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Tubulinea
Tubulinea
Tubulinea
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Tubulinea
Amoeba proteus
Scientific classification Edit this classification
Domain: Eukaryota
Clade: Amorphea
Phylum: Amoebozoa
Subphylum: Lobosa
Class: Tubulinea
Smirnov et al. 2005
Subclasses & Orders

The Tubulinea are a major grouping of Amoebozoa, including most of the more familiar amoebae genera like Amoeba, Arcella, Difflugia and Hartmannella.

Characteristics

[edit]

During locomotion most Tubulinea have a roughly cylindrical form or produce numerous cylindrical pseudopods. Each cylinder advances by a single central stream of cytoplasm, granular in appearance, and has no subpseudopodia. This distinguishes them from other amoeboid groups, although in some members this is not the normal type of locomotion.

Representation of a tubulinid
  1. Ectoplasm
  2. Endoplasm
  3. Lobopodia, a pseudopod or arm-like projection made of cytoplasm
  4. Phagocytic vacuole
  5. Mitochondrion, creates ATP (energy) for the cell (branched cristae)
  6. Endosymbiotic bacterium
  7. Triuret (nitrogen waste) crystal
  8. Prey
  9. Nucleus
  10. Honeycomb lamina
  11. Nucleolus
  12. Endoplasmic reticulum, the transport network for molecules going to specific parts of the cell
  13. Golgi apparatus, modifies proteins and sends them out of the cell
  14. Lysosome, holds enzymes
  15. Digestive vacuole
  16. Lipid granule
  17. Vesicle
  18. Contractile vacuole, regulates the quantity of water inside a cell
  19. Uroid

Classification

[edit]

This class was anticipated by some biologists such as Jahn, who grouped all amoebae with granular pseudopodia together,[1] but most split the lobose amoebae into testate Testacealobosia and naked Gymnamoebia. These latter are polyphyletic, but molecular trees by Bolivar et al.[2] identified a core monophyletic subgroup. Subsequent studies showed the testate lobose amoebae belong to the same group, which was thus renamed Lobosea sensu stricto[3] or Tubulinea.[4]

Taxonomy

[edit]

The class Tubulinea, as of 2022, is classified into three major groups: Corycida, Echinamoebida and Elardia. The most taxonomically abundant group is Elardia, which contains the testate amoebae of Arcellinida and the naked amoebae of orders Leptomyxida and Euamoebida.[5][6][7][8]

References

[edit]
Revisions and contributorsEdit on WikipediaRead on Wikipedia

Tubulinea

View on Grokipedia
from Grokipedia
Tubulinea is a class of amoeboid protists belonging to the phylum and subphylum Lobosa, characterized by the production of discrete, tubular or cylindrical known as , which feature a monoaxial flow of along a single central axis. This class encompasses a diverse array of naked and that primarily inhabit freshwater, , and marine environments, playing crucial roles as bacterivores and decomposers in microbial ecosystems. The taxonomic framework for Tubulinea was established by Smirnov et al. in 2005 as part of a revised of lobose amoebae, distinguishing it from other amoebozoan groups like Discosea based on pseudopodial morphology and . Subsequent revisions, such as Smirnov et al. in 2011, refined the class to include five orders: Euamoebida, Leptomyxida, Arcellinida, Echinamoebida, and Tubulinea sensu stricto, reflecting deeper evolutionary divergences within supported by and gene analyses. These updates highlight Tubulinea's , with early branching from other lobose lineages. Morphologically, tubulanean amoebae exhibit versatile locomotive forms, often shifting between flattened, expanded shapes for feeding and sub-cylindrical, monopodial extensions for migration, driven by actomyosin contractility rather than ciliary movement. Their are blunt and eruptive, lacking the branching or anastomosing patterns seen in related groups, and the granular contrasts with a clearer region at the advancing front. Testate species within orders like Arcellinida construct protective shells from environmental materials such as silica or organic debris, enhancing survival in varied habitats. Tubulinea displays significant diversity, with over 500 described species across its orders, including well-known naked genera such as , Hartmannella, and Leptomyxa, alongside testate forms like Arcella and Difflugia. Recent phylogenetic studies have revealed additional lineages, such as small-genome species like Echinamoeba silvestris, underscoring ongoing discoveries in their . Ecologically, Tubulinea species are dominant in benthic and communities, facilitating nutrient cycling through predation on and contributing to and freshwater food webs. Medically, certain members pose risks to humans; for instance, spp. cause and . These pathogens highlight the class's biomedical relevance alongside its ecological prominence.

Description

Morphology

Tubulinea are characterized by their production of tubular or cylindrical pseudopods, known as , which are typically non-branching and exhibit monoaxial along a single central axis. These pseudopods are and blunt-ended, facilitating both locomotion through substrate and feeding via , where prey such as or small protists are engulfed. Unlike more complex pseudopodial networks in other amoebozoans, the in Tubulinea maintain a sub-cylindrical shape, enabling efficient directional movement without branching or anastomosing. The body form of Tubulinea consists of lobose amoebae with a distinct differentiation between ectoplasm, which forms the clear, fluid outer layer involved in pseudopod extension, and granular , the inner viscous containing organelles and food vacuoles. Cells are generally naked or enclosed in tests, lacking rigid scales or thecal plates in their basic , though some testate forms possess proteinaceous or agglutinated shells. Locomotion relies on an actomyosin-based , with cytoplasmic present but rare and never organized into bundles; centrosomes are absent, and no stages occur. Cell sizes typically range from 10 to 500 μm, allowing for a variety of ecological roles from microhabitats to visible macroaggregates in some species. Sensory structures are minimal, with no cilia or specialized organelles, emphasizing reliance on chemosensory responses mediated through the cell surface. Feeding in Tubulinea occurs exclusively through , where pseudopods surround and internalize particulate food without the aid of a or other specialized ingestive apparatus. The supports this process with a flexible plasma membrane often coated in a , which varies across taxa but lacks complex skeletal elements. In some members, such as leptomyxids, a distinctive flexible pellicle—a thin, proteinaceous cell coat—contributes to morphological plasticity and aids in formation under stress, allowing the cell to transition from active to dormant states while preserving structural integrity. These features underscore the adaptive simplicity of Tubulinea morphology, optimized for versatile environmental interactions.

Reproduction and Life Cycle

Tubulinea predominantly reproduce asexually through binary fission, a process in which the parent cell divides into two genetically identical daughter cells, typically along a longitudinal or oblique axis depending on the species and environmental conditions. In free-living species such as , division occurs longitudinally, with the cell first rounding up, duplicating its nucleus, and then cleaving to form two separate amoebae. This mode of allows for rapid population growth under favorable conditions, with fission cycles completing in hours to days. The life cycle of Tubulinea features two main stages: the active trophic () phase, during which the organism feeds, moves via , and reproduces by fission, and a dormant stage triggered by environmental stresses such as desiccation, nutrient scarcity, or temperature extremes. Encystment involves the retracting , rounding up, and secreting a protective wall, often double-layered with an outer ectocyst and inner endocyst; in genera like , mature cysts include multiple ostioles capped by an operculum for controlled excystment. Excystment resumes the trophic phase upon return to favorable conditions, with the emerging to feed and divide. The encystment process typically spans 24 to 72 hours, enabling survival in harsh environments. Sexual reproduction is rare and not well-documented in Tubulinea, with most relying solely on asexual mechanisms; however, genomic analyses of like reveal the presence of meiotic genes and evidence for ancient syngamy, suggesting a cryptic sexual capability that may occur sporadically but is not a dominant reproductive strategy.

Classification and

Historical Development

The initial discovery of tubulanean amoebae traces back to the 19th century, when Christian Gottfried Ehrenberg described the genus Amoeba, including the model species Amoeba proteus, as part of his broader classification of pseudopodial protists within the Infusoria, emphasizing their locomotive extensions as key features. Ehrenberg's work in 1830 established foundational genera for naked lobose amoebae, later recognized as core members of Tubulinea, though initially grouped under the artificial phylum Rhizopoda alongside diverse sarcodine forms based on light microscopy observations of pseudopodia. This era relied heavily on gross morphology, leading to early misclassifications where some filose or testate amoebae were conflated with heliozoans due to superficial similarities in radial extensions visible under compound microscopes. Key milestones in the mid-20th century advanced the recognition of tubulanean-like groups through cytology. In the , T.L. Jahn and E.C. Bovee proposed the suborder Tubulina within Sarcodina, uniting naked and based on the structure of cylindrical or tubular , marking an early attempt to define the group by locomotive morphology rather than or size alone. The 1970s saw electron (EM) studies reveal ultrastructural details, such as the absence of cilia and the presence of specific mitochondrial cristae, confirming affinities among lobose amoebae and distinguishing them from other sarcodines, thus supporting their isolation as a cohesive assemblage within protozoan phylogeny.00278-4) These EM investigations, including examinations of and related forms, highlighted shared cytological traits like dense droplets and ectoplasmic layers, which foreshadowed their amoebozoan relationships. Influential researchers in the 1980s further refined pre-molecular through detailed monographs on naked amoebae. F.C. Page's comprehensive works, such as his 1986 classification of gymnamoebae and 1988 key to freshwater and soil species, shifted emphasis from the polyphyletic Rhizopoda to more coherent groupings based on pseudopodial types and locomotion, effectively outlining the morphological basis for what would become Tubulinea while excluding unrelated filose forms.80002-5) Page's systems documented over 200 species, prioritizing seminal traits like uroidal structures, and facilitated the transition away from outdated Rhizopoda toward protozoan subphyla like Lobosa, though still challenged by light microscopy's limitations in resolving fine pseudopodial variations.90001-9) The pre-molecular era's reliance on optical and early EM methods perpetuated challenges, including artificial groupings where some tubulanean testate amoebae were misclassified as heliozoans or foraminiferans due to shell-like tests and axial pseudopodia indistinguishable without ultrastructural analysis. By the 1990s and early 2000s, the advent of small subunit ribosomal RNA (SSU rRNA) sequencing began integrating lobose amoebae into higher amoebozoan phylogeny, with early studies like those on Amoeba and Chaos revealing their monophyly with myxogastrids and excluding polyphyletic sarcodines. This molecular transition culminated in the formal establishment of Tubulinea as a class in 2005 by Smirnov et al., based on SSU rRNA data corroborating tubular pseudopod cytology across naked and testate lineages.

Current Classification

Tubulinea is classified as a class within the phylum , which belongs to the supergroup . This placement reflects the group's position among amoeboid protists characterized by lobose , distinguishing it from other eukaryotic lineages. Diagnostic traits of Tubulinea include tubular, cylindrical, or subcylindrical that support monoaxial , with forms that are typically apodous or ending in fine points; branching pseudopodial types are excluded. These morphological features, combined with the absence of stages and tubular mitochondrial cristae, define the class and separate it from related groups like Discosea. Some taxa form tests or sorocarps, but the core locomotion relies on these non-branching, tube-like extensions. The higher structure of Tubulinea encompasses several orders, including Corycida as a basal group, Echinamoebida (with genera like Echinamoeba), Euamoebida (including ), Leptomyxida (featuring variable, often flattened forms like Leptomyxa), and elements within Elardia such as Arcellinida. The of Tubulinea is robustly supported by molecular phylogenies utilizing and genes, as demonstrated in 2010s analyses by Tekle et al. that incorporated multigene datasets including α- and β-tubulin, 2, and 14-3-3 proteins across 22 taxa. These studies confirmed four major lineages—Echinamoeboidea, Leptomyxida, Amoebida, and Poseidonida—while highlighting inconsistencies in traditional groupings like Arcellinida. In the 2020s, revisions based on extensive multi-gene analyses, such as a 2022 supermatrix of 824 genes from 113 taxa, have further solidified Tubulinea's and resolved in orders like Arcellinida by emphasizing test morphology over composition; they also established Corycida as the deepest-branching lineage and resolved monophyletic orders like Thecamoebida. These updates integrate broader phylogenomic data to refine the hierarchical without altering the class's core definition.

Diversity

Major Subgroups

The major subgroups of Tubulinea, as of 2022, are classified into three groups: Corycida, Echinamoebida, and Elardia, encompassing over 500 described , with Elardia being the most diverse. Corycida includes with proteinaceous shells, representing a smaller lineage with limited described species, primarily inhabiting freshwater and environments. The order Echinamoebida consists of naked amoebae distinguished by short, spine-like subpseudopodia (acanthopodia) and the ability to form cysts. It includes the family Echinamoebidae, with genera such as Echinamoeba and Vermamoeba (including pathogenic and non-pathogenic forms found in aquatic and habitats); the order comprises approximately 20–30 species. Elardia is the largest group, including both naked and testate amoebae across several orders. The order Euamoebida consists of free-living naked amoebae characterized by smooth, cylindrical pseudopods and monoaxial cytoplasmic flow, lacking complex branching or spiny extensions. This order includes the family Amoebidae, with genera such as Amoeba and Chaos featuring large, highly vacuolated cells, and the family Hartmannellidae, which contains smaller, more typical amoeboid forms like Saccamoeba that are non-pathogenic and widespread in aquatic and soil environments. The order Leptomyxida features amoebae with slender, sometimes branching pseudopods that enable a polymorphic lifestyle, shifting from monopodial locomotion forms to flattened, reticulose spreading stages. The primary family, Leptomyxidae, includes genera such as Leptomyxa and Flabellula, which are common in and freshwater habitats and exhibit distinctive cyst walls; this order includes 23 confirmed . The order Arcellinida comprises that construct shells (tests) from environmental materials like silica or organic debris. It includes families such as Arcellidae (e.g., ) and Difflugiidae (e.g., Difflugia), with over 600 described , making it one of the most diverse groups in Tubulinea. These amoebae are prevalent in freshwater, , and habitats.

Representative Species

Vermamoeba vermiformis, previously known as Hartmannella vermiformis, is a common free-living in the Echinamoebidae (order Echinamoebida), frequently found in distribution systems and biofilms. It acts as a host for pathogenic bacteria such as , protecting them from disinfectants and facilitating their replication within amoebic vacuoles. This symbiotic relationship has positioned V. vermiformis as an important subject in research on intracellular bacterial and environmental of waterborne diseases. Leptomyxa fragilis represents the family Leptomyxidae in Tubulinea and is a soil-dwelling characterized by its elongated, branching and a thin pellicle that enables gliding locomotion while attached to the substrate. This species exhibits polypodial movement and can grow to lengths exceeding 500 μm, often covered in adhering particles. Its unique mode of substrate-attached locomotion highlights adaptive strategies for navigating terrestrial environments. Chaos carolinense, formerly classified as Pelomyxa carolinensis, is a large, multinucleate amoeba in the family Amoebidae (order Euamoebida), capable of reaching sizes up to 5 mm in diameter. As a historical , it has been instrumental in studies of , where rapid flow drives pseudopodial extension and cellular . Observations of its fibrillar cytoplasmic structures have provided foundational insights into the contractile apparatus of amoeboid cells. Arcella vulgaris, a representative testate amoeba in the family Arcellidae (order Arcellinida), forms a proteinaceous shell and is common in freshwater habitats. It feeds on bacteria and small eukaryotes using extended through shell apertures, contributing to nutrient cycling in aquatic ecosystems.

Ecology and Evolution

Habitats and Distribution

Tubulinea, a class of lobose amoebae within the , are ubiquitous inhabitants of terrestrial and aquatic environments worldwide. They primarily occupy soils, freshwater sediments, and biofilms, where they thrive as key microbial consumers. In soil ecosystems, densities can reach up to 17,700 individuals per gram of dry weight, as observed in Scottish soils. Freshwater habitats, including sediments and peatlands, support a significant portion of their diversity, particularly testate forms like arcellinids. While predominantly limnic and terrestrial, some taxa, such as leptomyxids, extend into marine and brackish waters, though marine representation remains limited compared to other amoebozoan groups. Their distribution is cosmopolitan, with global presence across diverse biomes, but patterns reveal higher in tropical soils, where environmental stability and resource availability foster greater protistan diversity. For instance, soil protist communities, including Tubulinea, exhibit peak diversity in tropical regions, approaching levels comparable to bacterial counterparts. Certain species, like , demonstrate remarkable thermal tolerance, persisting in hot springs and thermal waters up to 45°C, enabling occupation of niche environments beyond typical temperate zones. Tubulinea exhibit broad abiotic tolerances that underpin their ecological success. They endure pH ranges from 4 to 9, accommodating acidic soils and alkaline sediments, and temperatures from -10°C to 50°C, with some strains surviving subzero conditions in s. resistance is achieved through cyst formation, allowing viability for decades under dry or nutrient-poor conditions, which facilitates persistence in fluctuating habitats like surface soils. These tolerances enable widespread colonization, from arctic tundras to arid zones. Ecologically, Tubulinea serve as predators of and fungi, grazing on microbial prey via and channeling energy through and aquatic food webs. This bacterivory and fungivory regulates microbial populations, with some species also consuming or smaller protists. Notably, they act as vectors for , harboring like species within their cysts, which enhances pathogen survival and dissemination in environments. In human-impacted niches, Tubulinea proliferate in biofilms within plants and systems, where moisture and organic substrates abound. , for example, is frequently isolated from cooling coils and distribution systems, contributing to complexity and potential health risks through . These engineered environments mimic natural moist niches, amplifying their presence in urban settings.

Evolutionary Relationships

Tubulinea constitutes a within the Lobosa of , positioned as the sister group to Discosea, a relationship robustly supported by analyses of 18S rRNA gene sequences and multigene phylogenomic datasets. This placement reflects the shared lobose morphology of both classes, characterized by non-branching , distinguishing them from other amoebozoan lineages. Early molecular studies using confirmed the monophyly of Tubulinea, while subsequent phylogenomic reconstructions, incorporating hundreds of genes, have reinforced this and highlighted its deep divergence within Lobosa. For instance, a comprehensive multigene analysis of Amoebozoa placed Tubulinea as a stable encompassing orders such as Arcellinida and Leptomyxida. The evolutionary origins of Tubulinea trace back to the Eon, with estimates suggesting the of major lineages, including ancestors of Tubulinea, around 1 billion years ago during the mid-. evidence supports this ancient history, with vase-shaped microfossils from deposits approximately 750 million years old in age exhibiting protist-like features attributable to early Tubulinea or closely related forms, such as arcellinid . These fossils indicate that testate lobose amoebae with tubular had already diversified by the late Period, predating the glaciations. Key evolutionary innovations in Tubulinea include the development of tubular, monoaxial from more generalized lobose ancestors within , enabling efficient and locomotion in diverse substrates. Most lineages also exhibit a secondary loss of flagella, a trait retained in some basal but absent across Lobosa, which likely facilitated adaptations to benthic and environments by emphasizing amoeboid over . In comparative phylogeny, Tubulinea contrasts sharply with Discosea, the other major lobosan class, where are often branching or discoid, reflecting divergent locomotor strategies that contributed to the broader radiation of during the . This bifurcation underscores how pseudopod morphology influenced ecological diversification within the phylum. Recent multi-omics approaches in the 2020s have uncovered extensive horizontal transfers from to , including Tubulinea relatives, enhancing physiological capabilities such as through acquired s for compatible solute synthesis. For example, genomic analyses of lobose amoebae reveal bacterial-derived permeases and stress-response pathways that bolster survival in fluctuating osmotic conditions, highlighting HGT's role in evolutionary resilience. These insights from transcriptomics and emphasize ongoing gene flux as a driver of adaptation in ancient eukaryotic lineages like Tubulinea.

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