and has received considerable attention in the context of its functions. By contrast much less is known about the mechanisms by which grouping predators hunt their prey. This is primarily due to a lack of experimental manipulation alongside logistical difficulties quantifying the behaviour of multiple predators at high spatiotemporal resolution as they search, select, and capture wild prey. However, the use of new remote-sensing technologies and a broadening of the focal taxa beyond apex predators provides researchers with a great opportunity to discern accurately how multiple predators hunt together and not just whether doing so provides hunters with a per capita benefit. We incorporate many ideas from collective behaviour and locomotion throughout this review to make testable predictions for future researchers and pay particular attention to the role that computer simulation can play in a feedback loop with empirical data collection. Our review of the literature showed that the breadth of predator:prey size ratios among the taxa that can be considered to hunt as a group is very large (<10⁰ to >10²). We therefore synthesised the literature with respect to these predator:prey ratios and found that they promoted different hunting mechanisms. Additionally, these different hunting mechanisms are also related to particular stages of the hunt (search, selection, capture) and thus we structure our review in accordance with these two factors (stage of the hunt and predator:prey size ratio). We identify several novel group-hunting mechanisms which are largely untested, particularly under field conditions, and we also highlight a range of potential study organisms that are amenable to experimental testing of these mechanisms in connection with tracking technology. We believe that a combination of new hypotheses, study systems and methodological approaches should help push the field of group-hunting in new directions.     

Is the Tonal Quality of Birdsong Learned? Evidence from Song Sparrows

Song sparrow (Melospiza melodia) songs are composed largely of pure-tonal sounds. This paper investigates the role that learning plays in the development of the tonal structure of song sparrow songs, as well as the role that tonal quality plays in determining the suitability of songs as models for learning. 20 birds were trained with both normal pure-tonal songs and modified songs that included harmonic overtones. The harmonic-modified songs were obtained from birds singing in a helium atmosphere, the result of which is to perturb vocal tract resonances and thus alter a song's tonal quality. Subjects learned equally well from normal and harmonic models. Birds that learned material from harmonic models reproduced some of this material with harmonic overtones, but the majority of notes learned from harmonic models were subsequently reproduced as pure-tonal copies. Thus, the tonal structure of songs does not influence young song sparrows in their selection of song models, but there is a strong tendency to reproduce songs in a pure-tonal fashion, even if learned from harmonic models.     

Susceptibility of turbot (Scophthalmus maximus), coho salmon (Oncorhynchus kisutch, and rainbow trout of. my kiss) to strains of Vibrio anguillarum and their exotoxins

The susceptibility of turbot, coho salmon, and rainbow trout to strains of Vibrio anguillarum of serotypes 01 and 02 and their extracellular products (ECP) was investigated in order to clarify the role of exotoxins in the mechanism of virulence of both serotypes. All V. anguillarum isolates were virulent for trout, salmon, and turbot. Despite the origin of the strains tested, rainbow trout was the most susceptible fish species to experimentally induced vibriosis. Coho salmon and turbot did not differ significantly in their susceptibility to V. anguillarum live cells. In contrast, the ECP from Vibrio strains of serotypes 01 and 02 exhibited similar lethal dose for turbot, salmon, and trout (ranging from 4.52 to 7.32 μg protein/g fish). Therefore, differences in susceptibility to vibriosis are not completely due to a differential sensitivity of fish to the extracellular products of Vibrio strains. The ECP from 7 of 10 V. anguillarum strains possessed vascular permeability factors, and all the extracts displayed proteolytic, hemolytic and cytotoxic activities. All the biological activities of ECP were lost after heat treatment at 80° C/10 min.     

Litter quality and nutrient cycling affected by grazing-induced species replacements along a precipitation gradient

One of the potential mechanisms for the impact of herbivores on nutrient cycling is the effect of selective grazing on litter quality through changes in species composition. However, the scarce evidence collected on this mechanism is controversial and seemingly influenced by site-specific variables. In this paper, we explored the consequences of grazing-induced changes in species composition on litter quality and nitrogen cycling with a regional perspective. Along a 900-mm of mean annual rainfall gradient, we selected species promoted and diminished by grazing from three natural rangelands of Argentina, analyzed their litter quality, and determined their decomposition and nutrient release kinetics under common greenhouse conditions. Litter quality and decomposition rates were strongly associated with plant response to grazing. However, the magnitude and direction of these differences depended on the ecosystem considered. In the wettest site, the species promoted by grazing (forbs) had higher nitrogen and phosphorus contents, faster decomposition rates, and higher release of nitrogen to the soil than species diminished by grazing (C₃ and C₄ grasses). In the intermediate and dry sites, species promoted by grazing had lower nitrogen and phosphorus contents, and slower decomposition rates than those diminished by grazing (C₃ grasses in both cases). Decomposition of the entire group of species was not correlated with mean annual rainfall, but when litter of the species diminished by grazing was analyzed, it was negatively correlated with precipitation. Nitrogen was immobilized more often than mineralized, even after one year of incubation. Immobilization was negatively correlated with precipitation. All these results indicate that grazing may significantly alter nutrient cycling by affecting litter quality through changes in species composition. These effects seem to be larger when species replacements induced by grazing either involve functional groups, as it was the case in our wettest site, or change root to shoot ratios. Therefore, the functional groups involved in the replacement of species as well as shifts between belowground and aboveground allocation should play a key role in grazing-induced changes on nitrogen cycling.     

Do Experience and Body Size Play a Role in Responses of Larval Ringed Salamanders,Ambystoma annulatum,to Predator Kairomones? Laboratory and Field Assays

Prey may experience ontogenetic changes in vulnerability to some predators, either because of changes in morphology or experience. If prey match their level of antipredator behavior to the level of predatory threat, prey responses to predators should reflect the appropriate level of threat for their stage of development. For larval salamanders, responses to predators may change with body size because larger larvae are less vulnerable to predation by gape‐limited predators or because fleeing responses by large salamanders may be more effective than for smaller salamanders. In a field experiment, small larval ringed salamanders, Ambystoma annulatum, responded to chemical stimuli (‘kairomones’) from predatory newts, Notophthalmus viridescens, with an antipredator response (decreased activity). Laboratory‐reared larvae decreased their activity following exposure to newt kairomones, indicating that larval ringed salamanders do not require experience with newts to recognize them as predators. In both experiments, larvae distinguished between chemical stimuli from newts and stimuli from tadpoles (non‐predators) and a blank control. In a third experiment, field‐caught (experienced) larvae showed a graded response to newt kairomones based on their body size: small larvae tended to decrease their activity while larger larvae showed no change or an increase in activity. This graded response was not observed for neutral stimuli, indicating that it is predator‐specific. Therefore, ringed salamander larvae exhibit threat‐sensitive ontogenetic changes in their response to chemical stimuli from predatory newts.     

DAYLENGTH AND LIGHT RESPONSES IN GROWTH AND FERTILITY OF GLOSSOPHORA KUNTHII (PHAEOPHYTA,DICTYOTALES) FROM PACIFIC SOUTH AMERICA1

Excised ligulae of Glossophora kunthii (C. Ag.) J. Ag. were cultured in photoperiods of 4–24 h and photon fluence rates of 10–75 μmol.m^?2.s^?1. Daylength interacted with irradiance on the growth of the ligulae. Maximal growth of primary ligulae occurred in long-day regimens with high irradiances suggesting an effect of irradiance on photosynthesis and growth. In contrast, growth of secondary ligulae was greatest in short-day regimes. Differences were significant at the highest irradiance tested. Differentiation of tetrasporangia on the ligulae is a short-day photoperiodic response. Daylengths of 8.5 h or less induced a sharp increase in numbers of fertile ligulae and tetrasporangia attaining maturity. Interruptions of the dark period decreased the development of tetrasporangia; the number of interruptions had a cumulative inhibitory effect. Differentiation of reproductive structures was influenced by interactions of photoperiod and irradiance. Maximum numbers of tetrasporangia were formed at short-day regimes and low irradiances; differentiation was completely inhibited at long-day conditions and high irradiance.     

Ehrlich cell plasma membrane redox system is modulated through signal transduction pathways involvingcGMP and Ca2+ as second messengers

Ehrlich cell plasma membrane ferricyanide reductase activity increased in the presence of mastoparan, a generic activator of G proteins, using either whole cells or isolated plasma membrane fractions. Agents that increase intracellularcAMP also increased the rate of ferricyanide reduction by Ehrlich cells. For the first time, evidence is shown on a modulation of plasma membrane redox system bycGMP. In fact, permeant analogs ofcGMP, dibutyrylcGMP, and 8-bromo-cGMP increased the rate of ferricyanide reduction by the Ehrlich cell plasma membrane redox system. Furthermore, specific inhibition ofcGMP-phosphodiesterases by dipyridamole was also accompanied by an enhancement in the rate of ferricyanide reduction. On the other hand, treatments expected to increase cytoplasmic Ca²⁺ concentrations were accompanied by a remarkable stimulation of the reductase activity. Taking all these data together, it seems that the Ehrlich cell plasma membrane redox system is under a multiple and complex regulation by different signal transduction pathways involving G proteins, cyclic nucleotides, and Ca²⁺ ions.