Parasitization selection pressures

For much of its life cycle a parasite s environment is its host. A principal, and potentially lethal, feature of this environment is the host immune response. Diversity in parasites interactions with this aspect of their environment may therefore be expected. The host immune response varies between different hosts and so, if this is considered as a selection pressure, it may produce the ideal conditions for the generation of diversity in immune-mediated parasite-host interactions. 

Current understanding of parasitism by T. spiralis is compartmentalized, and so glycoprotein function has been considered in the context of one compartment or another. However, LI larvae of T. spiralis have evolved under selective pressure to parasitize both intestine and muscle. Biological economy may require a duality of function in larval glycoproteins such that they are able to perform distinct roles in each of the two niches. Dualism is common in proteins, and elucidation of such properties in parasitic nematode products would provide unique insights into the basis of host adaptation. 

The function of DMEs is also thought to include the detoxification of dietary products and the evolution of plant metabolites, including drugs [11]. The selective forces responsible for the maintenance of different alleles in different populations may include the fact that one allele may enable improved rates of implantation, improved prenatal growth and development, improved postnatal health in response to dietary or environmental selective pressures or improved resistance to bacteria, viruses or parasites [11, 14]. Allele frequencies may also reflect ethnic dietary differences that have evolved over thousands of years [15]. 

The parasite-host interaction continuously exerts selective pressure(s) on both insects to survive. The host insect presents a particularly challenging environment for endoparasites because of the rapid development and differentiation that characterize parasite and host life cycles (21,22, 80, 81). Endoparasites may exploit a variety of agents to suppress or avoid host defenses and to modify the normal development of the host to match their need polydnaviruses, venoms, oviduct secretions, and protective materials coating the parasite egg or produced by the endo-parasite as it develops within its host. 

In a permissive host, not all immature wasps are successful in surviving the host s defenses, indicating that selection pressures bearing on the survival strategies of both insects involve some balance that favors the parasite. To the extent that failed endoparasite development is not due to... 

The central role of mitochondria in mediating the biosynthesis of FeS clusters in unicellular eukaryotes is noteworthy. It is this physiological function which may provide a critical selective pressure to retain highly reduced mitochondria over the course of the protist s adaptation, either to a parasitic lifestyle or to inhabiting oxygen-poor or anaerobic niches. 

The evolutionary fate of parasitic flat-worms lies intimately intertwined with the taxa they currently utilize, the taxa they can but have not yet utilized and the selective pressures driving speciation within and between parasites and their hosts. As with any other biological system, if time and conditions permit, natural selection will maintain diversity. Host switching, host speciation, vicariant events affecting parasites and/or hosts, an increase in host population size and dispersal and drug resistance are just some of the factors that will maintain or even promote the success of parasites and subsequently drive diversity in the hosts. 

Queens of the slave-making ants depend on host species for nest foundation. Newly mated queens usurp host colonies and replace the resident queens. Parasitic queens may obtain immediate care by resident host workers. In the species where this occurs, we might expect intense selective pressures in favor of fast acquisition of recognition cues that mimic their hosts. 

Selection pressures on cuticular chemical resemblance between hosts and parasites are expected to be even weaker when parasite sexuals are taken into account. Indeed, sexuals are cared for by enslaved host workers and live inside the natal colonies until the mating flight. As expected, in Chalepoxenus muellerianus (a slave making ant which enslaves multiple hosts of the related Temnothorax species), the complex hydrocarbon profiles of sexuals depend on the host-rearing species, with a bias towards the host species used by each parasite population, but with differences among the chemical signatures of parasite sexuals and workers of the host species (Beibl et al., 2007). It is obvious, from the example... 

There it is quite likely that no degree of resistance existed in the native population. The selection pressure most likely had not been as intense, and even with the reduced rate of active ingredient Introduced by MMP (compared to MPEG), parasitic and predatory insect reduction was significant. 

This two-level selection works effectively, with the aid of minority control of specific molecules for a cell. Indeed, surviving cells satisfy the minority control. With the selection pressure for reproduction of cells, there appears a state that is not expected by the rate equation for reaction of molecules, where the number of inactive Y molecules that are parasitic to the catalytic reaction is suppressed. Furthermore, resistance against parasitic (inactive) Y molecules is established by this minority-controlled state. 

The selection pressure exerted by malaria is strong enough to maintain sickle-cell anaemia and the thalassaemias at a high frequency in populations across Africa and Asia. These conditions are not isolated curiosities, but part of a spectrum of adaptations that enable people to survive in areas where malaria is endemic. Among the most important of these adaptations is malarial tolerance. Tolerance develops after infections in early childhood and lasts a lifetime. It is not the result of a heightened ability to kill parasites (as in vaccination) but the triumph of realpolitik —... 

Taking advantage of the fact that the cost of adaptation slows growth and reproduction can lead to surprisingly practical results. For example, there is a parasitic mite, Varroa destructor, that causes heavy losses in honeybees. There are several different miticides available for beekeepers to use to kill the mites. If one such miticide kills 99.9% of the mites, selection pressure ensures that resistant mites will dominate the population in a few generations. Therefore, this particular miticide would soon be useless. 

During the evolution of bark beetles, their hosts and associated fungi, parasites, predators and organisms, the composition of bark beetle pheromones will probably have been modified as other species evolved responses to components of the pheromone and exerted selection pressures on the beetle population. There is only circumstantial evidence that this might have happened. For example, in the experiments on widely separated populations of 7. pini (Lanier et al., 1972), California beetles attracted more local 7. pini than did New York beetles, and vice versa in New York. However, New York beetles in California attracted far more of the local predator E. lecontei than did the local 7. pini. California and New York 7. pini use different ratios of the enantiomers of ipsdienol as their pheromones. The high resolution of (-)ipsdienol in California populations versus the blend in New York supports the idea of coevolution of chemical systems of predator and prey production of and response to (+ )ipsdienol being eliminated in California by a predator which had evolved specific responses to it. Perhaps in the absence of E. lecontei in New York there was no pressure to resolve the blend. 

A number of system approaches can be used to clean up the fuel feed. These include pressure swing adsorption, membrane separation, methanation, and selective oxidation. Although selective oxidation does not remove CO2, it is usually the preferred method for CO removal because of the parasitic system loads and energy required by the other methods. In selective... 

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