Phylogeny and Adaptive Radiation
Phylogeny and
Adaptive Radiation
Phylogeny
Protozoan groups represent early phylogenetic splits from all multicellular (metazoan) animals. The common ancestor of protozoan groups and metazoans was unicellular. Some protozoa, particularly among colonial and multicellular flagellates (Figure 11-11), show various degrees of cell aggregation and some differentiation that may parallel the body plans of early metazoa. Current evidence indicates that metazoans are the sister group of choanoflagellate protozoa, but available data do not strongly support this claim.
Traditionally, Sarcodina and Mastigophora were considered separate classes in phylum Protozoa. Observations that some flagellates could form pseudopodia, that some species of amebas had flagellated stages, and that a supposed ameba was really a flagellate without a flagellum, all seemed to support the concept of a phylum Sarcomastigophora. However, analyses of sequences of bases in genes, particularly the gene encoding the small subunit of ribosomal RNA , have provided strong evidence that neither Sarcodina nor Mastigophora are monophyletic groups. For example, unicellular green algae (members of class Phytomastigophorea, subphylum Mastigophora in the classification to follow) and higher plants are more closely related to animals than they are to other protozoan groups. Ameboid forms arose independently many times, and there is no justification for placing them all in a subphylum Sarcodina.
In fact, molecular evidence has almost completely revised our concepts of protozoan phylogeny. Sequence analyses suggest that the earliest branch from the eukaryote ancestor led to a group called Euglenozoa (including Euglena and its relatives, Trypanosoma and its relatives, and others), followed by divergence of slime molds. There was then a rapid radiation producing many algal and protozoan groups, in addition to fungi, plants, and animals. Two large algal/protozoan groups are now called stramenopiles and alveolates. Other major groups contain, for example, red algae and green algae and plants.
The stramenopile grouping includes brown algae, yellow algae, diatoms, and many others. Alveolates are dinoflagellates (many of which are phototrophic), apicomplexans (coccidians, Plasmodium spp., etc.), and ciliates. The nearest common ancestor of dinoflagellates and apicomplexans apparently acquired photosynthetic plastids by a secondary endosymbiotic event. (Secondary endosymbiosis is an endosymbiotic acquisition of an eukaryote by another eukaryote, rather than the combination of two prokaryotes, as in the origin of eukaryotes) The endosymbiosis in alveolates apparently occurred after divergence of ciliates, and photosynthetic ability was lost subsequently by ancestral apicomplexans, which are now all parasitic. It is believed that stramenopiles acquired their photosynthetic plastids by another secondary endosymbiosis, independent of other major autotrophic assemblages such as red and green algae.
Adaptive Radiation
We have described some of the wide range of adaptations of protozoan groups in the preceding pages. Ameboid forms range from bottom-dwelling, naked species to planktonic forms such as the foraminiferans and radiolarians with beautiful, intricate tests. There are many symbiotic species of amebas. Flagellated forms likewise show adaptations for a similarly wide range of habitats, with the added variation of photosynthetic ability in many species of Phytomastigophorea.
Within a single-cell body plan, the division of labor and specialization of organelles are carried farthest in the ciliates. These have become the most complex of all protozoa. Specializations for intracellular parasitism have been adopted by Apicomplexa and Microspora.
Classification of Protozoan Phyla
Four main protozoan groups were recognized traditionally: flagellates, amebas, spore formers, and ciliates. Biologists then realized that the phylum “Protozoa” was not valid and was composed of several more-or-less unrelated phyla. They further believed that there was sufficient evidence of a relationship between flagellates and amebas to support a phylum Sarcomastigophora. These concepts are reflected in the following classification. However, molecular data do not support this arrangement, and when new classification of protozoan groups becomes formalized, we will incorporate it. Groups for which there is evidence of polyphyly and/or paraphyly are marked with an asterisk (*).
Phylum Sarcomastigophora* (sar´komas- ti-gof´o-ra) (Gr. sarkos, flesh, + mastix, whip, + phora, bearing). Flagella, pseudopodia, or both types of locomotory organelles; usually with only one type of nucleus; typically no spore formation; sexuality, when present, essentially syngamy.
Subphylum Mastigophora* (mas-tigof´o-ra) (Gr. mastix, whip, + phora, bearing). One or more flagella typically present in adult stages; autotrophic or heterotrophic or both; reproduction usually asexual by fission.
Class Phytomastigophorea* (fi´to-mas-ti-go-for´e-a) (Gr. phyton, plant,+ mastix, whip, + phora, bearing). Plantlike flagellates, usually bearing chromoplasts (pigment-bearing bodies; chromoplasts with chlorophyll are chloroplasts), which contain chlorophyll. Examples: Chilomonas, Euglena, Volvox, Ceratium, Peranema, Noctiluca.
Class Zoomastigophorea* (zo'o-mas-ti-go-for´e-a) (Gr. zoon, animal,+ mastix, whip, + phora, bearing). Flagellates without chromoplasts; one to many flagella; ameboid forms with or without flagella in some groups; species predominantly symbiotic. Examples: Trichomonas, Trichonympha, Trypanosoma, Leishmania, Dientamoeba.
Subphylum Opalinata (o´pa-lina ´ta) (N.F. opaline, like opal in appearance,+ata, group suffix). Body covered with longitudinal rows of cilium-like organelles; parasitic; cytostome (cell mouth) lacking; two to many nuclei of one type. Examples: Opalina, Protoopalina.
Subphylum Sarcodina* (sar-ko-di´na) (Gr. sarkos, flesh,+ ina, belonging to). Pseudopodia typically present; flagella present in developmental stages of some; free living or parasitic.
Superclass Rhizopoda* (ri-zop´oda) (Gr. rhiza, root, + pous, podos, foot). Locomotion by lobopodia, filopodia, or reticulopodia, or by cytoplasmic flow without production of discrete pseudopodia. Composed of eight classes, some of which are listed here.
Class Lobosea* (lo-bo´se-a) (Gr. lobos, lobe). Pseudopodia lobose or filiform but produced from broader lobe; usually uninucleate; no fruiting bodies. Examples: Amoeba, Entamoeba, Acanthamoeba, Naegleria, Chaos, Arcella, Difflugia.
Class Eumycetozoea* (yu´miset- o-zo´e-a) (Gr. eu, good, true, + mykes, fungus, +zoon, animal). Ameboid feeding stage, flagellated stage present or absent; produce aerial fruiting bodies with one to thousands of spores. Examples: Dictyostelium, Physarum.
Class Filosea (fi-los´e-a) (L. filum, thread). Hyaline, filiform pseudopodia, often branching, sometimes rejoining; no spores or flagellated stages known. Examples: Vampyrella, Euglypha, Gromia.
Class Granuloreticulosea (gran´yu-lo-re-tik´yu-los´e-a) (L. granulum, dim. of granum, grain, + reticulum, dim. of rete, net). Delicate, finely granular or hyaline reticulopodia or, rarely, finely pointed, granular but nonrejoining pseudopodia. Examples: Allogromia, Fusulina, Textularia, Elphidium, Globigerina, other foraminiferans.
Superclass Actinopoda (ak´tinop ´o-da) (Gr. aktis, aktinos, ray, + pous, podos, foot). Often spherical, usually planktonic; pseudopodia in form of axopodia, with microtubular supporting structure.
Class Acantharea (a´kanthar ´e-a) (Gr. akantha, spine or thorn). Strontium sulfate skeleton composed of 20 or more radiating spines more or less joined in cell center; marine, usually planktonic. Examples: Acanthometra, Lithoptera.
Class Polycystinea (pol´e-sistin ´e-a) (Gr. polys, many, + kystis, bladder). Siliceous skeleton in most species, usually of solid elements, consisting of one or more latticed shells with or without radial spines, or of spicules; capsular membrane usually of grossly polygonal plates with many more than three pores; marine, planktonic. Example: Thalassicolla.
Class Phaeodarea (fe´o-dar´- e-a) (Gr. phaios, dusky, + daria, suffix). Skeleton of mixed silica and organic matter, consisting of usually hollow spines and shells; very thick capsular membrane with three pores; marine, planktonic. Examples: Aulacantha, Challengeron.
Class Heliozoea (he´le-o-zo´- e-a) (Gr. helios, sun,+ zoon, animal). Without central capsule; skeletal structures, if present, siliceous or organic; axopodia radiating on all sides; most species freshwater. Examples: Clathrulina, Actinophrys, Actinosphaerium.
Phylum Labyrinthomorpha (la´birinth- o-morf´a) (Gr. labyrinth, maze, labyrinth, + morph, form; + a, suffix). Small group living on algae; mostly marine or estuarine. Example: Labyrinthula.
Phylum Apicomplexa (a´pi-complex ´a) (L. apex, tip or summit, + complex, twisted around, + a, suffix). Characteristic set of organelles (apical complex) associated with anterior end present in some developmental stages; cilia and flagella absent except for flagellated microgametes in some groups; cysts often present; all species parasitic.
Class Sporozoa (spor´o-zo´e-a) (Gr. sporos, seed, + zoon, animal). Spores or oocysts typically present that contain infective sporozoites; flagella present only in microgametes of some groups; pseudopods ordinarily absent, if present they are used for feeding, not locomotion; one or two host life cycles. Examples: Monocystis, Gregarina, Eimeria, Plasmodium, Toxoplasma, Babesia, Pneumocystis. [Note: taxonomic position of Pneumocystis not known with certainty.]
Phylum Microspora (mi-cros´por-a) (Gr. micro, small,+ sporos, seed). Parasites of invertebrates, especially arthropods, and lower vertebrates.
Phylum Ascetospora (as-e-tos´por-a) (Gr. asketos, curiously wrought,+ sporos, seed). Small group that is parasitic in invertebrates and a few vertebrates.
Phylum Ciliophora (sil-i-of´or-a) (L. cilium, eyelash, + Gr. phora, bearing). Cilia or ciliary organelles in at least one stage of life cycle; two types of nuclei, with rare exception; binary fission across rows of cilia, budding and multiple fission also occur; sexuality involving conjugation, autogamy, and cytogamy; nutrition heterotrophic; contractile vacuole typically present; most species free living, but many commensal, some parasitic. (This is a very large group, now divided by the Society of Protozoologists classification into three classes and numerous orders and suborders. The classes are separated on the basis of technical characteristics of the ciliary patterns, especially around the cytostome, the development of the cytostome, and other characteristics.) Examples: Paramecium, Colpoda, Tetrahymena, Balantidium, Stentor, Blepharisma, Epidinium, Euplotes, Vorticella, Carchesium, Trichodina, Podophrya, Ephelota.
Phylogeny
Protozoan groups represent early phylogenetic splits from all multicellular (metazoan) animals. The common ancestor of protozoan groups and metazoans was unicellular. Some protozoa, particularly among colonial and multicellular flagellates (Figure 11-11), show various degrees of cell aggregation and some differentiation that may parallel the body plans of early metazoa. Current evidence indicates that metazoans are the sister group of choanoflagellate protozoa, but available data do not strongly support this claim.
Traditionally, Sarcodina and Mastigophora were considered separate classes in phylum Protozoa. Observations that some flagellates could form pseudopodia, that some species of amebas had flagellated stages, and that a supposed ameba was really a flagellate without a flagellum, all seemed to support the concept of a phylum Sarcomastigophora. However, analyses of sequences of bases in genes, particularly the gene encoding the small subunit of ribosomal RNA , have provided strong evidence that neither Sarcodina nor Mastigophora are monophyletic groups. For example, unicellular green algae (members of class Phytomastigophorea, subphylum Mastigophora in the classification to follow) and higher plants are more closely related to animals than they are to other protozoan groups. Ameboid forms arose independently many times, and there is no justification for placing them all in a subphylum Sarcodina.
In fact, molecular evidence has almost completely revised our concepts of protozoan phylogeny. Sequence analyses suggest that the earliest branch from the eukaryote ancestor led to a group called Euglenozoa (including Euglena and its relatives, Trypanosoma and its relatives, and others), followed by divergence of slime molds. There was then a rapid radiation producing many algal and protozoan groups, in addition to fungi, plants, and animals. Two large algal/protozoan groups are now called stramenopiles and alveolates. Other major groups contain, for example, red algae and green algae and plants.
The stramenopile grouping includes brown algae, yellow algae, diatoms, and many others. Alveolates are dinoflagellates (many of which are phototrophic), apicomplexans (coccidians, Plasmodium spp., etc.), and ciliates. The nearest common ancestor of dinoflagellates and apicomplexans apparently acquired photosynthetic plastids by a secondary endosymbiotic event. (Secondary endosymbiosis is an endosymbiotic acquisition of an eukaryote by another eukaryote, rather than the combination of two prokaryotes, as in the origin of eukaryotes) The endosymbiosis in alveolates apparently occurred after divergence of ciliates, and photosynthetic ability was lost subsequently by ancestral apicomplexans, which are now all parasitic. It is believed that stramenopiles acquired their photosynthetic plastids by another secondary endosymbiosis, independent of other major autotrophic assemblages such as red and green algae.
Adaptive Radiation
We have described some of the wide range of adaptations of protozoan groups in the preceding pages. Ameboid forms range from bottom-dwelling, naked species to planktonic forms such as the foraminiferans and radiolarians with beautiful, intricate tests. There are many symbiotic species of amebas. Flagellated forms likewise show adaptations for a similarly wide range of habitats, with the added variation of photosynthetic ability in many species of Phytomastigophorea.
Within a single-cell body plan, the division of labor and specialization of organelles are carried farthest in the ciliates. These have become the most complex of all protozoa. Specializations for intracellular parasitism have been adopted by Apicomplexa and Microspora.
Classification of Protozoan Phyla
Four main protozoan groups were recognized traditionally: flagellates, amebas, spore formers, and ciliates. Biologists then realized that the phylum “Protozoa” was not valid and was composed of several more-or-less unrelated phyla. They further believed that there was sufficient evidence of a relationship between flagellates and amebas to support a phylum Sarcomastigophora. These concepts are reflected in the following classification. However, molecular data do not support this arrangement, and when new classification of protozoan groups becomes formalized, we will incorporate it. Groups for which there is evidence of polyphyly and/or paraphyly are marked with an asterisk (*).
Phylum Sarcomastigophora* (sar´komas- ti-gof´o-ra) (Gr. sarkos, flesh, + mastix, whip, + phora, bearing). Flagella, pseudopodia, or both types of locomotory organelles; usually with only one type of nucleus; typically no spore formation; sexuality, when present, essentially syngamy.
Subphylum Mastigophora* (mas-tigof´o-ra) (Gr. mastix, whip, + phora, bearing). One or more flagella typically present in adult stages; autotrophic or heterotrophic or both; reproduction usually asexual by fission.
Class Phytomastigophorea* (fi´to-mas-ti-go-for´e-a) (Gr. phyton, plant,+ mastix, whip, + phora, bearing). Plantlike flagellates, usually bearing chromoplasts (pigment-bearing bodies; chromoplasts with chlorophyll are chloroplasts), which contain chlorophyll. Examples: Chilomonas, Euglena, Volvox, Ceratium, Peranema, Noctiluca.
Class Zoomastigophorea* (zo'o-mas-ti-go-for´e-a) (Gr. zoon, animal,+ mastix, whip, + phora, bearing). Flagellates without chromoplasts; one to many flagella; ameboid forms with or without flagella in some groups; species predominantly symbiotic. Examples: Trichomonas, Trichonympha, Trypanosoma, Leishmania, Dientamoeba.
Subphylum Opalinata (o´pa-lina ´ta) (N.F. opaline, like opal in appearance,+ata, group suffix). Body covered with longitudinal rows of cilium-like organelles; parasitic; cytostome (cell mouth) lacking; two to many nuclei of one type. Examples: Opalina, Protoopalina.
Subphylum Sarcodina* (sar-ko-di´na) (Gr. sarkos, flesh,+ ina, belonging to). Pseudopodia typically present; flagella present in developmental stages of some; free living or parasitic.
Superclass Rhizopoda* (ri-zop´oda) (Gr. rhiza, root, + pous, podos, foot). Locomotion by lobopodia, filopodia, or reticulopodia, or by cytoplasmic flow without production of discrete pseudopodia. Composed of eight classes, some of which are listed here.
Class Lobosea* (lo-bo´se-a) (Gr. lobos, lobe). Pseudopodia lobose or filiform but produced from broader lobe; usually uninucleate; no fruiting bodies. Examples: Amoeba, Entamoeba, Acanthamoeba, Naegleria, Chaos, Arcella, Difflugia.
Class Eumycetozoea* (yu´miset- o-zo´e-a) (Gr. eu, good, true, + mykes, fungus, +zoon, animal). Ameboid feeding stage, flagellated stage present or absent; produce aerial fruiting bodies with one to thousands of spores. Examples: Dictyostelium, Physarum.
Class Filosea (fi-los´e-a) (L. filum, thread). Hyaline, filiform pseudopodia, often branching, sometimes rejoining; no spores or flagellated stages known. Examples: Vampyrella, Euglypha, Gromia.
Class Granuloreticulosea (gran´yu-lo-re-tik´yu-los´e-a) (L. granulum, dim. of granum, grain, + reticulum, dim. of rete, net). Delicate, finely granular or hyaline reticulopodia or, rarely, finely pointed, granular but nonrejoining pseudopodia. Examples: Allogromia, Fusulina, Textularia, Elphidium, Globigerina, other foraminiferans.
Superclass Actinopoda (ak´tinop ´o-da) (Gr. aktis, aktinos, ray, + pous, podos, foot). Often spherical, usually planktonic; pseudopodia in form of axopodia, with microtubular supporting structure.
Class Acantharea (a´kanthar ´e-a) (Gr. akantha, spine or thorn). Strontium sulfate skeleton composed of 20 or more radiating spines more or less joined in cell center; marine, usually planktonic. Examples: Acanthometra, Lithoptera.
Class Polycystinea (pol´e-sistin ´e-a) (Gr. polys, many, + kystis, bladder). Siliceous skeleton in most species, usually of solid elements, consisting of one or more latticed shells with or without radial spines, or of spicules; capsular membrane usually of grossly polygonal plates with many more than three pores; marine, planktonic. Example: Thalassicolla.
Class Phaeodarea (fe´o-dar´- e-a) (Gr. phaios, dusky, + daria, suffix). Skeleton of mixed silica and organic matter, consisting of usually hollow spines and shells; very thick capsular membrane with three pores; marine, planktonic. Examples: Aulacantha, Challengeron.
Class Heliozoea (he´le-o-zo´- e-a) (Gr. helios, sun,+ zoon, animal). Without central capsule; skeletal structures, if present, siliceous or organic; axopodia radiating on all sides; most species freshwater. Examples: Clathrulina, Actinophrys, Actinosphaerium.
Phylum Labyrinthomorpha (la´birinth- o-morf´a) (Gr. labyrinth, maze, labyrinth, + morph, form; + a, suffix). Small group living on algae; mostly marine or estuarine. Example: Labyrinthula.
Phylum Apicomplexa (a´pi-complex ´a) (L. apex, tip or summit, + complex, twisted around, + a, suffix). Characteristic set of organelles (apical complex) associated with anterior end present in some developmental stages; cilia and flagella absent except for flagellated microgametes in some groups; cysts often present; all species parasitic.
Class Sporozoa (spor´o-zo´e-a) (Gr. sporos, seed, + zoon, animal). Spores or oocysts typically present that contain infective sporozoites; flagella present only in microgametes of some groups; pseudopods ordinarily absent, if present they are used for feeding, not locomotion; one or two host life cycles. Examples: Monocystis, Gregarina, Eimeria, Plasmodium, Toxoplasma, Babesia, Pneumocystis. [Note: taxonomic position of Pneumocystis not known with certainty.]
Phylum Microspora (mi-cros´por-a) (Gr. micro, small,+ sporos, seed). Parasites of invertebrates, especially arthropods, and lower vertebrates.
Phylum Ascetospora (as-e-tos´por-a) (Gr. asketos, curiously wrought,+ sporos, seed). Small group that is parasitic in invertebrates and a few vertebrates.
Phylum Ciliophora (sil-i-of´or-a) (L. cilium, eyelash, + Gr. phora, bearing). Cilia or ciliary organelles in at least one stage of life cycle; two types of nuclei, with rare exception; binary fission across rows of cilia, budding and multiple fission also occur; sexuality involving conjugation, autogamy, and cytogamy; nutrition heterotrophic; contractile vacuole typically present; most species free living, but many commensal, some parasitic. (This is a very large group, now divided by the Society of Protozoologists classification into three classes and numerous orders and suborders. The classes are separated on the basis of technical characteristics of the ciliary patterns, especially around the cytostome, the development of the cytostome, and other characteristics.) Examples: Paramecium, Colpoda, Tetrahymena, Balantidium, Stentor, Blepharisma, Epidinium, Euplotes, Vorticella, Carchesium, Trichodina, Podophrya, Ephelota.
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