Obligate parasite

Obligate parasitism is exhibited in a range of organisms, with examples in viruses, bacteria, fungi, plants, and animals. They are unable to complete their development without passing through at least one parasitic stage which is necessary to their life-cycle. Whether one regards viruses as living organisms or not, they cannot reproduce except by means of resources within living cells. Accordingly, it is convenient and customary to regard them as obligate intracellular parasites. Among the Vespidae family, Vespula austriaca is an example of an obligate reproductive parasite; its common host is Vespula acadica. In the genus Bombus, B. bohemicus is an obligate parasite of B. locurum, B. cryptarum, and B. terrestris. ==Host-parasite interaction==

Life-cycle Parasitic life cycles involve the exploitation of at least one host. Parasites that infect a single species are said to have direct life-cycles. For example, the hookworm species Necator americanus. Parasites that infect more than one host are said to have a complex or indirect life-cycle. Parasitic permanence Obligate parasites may not necessarily spend all of their time behaving as parasites. When a parasite is permanent, a number of generations occur in or on the host of an infested individual. Head lice are an example of this. Temporary parasites are organisms whose parasitic mode of life is limited to a few or even one stage of development. Due to the wide range of obligate parasite types, it is impossible to identify a general invasion strategy. Intracellular parasites use various strategies to invade cells and subvert cellular signalling pathways. Most bacteria and viruses undergo passive uptake, where they rely on the host cell for uptake. However, apicomplexans engage in active entry. One obligate wasp parasite, Polistes atrimandibularis, infiltrates its hosts' colony by modifying its chemical signature to match that of the hosts. This tricks the host wasps into thinking the parasite is one of their own. Evasion of host defences A number of obligate intracellular parasites have evolved mechanisms for evading their hosts' cellular defences, including the ability to survive in distinct cellular compartments. One of the mechanisms that hosts employ in their attempt to reduce the replication and spread of pathogens is apoptosis (programmed cell death). Some obligate parasites have developed ways to suppress this phenomenon, for example Toxoplasma gondii, although the mechanism is not yet fully understood. ==Manipulation of host behaviour==

Changes in a host's behaviour following infection with obligate parasites are extremely common. Unusual behaviour observed in infected individuals is noted, and if its complexity suggests that this behaviour will benefit the transmission of the parasite, then this is said to be an example of adaptive manipulation. However, there is a difficulty in demonstrating changes in behaviour are the result of a selective process favouring transmission of the parasite. It has been suggested that these changes may merely be a side-effect of infection. Most behaviour changes have not been demonstrated to lead to fitness gains in either the host or the parasite. However, the "scientific metaphors, including anthropomorphisms" sometimes used in "popular media and the scientific literature" to describe the manipulation of host behavior have been described as "catchy, yet misleading". Extended phenotype In some cases the behaviour we observe in an organism is not due to the expression of its genes, but rather to the genes of parasites infecting it. This behaviour is an extended phenotype. • The mafia-like strategy- retaliation for non-compliance (eg.great spotted cuckoo and magpie) magpies that eject the cuckoos eggs from their nests suffer a much greater rate of cuckoo predation. It has been suggested that extended phenotype behaviours are not adaptive, but are Exaptative. While they may have a benefit for the parasitic organism, they did not arise with the intention of this benefit. ==Parasitic mimicry in brood parasites==

The cowbird and cuckoo require the nests and parental care of other passerines in order for their young to fledge. These are known as brood parasites. The parasitic bird species mimics egg patterns and colours of the host species, which reduces egg rejection. The chicks of some species are able to manipulate host behaviour by making rapid calls that mimic the sound made by up to four of the host chicks. Mimicry of the host species also occurs in the paper wasp species Polistes semenowi and Polistes sulcifer and the bumblebee species Bombus bohemicus, with the parasite changing its proportions of cuticular hydrocarbons, species- and colony-specific identifying chemicals, to match that of the usurped host species. It is proposed that this mimicry has evolved through two processes: either as coevolutionary responses to host defences against brood parasites or modifying pre-existing host provisioning strategies. Competition between the parasite and host young for parental resources might lead to exaggeration of the aspects of the signal that most effectively exploit host parents. The parasitic young are likely to experience stronger selection for exaggerated signals than host young, because they are unrelated to the other chicks in the nest and therefore under selection to behave more selfishly. ==Evolution of obligate parasitism==

Current theory in evolutionary biology indicates that host-parasite relationships may evolve towards equilibrial states of severe disease. This differs from the conventional belief that commensalism is the ideal equilibrium for both the host and parasite. ==See also==