Chemo- and thermosensory neurons: structure and function in animal parasitic nematodes.

Nematode parasites of warm-blooded hosts use chemical and thermal signals in host-finding and in the subsequent resumption of development. The free-living nematode Caenorhabditis elegans is a useful model for investigating the chemo- and thermosensory neurons of such parasites, because the functions of its amphidial neurons are well known from laser microbeam ablation studies. The neurons found in the amphidial channel detect aqueous chemoattractants and repellants; the wing cells-flattened amphidial neurons-detect volatile odorants. The finger cells-digitiform amphidial neurons-are the primary thermoreceptors. Two neuron classes, named ADF and ASI, control entry into the environmentally resistant resting and dispersal dauer larval stage, while the paired ASJ neurons control exit from this stage. Skin-penetrating nematode parasites, i.e. the dog hookworm Ancylostoma caninum, and the threadworm, Strongyloides stercoralis, use thermal and chemical signals for host-finding, while the passively ingested sheep stomach worm, Haemonchus contortus, uses environmental signals to position itself for ingestion. Amphidial neurons presumably recognize these signals. In all species, resumption of development, on entering a host, is probably triggered by host signals also perceived by amphidial neurons. In the amphids of the A. caninum infective larva, there are wing- and finger-cell neurons, as well as neurons ending in cilia-like dendritic processes, some of which presumably recognize a sequence of signals that stimulate these larvae to attach to suitable hosts. The functions of these neurons can be postulated, based on the known functions of their homologs in C. elegans. The threadworm, S. stercoralis, has a complex life cycle. After leaving the host, soil-dwelling larvae may develop either to infective larvae (the life-stage equivalent of dauer larvae) or to free-living adults. As with the dauer larva of C. elegans, two neuron classes control this developmental switch. Amphidial neurons control chemotaxis to a skin extract, and a highly modified amphidial neuron, the lamellar cell, appears to be the primary thermoreceptor, in addition to having chemosensory function. The stomach worm, Haemonchus contortus, depends on ingestion by a grazing host. Once ingested, the infective larva is exposed to profound environmental changes in the rumen. These changes stimulate resumption of development in this species. We hypothesize that resumption of development is under the control of the ASJ neuronal pair. Identification of the neurons that control the infective process could provide the basis for entirely new approaches to parasite control involving interference with development at the time and place of initial host-contact.     

Localization of metabolic sites of action of herbicides

Time- and concentration-course studies were conducted to determine the effect of thirteen herbicides on photosynthesis, respiration, RNA synthesis, protein synthesis, and lipid synthesis using isolated single leaf cells. Each herbicide was from a different chemical class. Appropriate ¹⁴C-substrates and product purification procedures were used for each process prior to liquid scintillation counting. The most sensitive metabolic site of inhibition was photosynthesis for atrazine, bromacil, dichlobenil, monuron, and paraquat; RNA synthesis for dalapon and dinoseb; protein synthesis for chlorpropham; and lipid synthesis for CDAA, chloramben, 2,4-D, EPTC, and trifluralin. However, with several herbicides, one or more process was almost as sensitive as the one mentioned above. All herbicides inhibited more than one process, and the most sensitive site of inhibition may not be the same process that was inhibited the greatest at the maximum concentration and maximum exposure time used. Therefore, a concept of metabolic sites of action, rather than a primary site of action, appears to be more meaningful for herbicides.     

An expanded eukaryotic genetic code

We describe a general and rapid route for the addition of unnatural amino acids to the genetic code of Saccharomyces cerevisiae. Five amino acids have been incorporated into proteins efficiently and with high fidelity in response to the nonsense codon TAG. The side chains of these amino acids contain a keto group, which can be uniquely modified in vitro and in vivo with a wide range of chemical probes and reagents; a heavy atom-containing amino acid for structural studies; and photocrosslinkers for cellular studies of protein interactions. This methodology not only removes the constraints imposed by the genetic code on our ability to manipulate protein structure and function in yeast, it provides a gateway to the systematic expansion of the genetic codes of multicellular eukaryotes.     

Trematode cataract in fresh water fish

Abstract— —The occurrence of cataract in fresh-water fish has recently been the cause for increasing concern amongst anglers who have been catching a steadily falling number of fish in several regions. It has been known for over 100 years that certain fish may become infested with a trematode (or fluke) which has a remarkable predilection for the lens. The life cycle begins in the intestine of the seagull where the adult trematode produces ova which are excreted on to the waters of reservoirs and lakes. The ova hatch out into miracidia which in turn invade water snails to form a sporocystic stage. Cercariae emerge from the snail and swim free to burrow into the body of a fish and find their way to the crystalline lens. The cycle is completed by the consumption of the fish by the gull in which the adult trematode once more develops. The purpose of this communication is to demonstrate the histological changes produced in the lens by the trematode. Résumé— —L'augmentation de l'incidence de la cataracte du Poisson d'eau douce a été récemment la cause de l'inquiétude croissante des pécheurs qui prennent une quantité sans cesse diminuée de Poisson dam différentes régions. Il est connu depuis plus de cent ans que certains poissons peuvent être infectés par les trématodes (ou douves) qui se logent avec une nette prédilection dans le cristallin. Le biocycle débute dam l'intestin de la mouette ou le trématode adulte pond des oeufs qui sont excrétés dans les eau des réservoirs et des lacs. Les oeufs donnent naissance am miracidies qui à leur tour envahissent les escargots d'eau et s'y transforment en sporocystes. La cercaire se libère de l'escargot et nage librement pour se loger dans le corps d'un Poisson et se frayer un chemim jusqu'au cristallin. Le cycle est achevé par l'ingestion du Poisson par la mouette chez laquelle le trématode adulte continue à se développer. Le but de cet article est de décrire les lésions histologiques du cristallin produites par le trématode. Zusammenfassung— —Das Auftreten von Katarakten bei Süsswasserfischen gab vor kurzem Anlass für steigende Besorgnis unter Anglern, die in mehreren Gebieten nur eine ständig abnehmende Zahl von Fischen fingen. Es ist seit über 100 Jahren bekannt, dass bestimmte Fische von Trematoden (Saugwürmer) befallen werden können, die eine merkliche Bevorzugung der Augenlinse zeigen. Der Lebenszyklus beginnt im Darm der Seemöwe, wo die envachsenen Trematoden Eier legen, die auf Wasserflächen von Staubecken und Seen ausgeschieden werden. Aus den Eiern gehen Mirazidien hervor, die ihrerseits Wasserschnecken befallen und dort ein sporozystisches Stadium bilden. Aus den Schnecken gehen dann Zerkarien hervor, die frei umherschwimmen und nach dem Eindringen in den Körper eines Fisches den Weg in die Augenlinse finden. Der Zyklus wird dadurch gesch-lossen, dass die Möwe den Fisch frisst und dass sich in dem Vogel abermals die erwachsenen Trematoden entwickeln. Der Zweck dieser Mitteilung besteht in der Beschreibung der histologischen Veranderungen, die von den Trematoden in der Linse hervorgerufen werden.     

Influences of gap position, vegetation management and herbivore control on survival and growth of white spruce (Picea glauca (Moench) Voss) seedlings

The boreal mixedwood forest type of the Canadian interior boreal is largely comprised of two dominant tree species: white spruce and trembling aspen (Picea glauca and Populus tremuloides). This forest type is expansive, providing important ecosystem services and economic production, yet such mixtures are difficult to establish after harvests. While aspen resprouts and grows vigorously following disturbance, spruce growth is relatively slow and is often limited by intense competition from associated vegetation, including aspen. To improve management, it is important to understand how environmental and vegetative conditions vary in relation to the competitive-facilitative relationship of spruce-aspen mixtures. In this study white spruce was planted across large canopy openings to determine whether survival and height growth is influenced by position within gap and by differing levels of competing vegetation control of aspen and understory plants. In addition, we addressed the issue of herbivory, which can pose a significant threat to planted spruce seedlings. Within each of four sites, linear gaps were created and five gap positions were recognized spanning the southern and northern forest understories, and southern, center and northern positions within each opening. Three different levels of vegetation management were implemented: a brush saw treatment in which all vegetation was cut to ground level, a mixing treatment in which all vegetation and rootstock was ground up, and a control with no vegetation management. The three herbivory treatments excluded large ungulates, small herbivores (rabbits, hares) or had no herbivore exclusion. Growth and survival of white spruce seedlings were measured for four years (1997-2000). Understory survival was significantly lower than within the gap, with the sheltered southern edge position providing the best initial environmental conditions and or ameliorative cover for spruce establishment. However, after four years the shelter effect starts to be inhibitive relative to center and northern gap positions, suggesting the removal of the canopy is necessary before spruce productivity declines. The optimal vegetation management treatment also changed over the study period. The most intensive treatment (mixing) initially showed a negative influence on survival and growth, but by year four, survival converged to approximately 75% for all treatments, and the mixing treatment produced the best height growth. The growth advantage became most evident in the center gap positions, which initially lagged the brushsaw and control treatments. Lastly, some growth losses from herbivory must be expected in boreal mixedwoods, although not enough to merit control. Results have implications for the timing and intensity of silvicultural treatments for harvesting and planting.