Description
by GUY BRUGEROLLE and DAVID PATIERSON The order Pelobiontida includes four genera of mostly free-living heterotrophic protists. They are usually found in anaerobic or microaerophilic habitats such as stagnant water which is rich in organic matter. They occur in freshwater and marine habitats. One species is endocommensal. The most usual trophic form in all but Pelomyxa is a flagellated cell , but amoeboid and encysted forms are known from some species and may be characteristic of all species . Flagellated cells have one or many flagella and move by flagellar motion and/or by (a sometimes distinctive type of) amoeboid streaming . Brugeroll e, 1991 a). Upper left : typi ca l uniflagell a ce ll. Th e si ngl e flagellum /basal body (b) is connect to the nu cle us by a cone of mi c rotubules . T microtubul es arise from a microtubule-organizing- center stru cture that is attached to the base of the basal body . A lateral band of microtubul es extends towards th e ce ll surface . IONTIDA The order has been more clearly circumscribed after the discovery of flagella in Pe/omyxa by Griffin (1988) and as a result of ultrastructural studies of various species (Brugerolle 1991 a, 1991 b; Mylnikov, 1991; Simpson et al ., 1997) . Light and electron microscopic studies have shown that flagella/basal bodies occur singly . In some cases the flagella have an 'n+n' substructure rather than the usual '9+2 ' arrangements of microtubules , and there may also be abnormalities in the basal body . Flagella are often attached to the nucleus by a cone or cape of microtubules radiating from the basal body (Figs. 1, 2a,b ,c) . The microtubules arise from a region at the base of or around the basal body . This has been demonstrated in Mastigina (Brugerolle , 1982) , Pe/omyxa (Griffin , 1988), Mastigamoeba (Brugerolle , 1991 a, 1991 b; Simpson et al. , 1997), and Mastigella (A . G. B. Simpson and C. Bernard , unpubl. observ .). The cytoplasm is rich in microfilaments , and cytoplasmic movements may lead to locomotion (especially in Mastigina and Pelomyxa) or to the ingestion of food . Most species move by the slow, undulating actions of the flagella . The endoplasmic reticulum is sometimes concentrated in large stacks of cisternae . There are no dictyosomes nor mitochondria; glycogen is used as a reserve material. Cells contain one to many nuclei . These contain large nucleol i. Mitosis involves an intranuclear spindle ; the nucleoli are persistent and divide , but without ultrastructural studies of division it is unclear if the nuclear envelope is pers istent or if it breaks down . Because of the absence of mitochondria and Golgi apparatus and because of the simplicity of the mastigont system , pelobionts have been regarded as an early offshoot in eukaryotic evolution and ted have also been classified as Archeozoa (Cavalier- ed Smith , 1991 ). The term Archezoa was h e previously used by Perty and Haeckel with different meanings and therefore is not adopted here . The primitive status of the Pelobionts is not supported by current molecular studies (Hinkle et al. , 1994) . General reviews of the group are found in Lemmermann , 1914 (species PELOBIONT diversity) , Mylnikov , 1991 (generic diversity) , and Brugerolle , 1991 a , 1991 b (ultrastruct- ure). KEY CHARACTERS All pelobionts have an amoeboid body which may give rise to pseudopodia and have one (or more , e.g. in Pelomyxa) flagella. The body gives rise to blunt pseudopodia , the form of which may have some diagnostic value-although several species are known to have variable pseudopodia! form. Amoeboid motion occurs in Mastigina and Pelomyxa . This motion is distinctive with the cytoplasm flowing along the center of the body and spreading out , fountain-like , at the anterior end. The posterior end of the cell has a bulbous or filose uroid , and food particles may be ingested here. In most cells of genera other than Pe/omyxa a flagellum is located anteriorly in moving cells . The relationship between the nucleus and flagellum is used to segregate the genera Mastigamoeba and Mastigella-the latter genus being sa id to have a greater relative difference between the two organelles. The flagellum is often very long and beats languidly , typically with a single wave passing from base to tip-the beat pattern is distinctive . Members of this genus may be confused with some other taxa (e .g. Cercomonas) , and electron microscopy is desirable to confirm that the organ ism is a pelobiont. Taxa are assigned to
Type species
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by GUY BRUGEROLLE and DAVID PATIERSON The order Pelobiontida includes four genera of mostly free-living heterotrophic protists. They are usually found in anaerobic or microaerophilic habitats such as stagnant water which is rich in organic matter. They occur in freshwater and marine habitats. One species is endocommensal. The most usual trophic form in all but Pelomyxa is a flagellated cell , but amoeboid and encysted forms are known from some species and may be characteristic of all species . Flagellated cells have one or many flagella and move by flagellar motion and/or by (a sometimes distinctive type of) amoeboid streaming . Brugeroll e, 1991 a). Upper left : typi ca l uniflagell a ce ll. Th e si ngl e flagellum /basal body (b) is connect to the nu cle us by a cone of mi c rotubules . T microtubul es arise from a microtubule-organizing- center stru cture that is attached to the base of the basal body . A lateral band of microtubul es extends towards th e ce ll surface . IONTIDA The order has been more clearly circumscribed after the discovery of flagella in Pe/omyxa by Griffin (1988) and as a result of ultrastructural studies of various species (Brugerolle 1991 a, 1991 b; Mylnikov, 1991; Simpson et al ., 1997) . Light and electron microscopic studies have shown that flagella/basal bodies occur singly . In some cases the flagella have an 'n+n' substructure rather than the usual '9+2 ' arrangements of microtubules , and there may also be abnormalities in the basal body . Flagella are often attached to the nucleus by a cone or cape of microtubules radiating from the basal body (Figs. 1, 2a,b ,c) . The microtubules arise from a region at the base of or around the basal body . This has been demonstrated in Mastigina (Brugerolle , 1982) , Pe/omyxa (Griffin , 1988), Mastigamoeba (Brugerolle , 1991 a, 1991 b; Simpson et al. , 1997), and Mastigella (A . G. B. Simpson and C. Bernard , unpubl. observ .). The cytoplasm is rich in microfilaments , and cytoplasmic movements may lead to locomotion (especially in Mastigina and Pelomyxa) or to the ingestion of food . Most species move by the slow, undulating actions of the flagella . The endoplasmic reticulum is sometimes concentrated in large stacks of cisternae . There are no dictyosomes nor mitochondria; glycogen is used as a reserve material. Cells contain one to many nuclei . These contain large nucleol i. Mitosis involves an intranuclear spindle ; the nucleoli are persistent and divide , but without ultrastructural studies of division it is unclear if the nuclear envelope is pers istent or if it breaks down . Because of the absence of mitochondria and Golgi apparatus and because of the simplicity of the mastigont system , pelobionts have been regarded as an early offshoot in eukaryotic evolution and ted have also been classified as Archeozoa (Cavalier- ed Smith , 1991 ). The term Archezoa was h e previously used by Perty and Haeckel with different meanings and therefore is not adopted here . The primitive status of the Pelobionts is not supported by current molecular studies (Hinkle et al. , 1994) . General reviews of the group are found in Lemmermann , 1914 (species PELOBIONT diversity) , Mylnikov , 1991 (generic diversity) , and Brugerolle , 1991 a , 1991 b (ultrastruct- ure). KEY CHARACTERS All pelobionts have an amoeboid body which may give rise to pseudopodia and have one (or more , e.g. in Pelomyxa) flagella. The body gives rise to blunt pseudopodia , the form of which may have some diagnostic value-although several species are known to have variable pseudopodia! form. Amoeboid motion occurs in Mastigina and Pelomyxa . This motion is distinctive with the cytoplasm flowing along the center of the body and spreading out , fountain-like , at the anterior end. The posterior end of the cell has a bulbous or filose uroid , and food particles may be ingested here. In most cells of genera other than Pe/omyxa a flagellum is located anteriorly in moving cells . The relationship between the nucleus and flagellum is used to segregate the genera Mastigamoeba and Mastigella-the latter genus being sa id to have a greater relative difference between the two organelles. The flagellum is often very long and beats languidly , typically with a single wave passing from base to tip-the beat pattern is distinctive . Members of this genus may be confused with some other taxa (e .g. Cercomonas) , and electron microscopy is desirable to confirm that the organ ism is a pelobiont. Taxa are assigned to