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  Evolution of Parasitic Association, Biology tutorial

Introduction:

Organisms often associate altogether, often closely. There are a number of reasons for such associations, comprising nutrition, protection and as a support to the dispersion (both temporally and geographically) of the organism. There are four major ways that animals of various species might be associated to one other: Symbiosis, Mutualism, Commensalism and Parasitism. These categorizations though, on closer inspection, might become blurred, one kind taking on the aspects of the other, for instance over time as the relationship evolves. Though, as a general guide such terms are still much helpful.

Types of Association:

1) Symbiosis:

In this association, both organisms are reliant on each other. Illustrations being the association of flagellate protozoa in the gut of termites, in which the termites are dependent or reliant on the protozoa for breaking down their food materials and the protozoa are dependent or reliant on the termites as a host organisms. The other good illustration here which is frequently cited is the association among clown fish and anemones in the tropical reefs; where the fish is dependent or reliant on anemone for protection and food whereas the anemone doesn't appear to get anything from the association, apart from possibly cleaning. Though, it has been noticed that in some conditions, in the absence of fish partner the anemones tend to vanish from their reef home, pointing out a true symbiotic instead of a mutualistic or commensal relationship.

The other famous example is found in the lichens, symbiotic association comprised of algae and fungi. These associations might become much close and it is thought that the eukaryotes as a group evolved as an outcome of such an association.

2) Mutualism:

In this association, the associates might or might not be dependent on one other for their existence; however both benefit when they are related. A good illustration of this takes place by the association of sea anemones on the backs of the crabs. Both get benefited from the association (that is, the anemone giving some food for the crab, which in turn provides additional motility to the anemone), however both can survive on their own. 

The other less famous illustration is found among certain species of ants and the caterpillars of some of the Lycaenidae butterflies (specifically the Blues), where the caterpillar is protected through the ants in their nests, in return for which the caterpillar secretes a honeydew that the ants collect. In this condition from the view-point of the ant, it gets advantage from the association, however doesn't appear to require it, (that is, the association is facultative, or opportunistic). Though, from the view-point of the caterpillar, this association is needed for its survival (that is, the association is mandatory).

3) Commensalism:

Neither organism is reliant or dependent on the other for its existence, however in this case just one of the partners gets advantage from the association, the other being unaffected. An illustration of this, found in the humans, is the non-pathogenic obligate commensal protozoa like the amoebae Entamoeba gingivalis, generally found in the mouth, feeding of bacteria, dead epithelial cells and food particles. Purely commensal relationships tend to be instead rare, as on a closer inspection element of the mutualism or parasitism might become obvious. 

4) Parasitism:

In this association, one of the associates live either partially or wholly at the expenditure of the other associate, the other partner (that is, the host organism) not gaining something from the association. This association might give rise to great pathology in the host, or the parasitism might be usually not much pathogenic. Parasitism is taken out by most of the organisms, the major groups comprising bacteria, viruses, protozoa (these generally being endoparasitic) and different metazoan groups (that is, multicellular eukaryotic animals), these being mainly groups of helminths (generally endoparasitic) and arthropods (generally ectoparasitic) and also some higher organisms like ectoparasitic lampreys and hagfish. Generally though, for partly historical reasons, the word Parasitology usually refers to the study of infection with eukaryotic protozoan, and invertebrate metazoan parasites, not viruses, bacteria or the higher chordate parasites, even although these are parasites in the proper sense.   

Classification of the parasitic organism:

Organisms in such associations might as well be on the outer surface of the host organism, (in which condition the prefix 'Ecto' is utilized), or inside the host organism, (in which condition the prefix 'Endo' is utilized). These prefixes might be employed by any of the animal associations. For illustration the flagellate protozoa in the termite guts are Endosymbionts, whereas the anemone can proceed as an Ectocommensal with the crab. Parasites might act as both Ectoparasites and Endoparasites.

Parasites might also be categorized according to the closeness of the relationship. For illustration: Facultative Parasites (such as numerous bacteria) are those where the parasitic lifestyle is merely taken up opportunistically, while Obligate Parasites (like all viruses and most of the helminths parasites) are those in which the organism should parasitize the other organism.

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