Parasite-associated growth enhancement in a fish-cestode system.
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Parasites impose an energetic cost upon their hosts, yet, paradoxically instances have been reported in which infection is associated with enhanced, rather than diminished, host growth rates. Field studies of these parasite effects are problematic, since the pre-infection condition of the hosts is generally unknown. Here, we describe a laboratory experiment in which the growth rate and body condition of 76 laboratory-reared three-spined stickleback fishes were examined before, during and after each fish was fed the infective stage of the parasitic cestode Schistocephalus solidus. Twenty-one of these fishes went on to become infected by the cestode. Fishes were individually housed and provided with an abundant food supply to eliminate the potentially masking effects of variable competitive ability. Infection occurred independently of fish gender, size, body condition or pre-exposure growth rate. After exposure to the cestode, infected fishes grew faster (excluding parasite weight) and maintained a similar or better body condition compared with uninfected fishes, despite developing enlarged spleens. The accelerated growth could not be explained by reduced gonadal development. This result, one of few demonstrations of parasite-associated growth enhancement in fishes, is discussed with respect to other such parasite systems. (+info)
Experimentally activated immune defence in female pied flycatchers results in reduced breeding success.
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Traditional explanations for the negative fitness consequences of parasitism have focused on the direct pathogenic effects of infectious agents. However, because of the high selection pressure by the parasites, immune defences are likely to be costly and trade off with other fitness-related traits, such as reproductive effort. In a field experiment, we immunized breeding female flycatchers with non-pathogenic antigens (diphtheria-tetanus vaccine), which excluded the direct negative effects of parasites, in order to test the consequences of activated immune defence on hosts' investment in reproduction and self-maintenance. Immunized females decreased their feeding effort and investment in self-maintenance (rectrix regrowth) and had lower reproductive output (fledgling quality and number) than control females injected with saline. Our results reveal the phenotypic cost of immune defence by showing that an activated immune system per se can lower the host's breeding success. This may be caused by an energetic or nutritional trade-off between immune function and physical workload when feeding young or be an adaptive response to 'infection' to avoid physiological disorders such as oxidative stress and immunopathology. (+info)
Biochemical characterization of MI-ENG1, a family 5 endoglucanase secreted by the root-knot nematode Meloidogyne incognita.
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A beta-1,4-endoglucanase named MI-ENG1, homologous to the family 5 glycoside hydrolases, was previously isolated from the plant parasitic root-knot nematode Meloidogyne incognita. We describe here the detection of the enzyme in the nematode homogenate and secretion and its complete biochemical characterization. This study is the first comparison of the enzymatic properties of an animal glycoside hydrolase with plant and microbial enzymes. MI-ENG1 shares many enzymatic properties with known endoglucanases from plants, free-living or rumen-associated microorganisms and phytopathogens. In spite of the presence of a cellulose-binding domain at the C-terminus, the ability of MI-ENG1 to bind cellulose could not be demonstrated, whatever the experimental conditions used. The biochemical characterization of the enzyme is a first step towards the understanding of the molecular events taking place during the plant-nematode interaction. (+info)
Inhibition of Fas-mediated apoptosis by Trypanosoma cruzi infection.
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Trypanosoma cruzi-infected and normal control mammalian cells were subjected to analysis of Fas-mediated apoptosis stimulated by an agonistic anti-Fas monoclonal antibody. The infected cells showed markedly hampered apoptotic changes in nuclear morphology, phosphatidylethanolamine translocation from the inside to the outside of the plasma membrane, and DNA fragmentation into multiples of 180 bp, relative to normal control cells. Upstream of these morphological and biochemical consequences, the caspase-3 activity was elevated by the Fas stimulation in a significantly greater proportion of intact control cells, but at a highly reduced rate of infected cells. The rapid elevation of caspase-8 activity in control, apoptotic cells was completely inhibited in infected cells. In an examination of the specificity of other stimulants, X-ray radiation or chemicals such as hydrogen peroxide, colchicine or etoposide did not cause significant differences in apoptotic rates between control and infected cells; tumor necrosis factor-alpha, however, induced a high rate of apoptosis in control cells, with an extremely lowered rate in infected cells. This study demonstrates, for the first time, that T. cruzi infection inhibits one of the earliest steps of death receptor-mediated apoptosis, an effect that most probably involves the inhibition of caspase-8. Differential apoptotic responses in cells infected with T. cruzi and other intracellular parasites are discussed. (+info)
Neochlamydia hartmannellae gen. nov., sp. nov. (Parachlamydiaceae), an endoparasite of the amoeba Hartmannella vermiformis.
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Free-living amoebae are increasingly being recognized to serve as vehicles of dispersal for various bacterial human pathogens and as hosts for a variety of obligate bacterial endocytobionts. Several Chlamydia-like Acanthamoeba endocytobionts constituting the recently proposed family Parachlamydiaceae are of special interest as potential human pathogens. In this study coccoid bacterial endocytobionts of a Hartmannella vermiformis isolate were analysed. Infection of H. vermiformis with these bacteria resulted in prevention of cyst formation and subsequent host-cell lysis. Transfection experiments demonstrated that the parasites were not capable of propagating within other closely related free-living amoebae but were able to infect the distantly related species Dictyostelium discoideum. Electron microscopy of the parasites revealed typical morphological characteristics of the Chlamydiales, including the existence of a Chlamydia-like life-cycle, but indicated that these endocytobionts, in contrast to Chlamydia species, do not reside within a vacuole. Comparative 16S rRNA sequence analysis showed that the endocytobiont of H. vermiformis, classified as Neochlamydia hartmannellae gen. nov., sp. nov., is affiliated to the family Parachlamydiaceae. Confocal laser scanning microscopy in combination with fluorescence in situ hybridization using rRNA-targeted oligonucleotide probes confirmed the intracellular localization of the parasites and demonstrated the absence of other bacterial species within the Hartmannella host. These findings extend our knowledge of the phylogenetic diversity of the Parachlamydiaceae and demonstrate for the first time that these endocytobionts can naturally develop within amoebae of the genus Hartmannella. (+info)
Type I restriction systems: sophisticated molecular machines (a legacy of Bertani and Weigle).
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Restriction enzymes are well known as reagents widely used by molecular biologists for genetic manipulation and analysis, but these reagents represent only one class (type II) of a wider range of enzymes that recognize specific nucleotide sequences in DNA molecules and detect the provenance of the DNA on the basis of specific modifications to their target sequence. Type I restriction and modification (R-M) systems are complex; a single multifunctional enzyme can respond to the modification state of its target sequence with the alternative activities of modification or restriction. In the absence of DNA modification, a type I R-M enzyme behaves like a molecular motor, translocating vast stretches of DNA towards itself before eventually breaking the DNA molecule. These sophisticated enzymes are the focus of this review, which will emphasize those aspects that give insights into more general problems of molecular and microbial biology. Current molecular experiments explore target recognition, intramolecular communication, and enzyme activities, including DNA translocation. Type I R-M systems are notable for their ability to evolve new specificities, even in laboratory cultures. This observation raises the important question of how bacteria protect their chromosomes from destruction by newly acquired restriction specifities. Recent experiments demonstrate proteolytic mechanisms by which cells avoid DNA breakage by a type I R-M system whenever their chromosomal DNA acquires unmodified target sequences. Finally, the review will reflect the present impact of genomic sequences on a field that has previously derived information almost exclusively from the analysis of bacteria commonly studied in the laboratory. (+info)
Female barn owls (Tyto alba) advertise good genes.
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The good genes hypothesis of sexual selection postulates that ornamentation signals superior genetic quality to potential mates. Support for this hypothesis comes from studies on male ornamentation only, while it remains to be shown that female ornamentation may signal genetic quality as well. Female barn owls (Tyto alba) display more black spots on their plumage than males. The expression of this plumage trait has a genetic basis and it has been suggested that males prefer to mate with females displaying more black spots. Given the role of parasites in the evolution of sexually selected traits and of the immune system in parasite resistance, we hypothesize that the extent of female plumage 'spottiness' reflects immunological defence. We assessed the genetic variation in specific antibody production against a non-pathogenic antigen among cross-fostered nestlings and studied its covariation with the plumage spottiness of genetic parents. The magnitude of the antibody response was positively correlated with the plumage spottiness of the genetic mother but not of the genetic father. Our study thereby provides the first experimental support, to our knowledge, for the hypothesis that female ornamentation signals genetic quality. (+info)
Evolution of parasite virulence against qualitative or quantitative host resistance.
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We analysed the effects of two different modes of host resistance on the evolution of parasite virulence. Hosts can either adopt an all-or-nothing qualitative response (i.e. resistant hosts cannot be infected) or a quantitative form of resistance (i.e. which reduces the within-host growth rate of the parasite). We show that the mode of host resistance greatly affects the evolutionary outcome. Specifically, a qualitative form of resistance reduces parasite virulence, while a quantitative form of resistance generally selects for higher virulence. (+info)