2.5 a Flashcards
The number of parasite species is unknown, but estimates suggest that:
- all free-living species are thought to be hosts for several parasite species
- at least half of all species are parasites
How can parasites teach us about evolution?
- parasites/hosts show coevolution
- study of parasitic/non-parasitic genomes gives evidence of gene loss mechanisms and diversification
- parasites play major role in evolutionary appearance and maintenance of sexual reproduction in organisms
Definition of parasitism…
parasitism is a symbiotic interaction between parasite and host (+/-)
a parasite gains benefit in terms of nutrients at the expense of its host
Describe reproductive potential of parasite…
unlike in predator/prey relationship, the reproductive potential of the parasite is greater than that of the host
What is an ecological niche?
a multidimensional summary of tolerances and requirements of a species
What is a parasite niche like?
tend to have narrow niches as they are host specific
- as host provides many of parasite’s needs, many are degenerate, lacking in structure and organs found in other organisms
What is an ectoparasite?
parasite that lives on surface of the host
What is an endoparasite?
parasite that lives inside host’s tissues
What is a definitive (primary) host?
the organism on or in which the parasite reaches sexual maturity
What is the intermediate (secondary) host?
host needed for parasite to complete life cycle
What is a vector?
it plays an active role in the transmission of the parasite and may also be a host
What is a fundamental niche?
what a species occupies in absence of interspecific competing influences
What is a realised niche?
is occupied in response to interspecific competition
When does competitive exclusion occur?
where niches of two species are so similar that one declines to local extinction
When does resource partitioning occur?
where realised niches are sufficiently different and potential competitors can co-exist
What is resource partitioning?
where two competing species occupy different realised niches, compromising over resources and thus managing to exist simultaneously
What is a microparasite?
of microscopic size (must use microscope to see)
e.g. bacteria, viruses, protozoans and fungi
What is a macroparasite?
can be seen with the naked eye
e.g. ticks, nematodes, flukes
What are common parasites?
protists, platyhelminths, nematodes, arthropods, bacteria and viruses
What human diseases are caused by parasites?
schistosomiasis and malaria
What causes schistosomiasis and where?
schistosomes in sub-tropical/tropical world regions
Describe life cycle of schistosomes…
-schistosomes sexually reproduce in human intestine
- fertilised eggs pass out via faeces into water where they develop into larvae
- larvae then infect water snails where asexual reproduction occurs
- produces kind of motile larvae which escapes snail and penetrate human skin, entering the blood stream
What are symptoms of schistosomiasis?
rash and cough, readily treated in UK
What causes malaria?
plasmodium
Describe life cycle of plasmodium…
- infected mosquito (vector) bites human
- plasmodium enters blood stream
- asexual reproduction occurs in liver and then red blood cells
- when red blood cells burst gametocytes released into the blood stream
- another mosquito bites infected human and gametocytes enter mosquito maturing into male/female gametocytes, allowing sexual reproduction to occur
Why is it hard to create malaria vaccine?
plasmodium is very hard to culture
What are malaria symptoms?
presents as headache and fever, treatable if detected
How do parasites escape the host immune system?
sometimes mimic host antigens, hiding in liver cell so are undetected by immune system, allowing future recurrence
also can modify the immune response
What are viruses?
parasites that only replicate inside the host cell
What is the structure of a virus like?
- genetic material, both DNA and RNA, packaged in protein coat
- some viruses surrounded by phospholipid membrane derived from host cell materials
- outer virus surface contains antigens that host cells may/may not detect as foreign
What are retroviruses?
have genetic material that is RNA, use the enzyme reverse transcriptase to form DNA which is then inserted into host cell genome
the viral genes are then expressed to form new viral products
Give an example of a retrovirus…
HIV
Describe the life cycle of a virus…
- infection of host with genetic material
- host cell enzymes replicate viral genome
- transcription of viral genes and translation of viral proteins
- assembly and release of new viral particles
What do viruses depend upon the host for?
- ATP
- nucleotides
- amino acids
- enzymes
What do viruses also do to the host cells?
they suppress host’s own DNA replication and protein synthesis
What is transmission?
the spread of a parasite to a host
What is virulence?
the harm caused to host species by parasite
When are transmission rates greatest?
- when hosts overcrowd, areas with high population density (cities)
- when mechanisms, vectors/waterborne dispersal stages allow parasite to spread even if hosts (infected) are incapacitated
How are ectoparasites transmitted?
through direct contact
How are endoparasites transmitted?
by vectors or consumption of intermediate hosts
How do parasite use host behaviours?
often exploited to maximise transmission
e.g. foraging, movement, sexual behaviour, habitat choice, anti’predator behaviour
What is an extended phenotype?
where host behaviour become part of parasite phenotype
Give example of extended phenotype in toxoplasma..
the cat parasite toxoplasma gondii causes infected rodents to lose predator fear so cat catch easier, toxoplasma then sexually reproduces in cat gut
Give example of extended phenotype in plasmodium…
female infected mosquitoes are more attracted to humans than non-infected ones
Give example of extended phenotype in hairworm…
crickets jump into water when infected by hairworm, sot it can come out of cricket to continue its lifestyle
adult horsehair worms are free-living in fresh water and damp soil
parasitised crickets seek water as they think they are thirsty
What may parasites modify in a host?
- host immune system
- host size
- host reproductive rate
- host lifespan
What does host modification do for the parasite?
tend to benefit parasite growth, reproduction or transmission
host survival may actually increase with some levels of infection
