Voici les éléments 1 - 10 sur 2387
  • Publication
    Accès libre
    Experimental evidence that partner choice is a driving force in the payoff distribution among cooperators or mutualists : the cleaner fish case
    (2001) ;
    Grutter, Alexandra S.
    Supply and demand largely determine the price of goods on human markets. It has been proposed that in animals, similar forces influence the payoff distribution between trading partners in sexual selection, intraspecific cooperation and interspecific mutualism. Here we present the first experimental evidence supporting biological market theory in a study on cleaner fish, Labroides dimidiatus. Cleaners interact with two classes of clients: choosy client species with access to several cleaners usually do not queue for service and do not return if ignored, while resident client species with access to only one cleaning station do queue or return. We used plexiglas plates with equal amounts of food to simulate these behaviours of the two client classes. Cleaners soon inspected 'choosy' plates before 'resident' plates. This supports previous field observations that suggest that client species with access to several cleaners exert choice to receive better (immediate) service.
  • Publication
    Métadonnées seulement
    Le sol vivant. Bases de pédologie, biologie des sols.
    (Lausanne: Presses Universitaires et Polytechniques romandes, 1998) ; ;
  • Publication
    Métadonnées seulement
    Forêts "Les Roches", Orvin, canton de Berne. Carte des associations végétales.
    (Inspection de la Protection de la Nature du canton de Berne, 1998) ;
    Perrenoud, Alain
  • Publication
    Accès libre
    Toxins in chrysomelid beetles: Possible evolutionary sequence from de novo synthesis to derivation from food-plant chemicals
    (1990)
    Pasteels, Jacques M.
    ;
    Duffey, S.
    ;
    In the Chrysomelinae, it appears that de novo synthesis of chemicals for defense is the primitive state, and the sequestration of plant chemicals for defense the derived state. The derived state evolved through both the morphological and biochemical preadaptiveness of the homologous defensive glands. In the adults, we discuss one unique case of sequestration in exocrine defensive glands of host-plant pyrrolizidine alkaloids by Oreina cacaliae. However, hypericin is not sequestered either in the glands or elsewhere in the body of Chrysolina spp. feeding on Hypericum, which contradicts an earlier claim. In the larvae, we examine in more detail how the phenolglucoside salicin can be used as the precursor of the salicylaldehyde present in the defensive secretion of Phratora vitellinae and Chrysomela spp. with minimal changes in the biochemical mechanisms involved in the biosynthesis of iridoid monoterpenes in related species.
  • Publication
    Accès libre
    Genetic diversity of cyanobacterial communities in Lake Kinneret (Israel) using 16S rRNA gene, psbA and ntcA sequence analyses
    (2007) ;
    Witzel, Karl-Paul
    ;
    Hadas, Ora
    The genetic diversity of cyanobacterial communities was studied at various depths in Lake Kinneret (Israel). Denaturing gradient gel electrophoresis (DGGE) of specific 16S rRNA gene PCR products showed significant differences in the cyanobacterial community structure between epi- and hypolimnetic waters. Sequences of clone libraries prepared from 16S rRNA gene PCR products from epi- and hypolimnion revealed the presence of at least 11 different groups of cyanobacteria. Clones related to the unicellular cyanobacteria (Chroococcales and picocyanobacteria) dominated the clone libraries from both depths. New primers to amplify the gene coding for the photosystem II reaction centre (psbA) and the nitrogen regulator gene (ntcA) of cyanobacteria were developed and used for further characterization of the cyanobacterial communities from the lake. Sequences of psbA amplicons clustered with those from 2 different groups of marine Synechococcus and Chroococcales. Cloned ntcA amplicons from the lake were closely related and did not cluster with sequences from cultured cyanobacteria or other environmental sequences from this gene. All the molecular markers analyzed here showed similarity to sequences from some groups of cyanobacteria in the lake and those so far found in marine habitats.
  • Publication
    Métadonnées seulement
    Artificial-infection protocols allow immunodetection of novel Borrelia burgdorferi antigens suitable as vaccine candidates against Lyme disease
    (2003)
    Wallich, Reinhard
    ;
    Jahraus, Oliver
    ;
    Stehle, Thomas
    ;
    Tran, Thi Thanh Thao
    ;
    Brenner, Christiane
    ;
    Hofmann, Heidelore
    ;
    ;
    Simon, Markus M
    Vaccination with recombinant outer surface protein A (OspA) from Borrelia burgdorferi provides excellent antibody-mediated protection against challenge with the pathogen in animal models and in humans. However, the bactericidal antibodies are ineffective in the reservoir host, since OspA is expressed by spirochetes only in the vector, but rarely, if at all, in mammals. Using an artificially generated immune serum (anti-10(8) spirochetes) with high protective potential for prophylactic and therapeutic treatment, we have now isolated from an expression library of B. burgdorferi (strain ZS7) three novel genes, zs7.a36, zs7.a66 and zs7.a68. All three genes are located, together with ospA/B, on the linear plasmid lp54, and are expressed in vitro and in ticks. At least temporarily two of them, ZS7.A36 and ZS7.A66, are also expressed during infection. The respective natural antigens are poorly immunogenic in infected normal mice but elicited antibodies in Lyme disease patients. We show that recombinant preparations of ZS7.A36, ZS7.A66 and ZS7.A68 induce functional antibodies in rabbits capable of protecting immunodeficient mice against subsequent experimental infection. These findings suggest that all three recombinant antigens represent potential candidates for a 'second generation' vaccine to prevent and/or cure Lyme disease.
  • Publication
    Accès libre
    Antioxidant allocation modulates sperm quality across changing social environments
    In promiscuous species, male reproductive success depends on their ability to mate with fertile females and on the fertilizing ability of their sperm. In such species, theory predicts that, owing to a trade-off between pre- and post-copulatory reproductive traits, males with lesser access to females should increase resource investment into those sperm traits that enhance fertilization success–usually referred to as ejaculate quality. This prediction has been validated in several taxa, yet studies on the physiological mechanisms modulating ejaculate quality are lacking. Sperm cells are highly vulnerable to oxidative stress, which impairs male fertility. Therefore, males that better protect their sperm from oxidative stress are expected to achieve higher ejaculate quality. Based on theoretical expectations, and since social dominance is a major determinant of mating opportunity, we predicted that subordinate males should invest more into the antioxidant protection of their sperm in order to achieve higher ejaculate quality. We maintained 60 male and 60 female wild-caught house sparrows Passer domesticus in outdoor aviaries, where we experimentally manipulated male social status to test our predictions. We measured cellular oxidative stress and enzymatic antioxidant activity in blood and sperm both before and after manipulating social ranks. Before manipulating the social status, we found that ejaculate viability correlated with oxidative stress level in sperm, with dominant males producing more oxidized and less viable ejaculates. Further, males at the lower end of the hierarchy produced ejaculates of similar quality to those of dominant males, suggesting that restricted access to resources might limit male reproductive strategies. After experimentally manipulating the social status, males matched their ejaculate quality to their new rank, while increases in antioxidant investment into ejaculates paralleled increases in ejaculate viability. Oxidative stress has been proposed as a general constraint to the evolution of life histories. Our results highlight oxidative stress and strategic antioxidant allocation as important proximate physiological mechanisms underlying male reproductive strategies.
  • Publication
    Métadonnées seulement
    T helper cell priming of mice to Borrelia burgdorferi OspA leads to induction of protective antibodies following experimental but not tick-borne infection
    (1997)
    Zhong, Weimin
    ;
    ;
    Kramer, Michael
    ;
    Wallich, Reinhard
    ;
    Simon, Markus M
    Antibodies to the outer surface lipoprotein A (OspA) of Borrelia burgdorferi confer protection to SCID mice against subsequent tick-borne or experimental infection. However, OspA-specific antibodies are hardly detectable in naturally infected humans, dogs, hamsters and mice. This is most probably due to limited expression of OspA on spirochetes transmitted from the vector to the host. Here we have tested whether T cell priming of mice would lead to the induction of protective OspA-specific antibodies upon infection. It is shown that AKR/N mice, previously immunized with either a single T helper cell peptide of OspA, or a mixture of 27 peptides spanning the entire molecule, develop OspA-specific IgM or IgG antibodies, including those to a prominent protective B cell epitope of OspA, LA-2, within 7 days of infection with low doses (10(3)) of culture-derived spirochetes. In marked contrast, the same groups of pre-sensitized mice failed to generate any detectable OspA-specific antibodies after tick-borne infection for more than 40 days after infection. All mice, irrespective of their state of T cell immunity to OspA or the mode of infection, produced similar levels of OspC-specific IgM and IgG antibodies as early as day 14 after infection. None of the mice previously immunized with OspA peptides were protected against experimental infection, in spite of the appearance of protective antibodies. It is clear from these data that, in contrast to culture-derived spirochetes, the naturally transmitted pathogen fails to express OspA within the mammalian host at levels sufficient for induction of B cell responses, even in the presence of pre-activated T helper cells. Together with the fact that OspA-specific antibodies are mainly operative by eliminating spirochetes from the vector during infestation, the data suggest that OspA-vaccination for T helper cell immunity alone is not sufficient to prevent Lyme disease.