Let us learn about the top four important lineages of protists. The lineages are: 1. Euglenozoa 2. Alveolata 3. Stramenopila and Rhodophyta 4. Chlorophya.
Contents
Lineages of Protists # 1. Euglenozoa:
(a) Euglenoids:
The Euglenoids diverged early as free-living eukaryotes and had mitochondria in their cells. They have individuals that were difficult to distinguish in either the plants or animals (amongst the protists). And about one-third of 40 genera of euglenoids have chloroplasts and are fully autotrophic; and still others lack chloroplasts, ingest their food, and are heterotrophic.
It can, however, be mentioned that some euglenoids with chloroplast may become heterotrophic in dark (in this case chloroplasts become small and nonfunctional). The euglenoids vary in range from 10 to 500 µ and because of a flexible covering called pellicle (made up of interlocking proteinaceous strips arranged in a helical pattern), which lie within the plasma membrane, a euglenoid can change its shape rapidly.
The reproduction is performed by mitotic cell division and the nuclear envelope remains intact during mitosis. The Euglena is a characteristic organism of this phylum. It has two flageila, contractile vacuoles for maintaining osmotic pressure within the organism, numerous small chloroplasts (having chlorophyll a and b, together with carotenoids).
Certain paramylon granules are also seen as areas having stored food reserves.
The Euglenozoa include free-living and parasitic protists capable of moving with flagella. In Euglena the two flagella are attached at the base of a flask-shaped opening called reservoir (located at the anterior end of the cells). It has one light-sensitive organ called stigma, and helps these photosynthetic organisms move towards light.
The chloroplasts in Euglena are believed to have obtained via endosymbiosis and probably evolved from a symbiotic relationship through ingestion of green algae. The euglenoid species of Peranema and Euglena are among the most familiar organisms of all flagellates.
(b) Kinetoplastids:
The kinetoplastid flagellates form a second major group within the Euglenozoa. They are colourless heterotrophs. They have about 600 free-living species and most of them are parasites. The term kinetoplastid refers to a single mitochondrion in each ceil.
Their conspicuous mass of DNA, called kinetoplast, remains located within a single, large mitochondrion. Trypanosomes are kinetoplastids that cause familiar diseases like Trypanosomiasis, also known as African sleeping sickness (and brings extreme fatigue and lethargy).
The causative species are Trypanosoma brucei rhodasiense and T.b. gambiense and the disease is transmitted by the tsetse fly, belonging to the genus Glossina. Both male and female tsetse flies suck blood and can therefore transmit the disease. Once injected a tsetse fly remains a vector of trypanosomiasis for the remaining life. The life span of a tsetse fly is about 3 months.
Other diseases of genera Leishmania and T. cruzi are Leishmaniasis and chagas disease. East coast fever is yet another disease caused by trypanosomes. Leishmaniansis, transmitted by sand flies, causes skin sores and about 1.5 million new cases are reported each year and in some cases internal organs also get affected leading to death.
It is also cabled Kala-azar, and is reported from Eurasia, Africa and America. Tiny biting flies known as sand-flies or no-see-ums (Ceratopogonidae) are the blood sucking insect hosts of this protozoan.
Chagas disease of tropical America is caused by T. cruzi (whose parasites reside in small mammals and are transmitted by blood sucking bugs) causes extensive damage to liver, spleen and heart muscles in human. Kinetoplastids undergo binary fission and most trypanosomes .reproduce asexually, however mating, syngamy and meiosis (as reported first time in 1986) suggest sexual cycle.
Lineages of Protists # 2. Alveolata:
These organisms derive their name from one common trait, i.e., the presence of either a space or alveoli (hence the name alveolata) below their plasma membranes.
Its members include:
Dinoflagellates (photosynthetic unicells with two flageila and often thrive in both marine and freshwater environments, e.g., Noctiluca and Ceratiumetc.); apicomplexes (sporeforming parasite of animals, e.g., malarial parasite Plasmodium sp.) and ciliates (they are heterotrophic; unicellular protists, e.g., Paramecium sp.) with a large number of cilia on their body.
The dinolagellates, apicomplexes and ciliates have had a common lineage despite their diverse modes of locomotion.
Lineages of Protists # 3. Stramenopila and Rhodophyta:
The stramenopiles include brown algae, diatoms and water molds (oomycetes). The brown algae life-cycle shows alternation of generation having a sporophyte (diploid) and a gametophyte (haploid) stage. The kelps, as we know, the brown algae had some very large individuals developed out of repeated mitosis. The gametophytes are, on the contrary, smaller, filamentous individuals.
The members of the phylum chrysophyta, the Diatoms, are unicellular organisms with unique double shells made of opaline silica with unique patterns of arrangements. The diatoms also produce a unique carbohydrate called chrysolaminarin and often contain chl a and c.
All oomycetes are either parasitic in nature or saprobic (feeding on dead organic matter). They differ from other protists in having motile spores or zoospores having two unequal, opposite facing flageila.
Most of the oomycetes are aquatic, but some of their terrestrial forms are plant pathogens (e.g., Phytophthora infestans, is a causative agent of late blight of potatoes). The oomycetes lack photosynthesis.
The Rhodophyta (red algae) lack flageila and centrioles and have the photosynthetic pigments: phycoerythrin, phycocyanin and allophyco cyanin, which are arranged within the structures called phycobilisomes. They also reproduce using alternation of generations. The Rhodophyta have more than 7000 species.
Lineages of Protists # 4. Chlorophya:
As the ancestors of the plant kingdom, the multicellular green algae often find a special piece in the study of early life forms, because of their unusual diversity.
It had two distinct lineages: the chlorophytes (fossil record dating back to 900 million years) and the streptophyta (that later gave rise to the land plants). One of the common, most studied examples of chlorophyta include, Chlamydomonas sp., a microscopic, biflagellated alga, haploid, dividing asexually (also reproduces sexually) and uses rhodopsin molecules in its eyespot, to locate directions in swimming by receiving light.
Chlamydia:
Smallest recognized bacteria (0.25 nm). The chlamydias are gram negative eubacteria that are obligate intracellular parasite with little metabolic activity. Three species of chlamydia are recognized C. psittaci, the causative agent of Psittacosis; C. trachmatis, the causative agent of trachoma and other human diseases and C. pneumonia the causative agent of Respiratory syndrome.
Psittacosis:
Respiratory diseases is an epidemic of birds that is occasionally transmitted to human and (infection in lung) cause Pneumonia like symptoms.
Trachoma:
Disease of eye, endemic of hot, dry region of Northern Africa and South Western Asia. An infection of cornea and conjunctiva characterized by scarring of the cornea. Trachoma is the leading cause of blindness in human.
Epidemic- The occurrence of disease is usually high number in localized region.
Endimic- disease constantly present, usually in low number.
Molecular and Metabolic Properties of Chlamydia:
Chlamydias have gram-negative type cell walls and they have both DNA and RNA, that is, they are cells not viruses. Indeed, for some-time. It was thought that chlamydias were “energy parasites” obtaining not only biosynthetic intermediate from their host, but also ATP. 1 m B chromosome of C. trachomatis contain gene for ATP synthesis and even contain a complement of gene encoding peptidoglycan synthesis.
1. C. trachomatis chromosomess lacks a gene encoding the protein FtsZ (A key protein involve in septum formation during cell division)
2. C. trachomatis has picked up some host genes that may encode functions that assist it in its pathogenic life style.
Tow cellular types are seen in in life cycle of chlamydia that is a small, dense, cell, called an elementary body, which is relatively resistant to drying, and a large less dense cell, called a reticulate body, which divides by binary fission and is the vegetative form.
Elementary bodies are non-multiplying cells specialized for transmission. Whereas reticulate bodies are noninfectious forms that are specialized in intracullar multiplication. When an elementary body inters a cell, and enlarges to form a reticulate body that divides by binary fission to form a series of small dense parasites. These develop into highly infectious elementary bodies.
The development cycle alternates between two forms:
(I) The elementary body, 0.3 µm in diameter, is specilised for survival when released from the cell
(II) reticulate body, up-to 1 µm in diameter and engaged in intracellular multiplication (Fig. 15.1).
With-in few hours after entry into the host cells, the EBs begin to undergo profound changes in their cell envelopes and the characteristic central condensate begins to disperse to form a more homogenous cytoplasm in which nucleic acid and ribosome are present.
The resulting RBs continue to grow in size and in 10-15 hours after infection binary fission begins. At 20-30 hours after infection some of the RBs develop on central condensation of cytoplasmic contents decrease in size and become typical EBs that are released when the host cell disintegrates and can then infect other cells.
a. Chlamydial metabolism is very different from other Gram -negative bacteria. Chlamydias can-not catabolize carbohydrates and other substances and synthesize ATP.
b. C. psittaci lack both fiavo protein and cytochrome does have membrane translocate that acquire Host ATP in exchange for ADP.
c. EB lack much metabolic activity and can-not take in ATP or synthesized protein.
When virus infects a cell, it loses its structural integrity and liberate nucleic acid. When an EBs enters a cell, however, although it changes form, it remains a structural unit and enlarges and begins to undergo binary fission.
Lineages of Protists # 5. Choanoflagellida and Protists (Often Difficult to Categorize):
The choanoflagellates are believed to be the common ancestors of the sponges and animals. Their colonial forms resemble fresh water sponges and feed on bacteria separated by collars from water.
The strong homology between a surface receptor (a tyrosine kinase receptor) found in choanoflagellates and sponges also shows the close relationship of choanoflagellates to animals (Fig. 15.2). That surface receptor initiates a signaling pathway involving phosphorylation.
The 600 sp. of choanoflagellates are mostly tiny, where the feeding is enabled by the beating of the flagellum producing water current enabling the collar to filter bacteria and other organic particulate matter. It feeds by ingestion and phagocytosis. The choanoflagellates may be solitary, or colonial, attached or free swimming.
Amoebas:
The ameboid protozoans, (often placed in Sarcodina subphylum) have flowing extensions of the body coming out, called as pseudopodia. The three principal groups of ameboid protozoa are the amebas (Caryoblasta, Heterolobosa and Amoebozoa), the forams (Foraminiferea), and the Actinopods (Radiolaria, Acantharea and Heliozoa).
Foraminifera:
They are heterotrophic marine protists, often benthic (bottom dwelling) in nature. Informally they are often referred as ‘forams’ based on their building material, Foraminifera. The forams are used as excellent geological markers in searching for oil bearing strata in petroleum industry.
Their calcareous tests are well preserved in fossils; and 40,000 of the 45,000 described species of forams are fossil species. The largest forams, members of the deep-sea Xenophyophorea are relatively big in diameter (few centimeters).
Certain well known genera of forams are as follows:
Allogromiina:
Iridia, Myscotheca
Textulariina:
Allogromia, Textularia and Ammodiscus
Miliolina:
Their calcareous test resembles porcelain, e.g., Amphisorus (mermaid’s penny) and Pyrgo (Ooze former).
Rotaliina:
Calcareous test is hyaline (glassy appearance) having pores.
Bulimina, Lagena, Homotrema and Discorbis sp.
The Fungi (Mycota):
The term fungi is derived from the mushrooms (Greek, mykes, Latin, fungus). They are eukaryotes with cell wall, liquid filled intracellular vacuoles, and visible streaming of the cytoplasm (with microscope) and lack motility. They lack photosynthetic pigments; are chemo-organoheterotrophs and grow aerobically obtaining their energy by oxidation of organic substances.
Fungi (as compared to advanced plants) show relatively less developed morphological differentiation.
Some of the important characteristics of fungi are as follows:
All fungi are heterotrops by absorption. Fungal cell walls include chitin. Some fungi have filamentous bodies (hyphae). The vegetative body is called thallus, and the total of the hyphal mass of a fungal thallus is called the mycelium. The fungal hyphae elongate at their apices (apical growth).
In most fungi every part of the mycelium has the potential for growth (elongation) and a small piece of mycelium is sufficient for inoculation (to produce a new thallus). The fungi reproduce both asexually and sexually (by chemical signaling the hyphae get attracted to each other and fuse).
However, the forms and mechanisms used in reproduction are diverse, complex and are often used as the basis of classification. Most fungi can reproduce both ways asexually as well as sexually.
The fungi obtain nutrients by external digestion of dead or living organisms. Their metabolic pathways provide resources for human and are good source of minerals and proteins. And the fungi are viewed as an important decomposers in terrestrial ecosystems.
The fungi also participate in many symbiotic associations:
Lichens:
A lichen is a mutualistic association between a fungus and an algae or cyanobacterium.
Mycorrhizae:
They are mutualistic associations between fungi and the roots of the plants.
They are often divided into two broad categories:
(a) Arbuscular mycorrhizae (e.g., genera like Glomales sp.) form associations with about 70% plants sp. by inserting their hyphal tips inside the roots of plants;
(b) Ectomycorrhizae (often involve Basidiomycetes) and are formed in Orchids, Pines, Oaks and Eukalyptus etc. at the surface of the above species of plants.
Mutualistic Animal Symbioses:
They form internal gut flora of grazing animals in symbiotic relationships and are also formed by ants.
Fungal Parasites and Pathogens:
As they develop a variety of substances, they often affect agriculture and human health (through fungal toxins) causing several skin and other diseases.
Phylogenetic Relationships and Classification of Fungi:
The fungal phylogeny is experiencing a rapid and exciting change whereby we now believe that fungi are more closely related to animals than to plants.
Four major groups of fungi that are often mentioned are as follows:
(1) Chytridiomycota
(2) Zygomycota
(3) Basidiomycota
(4) Ascomycota
The Ascomycota and Basidiomycota are monophyletic, but the other two groups are not.
The chytrids are believed to be the closest living relatives of the first fungal ancestors.
Some of the most important model organisms of various fungi are being described here in brief:
(1) Acrasiomycetes (Cellular Slime Molds):
They are called cellular slime molds to distinguish them from the Plasmodia (or true slime molds). They are also called ‘social amoebae’, because of their curious formation of well-formed fruiting bodies. Its members are free-living soil inhabitants (e.g., Dictyostelium sp.).
(2) Myxomycetes (True Slime Moulds):
Its fruiting bodies are similar to those of the myxobacteria and of the Acrasiomycets, although they are larger in size than either of these. These fungi grow in damp places in wood, on fallen leaves, bark, timber etc. (e.g., Lycogala sp., Fuligo septica etc.)
Phycomycetes (Lower Fungi):
They comprise of a large group of fungi having unseptate and multinucleate vegetative boides, despite multiple branching of the hyphae. It is also called Coenocytic thallus. Most of the lower fungi generate spores in sporangia.
Chytridiomycetes :
They are predominantly aquatic fungi, but some forms are also present in soil. They are microscopic in size and cell wall is composed of chitin. Some forms are parasitic on plankton and aquatic plants. One species Rhizophidium pollinis parasitises pine pollens.
Oomycetes:
They are aquatic and terrestrial fungi that reproduce asexually by-means of biflagellated zoospores. Saprolegnia and Leptomitus are aquatic species and are called ‘water moulds’. Peronosporales show advanced terrestrial mode of life and complete their life cycle (as obligate parasites) inside the higher plants. They also form zoospores.
Phytophthora infestans causing Potato blight and Plasmopara viticola are noted pathogens causing great losses due to blight and false mildew of vine. Saprolegnia sp. is also widely distributed, sp. that can be easily isolated and cultured.
Zygomycetes:
It derives its name from the zygospore, formed during the sexual reproduction. The coenozygote or zygospore arises from the fusion of two gametangia (gametangiogamy) which connect the parental hyphae by a kind of ‘bridge’ or ‘yoke’. They are amongst the most developed forms of phycomycetes adapted for terrestrial life.
They can be further divided into three important orders:
Mucorales (growing on rotting organic material, e.g., Rhizopus nigricans, the common bread mould and Mucor racemosus etc.); Entomophthorales and Zoopagales.
(4) Ascomycetes:
The Ascomycetes, along with Basidiomycetes, form the higher fungi (Eumycetes), have a septated mycelium and show formation of conidiospores. They lack any flagellated cells. The name Ascomycetes, is from an ascus, bearing ascospores. It shows both karyogamy and rheiosis inside the ascus.
Since the ascus stage is the end stage of sexual reproduction, it is also designated as the perfect or main fruiting formation. The Ascomycetes also reproduce asexually by means of conidia. This subsidiary ‘fruiting formation’ is then called as the ‘fungi imperfecti’ or deuteromycetes.
Yeasts:
They are ‘budding fungi’ and belong to protoaxomycetes. The Saccharomycetaceae (the true yeasts) lack a mycelium. The brewer’s and baker’s yeast are physiological variants of Saccharomyces cerevisiae. In nature the yeasts are found in all major habitats where fermentable, sugar-rich liquid extracts and secretions are available.
Plectomycetes:
They are cleistothecial fungi (e.g., Aspergillus sp. and Penicillium sp. belong to this group) and are identified by their conidial stage. They bear highly branched, multinucleate mycelia having a large number of conidiophores, which arise individually on the hyphae.
Aspergilli and Penicillia can damage organic matter such as timber, fruits, leather etc. Penicillium notatum and P. chrysogenum are well-known penicillin producers.
Pyrenomycetes:
They are perithecial fungi. A typical perithecium has its own, true wall. The asci arise in the base or in the lower part of the flask-shaped fruiting body. A large number of pathogenic fungi belong to pyrenomycetes.
And they include obligate parasitic true mildews (e.g., Erysiphe, Uncinula sp. etc.) and also the common object of genetic studies, Neurospora.
(5) Basidiomycetes:
They are considered as the most highly developed group of fungi. Their unique organ is basidium, an upright fungal cell that is often corresponding to an ascus. Four basidiospores are usually given off from this structure. They are mononucleate and haploid. The mycelium of basidiomycetes consists of septate hyphae. Its white strands, of hyphal bundles are called as rhizomorphs.