The name ‘proto-zoa’ literally means ‘first animals’ and early classification systems grouped the protozoa as basal members of the animal kingdom. However, they were recognized as a discrete assemblage on the basis of their unicellularlity and were assigned to the taxon Protozoa (but still invariably figured as the trunk of the animal tree of life). Members of the subkingdom Protozoa are quite disparate; indeed the taxon has never been considered a natural assemblage of organisms but rather one of convenience. More recently, the protozoa have been classified together with several algal and fungal groups in the kingdom Protista (protozoa representing the motile protists). Irrespective of contemporary classification systems, most parasitological texts continue to use the name protozoa for historical reasons.
Protozoa are eukaryotic organisms (with a membrane-bound nucleus) which exist as structurally and functionally independent individual cells (including those species which are gregarious or form colonies). None have adopted multicellular somatic organisation characteristic of metazoan organisms. Instead, protozoa have developed relatively complex subcellular features (membranes & organelles) which enable them to survive the rigours of their environments. Most protozoa are microscopic organisms, only a few grow to a size large enough to be visible to the naked eye. As unicellular eukaryotes, protozoa display all the same essential life activities as higher metazoan eukaryotes: they move about to survive, feed and breed.
Biodiversity
Four main groups of protozoa are recognized on the basis of their locomotion using specialized subcellular and cytoskeletal features:
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Amoebae use pseudopodia (singular: pseudopodium) to creep or crawl over solid substrates. Pseudopodia (or ‘false feet’) are temporary thread-like or balloon-like extensions of the cell membrane into which the protoplasm streams. Similar amoeboid motion has been observed in cells of many life-forms, especially phagocytic cells (e.g. human macrophages).
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Flagellates use elongate flagella (singular: flagellum) which undulate to propel the cell through liquid environments. Flagella are ‘whip-like’ extensions of the cell membrane with an inner core of microtubules arranged in a specific 2+9 configuration (2 single central microtubules surrounded by 9 peripheral doublets). This configuration is conserved throughout eukaryotic biology, many organisms produce flagellated cells (e.g. human spermatozoa).
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Ciliates use numerous small cilia (singular: cilium) which undulate in waves allowing cells to swim in fluids. Cilia are ‘hair-like’ extensions of the cell membrane similar in construction to flagella but with interconnecting basal elements facilitating synchronous movement. Ciliated cells are found in specialized tissues and organs in many other higher life-forms (e.g. human bronchial epithelial cells).
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Sporozoa (‘spore-formers’) were originally recognized not on the basis of their locomotion, but because they all formed non-motile spores as transmission stages. Recent studies, however, have shown that many pre-spore stages move using tiny undulating ridges or waves in the cell membrane imparting a forward gliding motion, but the actual mechanisms involved are not yet known.
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Protozoan biodiversity (or species richness) includes counts (or estimates) of some 32,000 extant (living) species and another 34,000 extinct (fossil) species (especially foraminifera). Of those alive today, some 21,000 species occur as free-living organisms in aquatic or terrestrial environments, whereas the remaining 11,000 species are parasitic in vertebrate and invertebrate hosts. There are approximately 6,900 flagellate species (1,800 parasitic, 5,100 free-living), 11,550 amoebae species (250 parasitic, 11,300 free-living), 7,200 ciliate species (2,500 parasitic, 4,700 free-living) and 5,600 sporozoan species (all parasitic).
Life-cyclesMost protozoa have enormous reproductive potential because they have short generation times, undergo rapid sequential development and produce large numbers of progeny by asexual or sexual processes. These characteristics are responsible for many protozoan infections rapidly causing acute disease syndromes. Parasites may multiply by asexual division (fission/splitting or internal/endogenous budding) or sexual reproduction (formation of gametes and fertilization to form zygote, or unique process of conjugation where ciliates exchange micronuclei).
Protozoan developmental stages occurring within hosts generally consist of feeding trophozoites, and they may be found intracellularly (within host cells) or extracellularly (in hollow organs, body fluids or interstitial spaces between cells). While trophozoites are ideally suited to their parasitic mode of existence, they are not very resistant to external environmental conditions and do not survive long outside of their hosts. To move from host-to-host, protozoan parasites use one of four main modes of transmission: direct, faecal-oral, vector-borne and predator-prey transmission.
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direct transmission of trophozoites through intimate body contact, such as sexual transmission (e.g.Trichomonas spp. flagellates causing trichomoniasis in humans and bovine infertility in cattle).
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faecal-oral transmission of environmentally-resistant cyst stages passed in faeces of one host and ingested with food/water by another (e.g. Entamoeba histolytica, Giardia duodenalis and Balantidium coli all form faecal cysts which are ingested by new hosts leading to amoebic dysentery, giardiasis and balantidiasis, respectively).
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vector-borne transmission of trophozoites taken up by blood-sucking arthropods (insects or arachnids) and passed to new hosts when they next feed (e.g. Trypanosoma brucei flagellates transmitted by tsetse flies to humans where they cause sleeping sickness, Plasmodium spp. haemosporidia transmitted by mosquitoes to humans where they cause malaria).
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predator-prey transmission of zoites encysted within the tissues of a prey animal (e.g. herbivore) being eaten by a predator (carnivore) which subsequently sheds spores into the environment to be ingested by new prey animals (e.g. tissue cysts of the sporozoan Toxoplasma gondii being ingested by
cats
, and tissue cysts of the microsporan Thelohania spp. being ingested by crustaceans). |
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Taxonomic overview
Flagellates and amoebae are considered to be closely related, because some amoebae form transient flagellated stages (to aid in dispersal) and some flagellates exhibit intermittent amoeboid motion. Two groups of flagellates are recognized on the basis of the presence or absence of chloroplasts:
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Phytoflagellates with chloroplasts derive energy by photosynthesis. Most are free-living aquatic organisms and some exhibit periodic blooms (e.g. red tides). Others contain potent neurotoxins and cause paralytic shellfish poisoning.
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Zooflagellates without chloroplasts derive energy by the absorption of nutrients or the ingestion of food particles. Many species occur as free-living aquatic organisms whereas others live in insects and some vertebrates as symbiotes, commensals or parasites (several species cause major human diseases such as sleeping sickness, Chagas disease, kala azar and diarrhoea).
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