Symbiotic Relationships - Parasitism - Commensalism - Mutualism SYMBIOSIS refers to relationships between organisms of DIFFERENT species that show an intimate association with each other Symbiotic relationships provide at least ONE of the participating species with a nutritional advantage 3 types of symbiosis have been recognised depending on the nature of the relationship:
Interaction in which one organism, the parasite , derives nourishment from the other organism, the host
Parasites are therefore chemoautotrophs
This relationship is detrimental to the host, however a true parasite does normally not kill its host
Obligate vs Facultative
Most parasites are OBLIGATE - that is they must live parasitically and die when their host dies
Obligate parasites also have very few specialised structures for feeding or locomotion
Some fungi are FACULTATIVE parasites since they can continue to feed saprophytically once their host has died
Fewer facultative parasites have evolved as they must form complicated systems to detect, take in and digest food. Due to natural selection they would be at a disadvantage to obligate parasites.
- remain external to the host
e.g. ticks, fleas, leeches
- live inside the body of the host
e.g. liver flukes, tapeworms, malarial parasites
They are small and have a short generation time e.g. viruses, bacteria and protozoans [ Plasmodium spp ]
The duration of infection is short compared to the life span of the host
Longer generation time and tendency to persist causing continual reinfection e.g. roundworms tapeworms and fungi
Intermediate hosts more common
Many parasites complete their entire lifecycle on or in a single host organism
However, many alternate between 2 or more host species, specialising on a different host species at each stage in their lifecycle
2 general types of transmission:
VERTICAL : from mother offspring e.g. HIV
HORIZONTAL : between members of a population
Direct Contact e.g. Headlice
Resistant Stages e.g. Liver fluke
Secondary host species / Vectors e.g. Mosquitoes
Parasite Transmission Case Study: MALARIA
Malaria is a mosquito-borne disease caused by a parasite Plasmodium falciparum , P. vivax , P. ovale and P. malariae
People with malaria often experience fever, chills, and flu-like illness
Left untreated, they may develop severe complications and die
Almost 85% of the world's malaria occurs in sub-Saharan Africa
Each year 350-500 million cases of malaria occur worldwide, and over one million people die, most of them young children in sub-Saharan Africa
Humans are the INTERMEDIATE HOST and RESERVOIR of the parasite, and the mosquito is the DEFINITIVE HOST and VECTOR .
Female anopheline mosquitoes become infected only if they take a blood meal from a person whose blood contains mature male and female stages of the parasite.
A cycle of development and multiplication then begins with union of the male and female gametocytes in the stomach of the mosquito and ends with parasites, called sporozoites, in its salivary glands, which are infective to humans.
The time required for the complete maturation of the parasite in the mosquito varies and depends on the Plasmodium species and external temperature.
Lifecycle of Malaria Parasite 1 2 3
The gametocytes are ingested by the female mosquito in a bloodmeal from an infected human. The gametocytes fuse to produce a zygote.The zygote secrete a cyst containing sporozoites formed from meiotic divisions
Sporozites enters the liver cell and during the next two weeks the intracellular parasite reproduces by mitosis within a liver cell to form as many as 200,000 merozoites! On maturation, the merozoites rupture the liver cells and are are released into the blood where they invade human red blood cells
In the red blood cells, the parasite matures asexually to produce another 10-20 merozoites which in turn can rupture the red blood cell and invade more liver cells or red blood cells
[ Animation ]
Evolution of Host/Parasite relationship
Most parasitic relationships are very specific and complex
The parasite and the host have co-evolved
This means that the host has developed a defence mechanism e.g. immune system or hydrochloric acid in the stomach , to prevent the parasite from causing any harm if it has entered the body.
The longer the relationship has existed, the more host specific the parasite becomes
Modification of Parasites
- Absence/degeneration of feeding and locomotory organs
- Highly specialised mouth parts as in fluid feeders e.g. aphids
- Boring devices to aid entry into host
- Attachment organs e.g. hooks or suckers
- Resistant outer covering
- Degeneracy of sense organs associated with the constancy of the parasites environment
- Exoenzyme production to digest host tissue external to parasite
- Chemosensitivty to reach optimum location in hosts body
- Production of anti-enzymes
- Ability to respire in anaerobic conditions
REPRODUCTIVE - Hermaphrodites - Enormous numbers of reproductive bodies cysts and spores - Resistant reproduction bodies when external to the host - Use of secondary hosts as vectors
Host Responses to Parasite Infection
Organisms respond to parasites in different ways:
Vertebrate hosts infected with microparasites mount an immunological response
Vertebrate hosts infected with ectoparasites have other behavioural strategies e.g.
Preening or grooming each other to remove ectoparasites e.g. chimpanzees.
Move away from the infected area e.g. caribou move to higher altitudes during the summer months when the mosquito population is particularly dense to avoid attacks
Plants respond to parasitic infection in several ways:
e.g. in tobacco plants, if just one leaf is infected with the tobacco mosaic virus, there is an increase in the defensive chemicals throughout the plant-protects the plant from a variety of parasites and from the effects of grazing by herbivores. In addition, the plant will often kill the cells in the area that has been infected by the parasite, causing localised cell death. This deprives the parasite of its source of food and prevents parasitic spread to other cells.
Some particularly nasty parasites ...
A parasite for sore eyes!
Cymothoa exigua Biting Your Tongue, So You Don’t Have To! Sacculina carcini: Reasons You Shouldn’t Pick up a Hitchhiker Screw worms: Causing Trouble Right out of the Hatch
There may be several different organisms growing in an infected sample, although most will have appeared after the initial disease has weakened the host.
Koch's postulates need to be satisfied in order to identify the organism that is causing a disease
Koch was one of the original researchers into tuberculosis, in the 19th century. In an attempt to define what an infectious disease actually is, he formulated his famous postulates, which now bears his name. Basically if,
1. An organism can be isolated from a host suffering from the disease AND
2. The organism can be cultured in the laboratory AND
3. The organism causes the same disease when introduced into another host AND
4. The organism can be re-isolated from that host THEN
The organism is the cause of the disease and the disease is an infectious disease
An interaction between species where neither species is dependent on the other for its existence, but in this case only ONE of the partners benefits from the association
In the strictest truth very few of these relationships exist, as it is very unlikely the two organisms can live together without them affecting each other
Most examples of commensalism relationships are feeding or protection
Commensalism : An Example
PORCELAIN ANEMONE CRABS AND THEIR HOST ANEMONES
- These crabs are primarily suspension-feeding animals, and they use their large basket-like feeding appendages to sweep the water to get their food
- They don't harm the anemones, but they benefit by gaining protection from their host. Few fish will hazard getting eaten by an anemone simply for the chance to snack on the crab
An interspecific interaction that benefits BOTH species
They exchange food or provide shelter or protection, but may still be able to live an independent life
In return for shelter, the clownfish cleans the anemones, chasing away their predators and dropping scraps of food for the anemone to eat
There are many different examples of mutualistic relationships:
Plants and microbes e.g. rhizobium in root nodules
Protists and fungi e.g. lichen
Terrestrial plants and insects, e.g. pollination
Animals and protists/bacteria e.g. ruminants, corals
Animals and other animals e.g. crocodile and plover bird
All orchids depend on fungi called mycorrhizae at some point during their life cycle
The fungi grow partly on the root and aid the plant in the uptake of nutrients
The fungi benefit as they ingest some of the food from plant photosynthesis
Most plants have to search through the soil with their roots to find nitrogen which is a critical nutrient required for growth
Legumes, however, form symbiotic relationships with Rhizobium bacteria
The Rhizobium live in little nodules in the roots of the legumes and fix atmospheric nitrogen into ammonium or nitrate, forms of nitrogen that can be used by the plant i.e. Rhizobium turn air into fertiliser!
The plant benefits because it gains nitrogen.
The bacteria benefit because they get sugars and nutrients to survive