| Euglenid | |
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| Euglena viridis, byEhrenberg | |
Scientific classification | |
| Domain: | Eukaryota |
| Clade: | Discoba |
| Phylum: | Euglenozoa |
| Class: | Euglenida Butschli 1884,emend. Simpson 1997 |
| Major groups | |
| Synonyms | |
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Euglenids oreuglenoids are one of the best-known groups ofeukaryoticflagellates: single-celled organisms withflagella, or whip-like tails. They are classified in the phylumEuglenozoa,classEuglenida orEuglenoidea. Euglenids are commonly found in fresh water, especially when it is rich in organic materials, but they have a few marine andendosymbiotic members. Many euglenids feed byphagocytosis, or strictly bydiffusion. A monophyletic subgroup known asEuglenophyceae havechloroplasts and produce their own food throughphotosynthesis.[3][4][5] This group contains the carbohydrateparamylon.
Euglenids split from otherEuglenozoa (a larger group of flagellates) more than a billion years ago. Theplastids (membranous organelles) in all extant photosynthetic species result from secondaryendosymbiosis between a euglenid and a green alga.[6]
Euglenoids are distinguished mainly by the presence of a type of cell covering called apellicle. Within its taxon, the pellicle is one of the euglenoids' most diverse morphological features.[7] The pellicle is composed of proteinaceous strips underneath the cell membrane, supported by dorsal and ventralmicrotubules. This varies from rigid to flexible, and gives the cell its shape, often giving it distinctive striations. In many euglenids, the strips can slide past one another, causing an inching motion calledmetaboly. Otherwise, they move using their flagella.
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The first attempt at classifying euglenids was done byEhrenberg in 1830, when he described the genusEuglena and placed it in the Polygastrica of family Astasiae, containing other creatures of variable body shape and lackingpseudopods orlorica. Later, various biologists described additional characteristics forEuglena and established different classification systems for euglenids based on nutrition modes, the presence and number offlagella, and the degree ofmetaboly. The 1942 revision by A. Hollande distinguished three groups, Peranemoidées (flexible phagotrophs), Petalomonadinées (rigid phagotrophs) and Euglenidinées (phototrophs), and was widely accepted as the best reflection of the natural relationships between euglenids, adopted by many other authors.[8] Gordon F. Leedale expanded on Hollande's system, establishing six orders (Eutreptiales,Euglenales,Rhabdomonadales,Sphenomonadales,Heteronematales andEuglenamorphales) and taking into account new data on their physiology andultrastructure. This scheme endured until 1986, with the sequencing of theSSU rRNA gene fromEuglena gracilis.[8]
Euglenids are currently regarded as a highly diverse clade withinEuglenozoa, in theeukaryotic supergroupDiscoba.[9] They are traditionally organized into three categories based on modes of nutrition: thephototrophs (Euglenophyceae), theosmotrophs (mainly the 'primary osmotrophs' known asAphagea), and thephagotrophs, from which the first two groups have evolved.[10] The phagotrophs, althoughparaphyletic, have historically been classified under the name ofHeteronematina.[9]
In addition, euglenids can be divided into inflexible or rigid euglenids, and flexible or metabolic euglenids which are capable of 'metaboly' or 'euglenid motion'. Only those with more than 18 protein strips in their pellicle gain this flexibility. Phylogenetic studies show that various clades of rigid phagotrophic euglenids compose thebase of the euglenid tree, namelyPetalomonadida and the paraphyletic 'Ploeotiida'. In contrast, all flexible euglenids belong to amonophyletic group known asSpirocuta, which includes Euglenophyceae, Aphagea and various phagotrophs (Peranemidae,Anisonemidae andNeometanemidae). The current classification of class Euglenida, as a result of these studies, is as follows:[10][11][12][13][14]
The classification of euglenids is still variable, as groups are being revised to conform with their molecularphylogeny. Classifications have fallen in line with the traditional groups based on differences in nutrition and number of flagella; these provide a starting point for considering euglenid diversity. Different characteristics of the euglenids' pellicles can provide insight into their modes of movement and nutrition.[18]
As with otherEuglenozoa, the primitive mode of nutrition isphagocytosis. Prey such asbacteria and smaller flagellates is ingested through acytostome, supported by microtubules. These are often packed together to form two or more rods, which function in ingestion, and inEntosiphon form an extendable siphon. Mostphagotrophic euglenids have two flagella, one leading and one trailing. The latter is used for gliding along thesubstrate. In some, such asPeranema, the leading flagellum is rigid and beats only at its tip.
Osmotrophic euglenids are euglenids which have undergoneosmotrophy.
Due to a lack of characteristics that are useful fortaxonomical purposes, the origin of osmotrophic euglenids is unclear, though certain morphological characteristics reveal a small fraction of osmotrophic euglenids are derived from phototrophic and phagotrophic ancestors.[19]
A prolonged absence of light or exposure to harmful chemicals may causeatrophy and absorption of the chloroplasts without otherwise harming the organism. A number of species exists where a chloroplast's absence was formerly marked with separate genera such asAstasia (colourlessEuglena) andHyalophacus (colourlessPhacus). Due to the lack of a developed cytostome, these forms feed exclusively by osmotrophic absorption.
Although euglenids share several common characteristics with animals, which is why they were originally classified as so, no evidence has been found of euglenids ever usingsexual reproduction. This is one of the reasons they could no longer be classified as animals.[dubious –discuss]
For euglenids to reproduce,asexual reproduction takes place in the form ofbinary fission, and the cells replicate and divide duringmitosis andcytokinesis. This process occurs in a very distinct order. First, thebasal bodies and flagella replicate, then thecytostome and microtubules (the feeding apparatus), and finally the nucleus and remainingcytoskeleton. Once this occurs, the organism begins to cleave at the basal bodies, and this cleavage line moves towards the center of the organism until two separate euglenids are evident.[20] Because of the way that this reproduction takes place and the axis of separation, it is called longitudinalcell division or longitudinal binary fission.[21]
The earliest fossil of euglenids is attributed toMoyeria, which is interpreted as possessing a pellicle composed of proteinaceous strips, the defining characteristic of euglenids. It is found inMiddle Ordovician andSilurian rocks, making it the oldest fossil evidence of euglenids.[1][2]
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