Obligate parasite

Very few self-sufficient viruses have only 60 protein chains in their shells. The satellite viruses do not themselves encode all of the functions required for their replication and are therefore not self-sufficient. The first satellite virus to be discovered, satellite tobacco necrosis virus, which is also one of the smallest known with a diameter of 180 A, has a protein shell of 60 subunits. This virus cannot replicate on its own inside a tobacco cell but needs a helper virus, tobacco necrosis virus, to supply the functions it does not encode. The RNA genome of the satellite virus has only 1120 nucleotides, which code for the viral coat protein of 195 amino acids but no other protein. With this minimal genome the satellite viruses are obligate parasites of the viruses that parasitize cells. 

Rickettsia are more difficult to produce in quantity than bacteria. Similar to viruses, they are strict obligate parasites and require living cells for growth. They cannot survive long outside a host. They also have a selective affinity for specific types of cells in the body. They are normally transmitted by an arthropod vector (i.e., ticks, lice, fleas, mites), which also serves as either the primary or intermediate host. 

Heagle, A. S. Response of three obligate parasites to oeone. Environ. PoDut. 9 91-95, 1975. 

The cultivar Green Pod 407 that did not produce the polymer (Fig. 1) was well inoculated. The bean rust pathogen is an obligate parasite therefore, it is difficult to determine if an essential growth factor was immobilized and not available to the rust organism or whether the pathogen was inhibited by phenols or o-quinones. 

Mycoplasma genitalium 580 Obligate parasite Fraser etal., 1995... 

Figure 11.5 Prokaryotic genome size distribution (N = 641). Open boxes, free-living prokaryotes grey boxes, obligate parasites black boxes, thermophiles boxes with horizontal lines, endosymbionts. (Modified from Islas etal, 2004.)...

Let us now come back to nature as it is, and look at the size of the smallest organisms. Figure 11.5 compares the genome size distribution, calculated under a series of assumptions (Islas etal., 2004), of free-living prokaryotes, obligate parasites, thermophiles, and endosymbionts. The values of DNA content of free-hving... 

Classification of endosymbionts as a group by themselves shows that their DNA content can be significantly smaller, reaching a value of 450 kb for Buchnera spp. (Gil et al., 2002) or a value of 580 kb for the obligate parasite Mycoplasma genitalium (Fraser etal, 1995). 

One group of turbellarians, which have provided a compelling link to the obligate parasites, is the Temnocephalida. With various degrees of adaptation towards their ectocom-mensal existence with freshwater crustaceans,... 

Fig. 1.3. Four morphological and one molecular estimate of the interrelationships of the Platyhelminthes with emphasis on identifying the sister group to the obligate parasites (Neodermata). Currently, the two main competing hypotheses concern (c) a revertospermatan clade and (e) a large clade of neoophoran turbellarians.

Fig. 1.4. (cont d) Two main competing outgroups for the Neodermata are (a,c,e,g) a clade of parasitic turbellaria, which, with the obligate parasites, form the Mediofusata (see Fig. 1.3c) and (b,d,f,h) a clade of neoophoran turbellarians (see Fig. 1.3e). By mapping site of infection (ectoparasitic/endoparasitic) and identity of hosts used, few topologies offer unequivocal reconstructions of plesiomorphic states.

Whatever the functional advantages conferred by the neodermis might be, clearly these need to be elucidated further. The obligate parasitic flatworms share another important characteristic feature - with the exception of a few derived forms, they almost all parasitize vertebrates and use them as a host where they reach maturity, reproduce and complete their life cycle. Monogeneans are found predominantly on their respective hosts and the Trematoda and Cestoda are found predominantly in their hosts. 

Spore germination and appressoria formation during the pathogenesis of rust fungi is affected to a very low extent. The interference with haustorial formation is the most important effect. Colonized cells undergo a hypersensitivity reaction finally they become necrotic. Further development of the obligate parasite is therefore stopped. 

Culture Filtrates. SchBnbeck et al. (26) applied culture filtrates from various fungi and bacteria to beans and induced resistance to bean rust. The induction of resistance occurred at a distance from the site where the inducer was applied indicating it was systemic. A period of at least two days was required for the Initiation of the resistance response. The phenomenon appeared to be nonspecific since the culture filtrates induced resistance in other plants and against various pathogens however, only against obligate parasites. The active compounds in the culture filtrates have not been identified and their mechanism of action is unknown. 

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