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Taxis
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Taxis
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A taxis (from Ancient Greek τάξις (táxis) 'arrangement, order';[1] pl.: taxes /ˈtæksiːz/)[2][3][4] is the movement of an organism in response to a stimulus such as light or the presence of food. Taxes are innate behavioural responses. A taxis differs from a tropism (turning response, often growth towards or away from a stimulus) in that in the case of taxis, the organism has motility and demonstrates guided movement towards or away from the stimulus source.[5][6] It is sometimes distinguished from a kinesis, a non-directional change in activity in response to a stimulus. Taxis can be positive (moving towards the stimulus) or negative (moving away from the stimulus).

Classification

[edit]

Taxes are classified based on the type of stimulus, and on whether the organism's response is to move towards or away from the stimulus. If the organism moves towards the stimulus the taxis are positive, while if it moves away the taxis are negative. For example, flagellate protozoans of the genus Euglena move towards a light source. This reaction or behavior is called positive phototaxis since phototaxis refers to a response to light and the organism is moving towards the stimulus.

Terminology derived from type of stimulus

[edit]

Many types of taxis have been identified, including:

Depending on the type of sensory organs present, a taxis can be classified as a klinotaxis, where an organism continuously samples the environment to determine the direction of a stimulus; a tropotaxis, where bilateral sense organs are used to determine the stimulus direction; and a telotaxis, where a single organ suffices to establish the orientation of the stimulus.

Terminology derived from taxis direction

[edit]

There are five types of taxes based on the movement of organisms.

  • Klinotaxis occurs in organisms with receptor cells but not paired receptor organs. The cells for reception may be located all over the body, but often towards the anterior side. The organism detects the stimuli by turning its head sideways and comparing the intensity of the stimulus. Their direction of movement is then based on the stronger stimulus, either moving toward a desirable stimulus or away from an undesired one.[7] When the intensity of stimuli is balanced equally from all sides, the organism moves in a straight line. The movement of blowfly and butterfly larvae clearly demonstrates klinotaxis.
  • Tropotaxis is displayed by organisms with paired receptor cells, comparing the strength of the signals and turning toward the strongest signal.[7] The movement of grayling butterflies and fish lice clearly demonstrates tropotaxis.
  • Telotaxis also requires paired receptors. The movement occurs along the direction where the intensity of the stimuli is stronger. Telotaxis is clearly seen in the movement of bees when they leave their hive to look for food. They balance the stimuli from the sun as well as from flowers but land on the flower whose stimulus is most intense for them.
  • Menotaxis describes organisms' maintenance of a constant angular orientation. A clear demonstration is shown by bees returning to their hive at night and the movement of ants with respect to the sun.
  • Mnemotaxis is the use of memory to follow trails that organisms have left when travelling to or from their home.

Examples

[edit]
  • Aerotaxis is the response of an organism to variation in oxygen concentration, and is mainly found in aerobic bacteria.[8]
  • Anemotaxis is the response of an organism to wind. Many insects show a positive anemotactic response (turning/flying into the wind) upon exposure to an airborne stimulus cue from a food source or pheromones.[7] Cross-wind anemotactic search is exhibited by some olfactory animals in the absence of a target odor including moths, albatrosses, and polar bears.[9][10][11] Rats have specialized supra-orbital whiskers that detect wind and cause anemotactic turning.[12]
  • Chemotaxis is a response elicited by chemicals: that is, a response to a chemical concentration gradient.[8][7][13] For example, chemotaxis in response to a sugar gradient has been observed in motile bacteria such as E. coli.[14] Chemotaxis also occurs in the antherozoids of liverworts, ferns, and mosses in response to chemicals secreted by the archegonia.[8] Unicellular (e.g. protozoa) or multicellular (e.g. worms) organisms are targets of chemotactic substances. A concentration gradient of chemicals developed in a fluid phase guides the vectorial movement of responder cells or organisms. Inducers of locomotion towards increasing steps of concentrations are considered as chemoattractants, while chemorepellents result moving off the chemical. Chemotaxis is described in prokaryotic and eukaryotic cells, but signalling mechanisms (receptors, intracellular signaling) and effectors are significantly different.
  • Durotaxis is the directional movement of a cell along a stiffness gradient.
  • Electrotaxis (or galvanotaxis) is the directional movement of motile cells along the vector of an electric field. It has been suggested that by detecting and orienting themselves toward the electric fields, cells can move towards damages or wounds to repair them. It also is suggested that such a movement may contribute to directional growth of cells and tissues during development and regeneration. This notion is based on the existence of measurable electric fields that naturally occur during wound healing, development and regeneration; and cells in cultures respond to applied electric fields by directional cell migration – electrotaxis / galvanotaxis.
  • Energy taxis is the orientation of bacteria towards conditions of optimal metabolic activity by sensing the internal energetic conditions of cell. Therefore, in contrast to chemotaxis (taxis towards or away from a specific extracellular compound), energy taxis responds on an intracellular stimulus (e.g. proton motive force, activity of NDH- 1) and requires metabolic activity.[15]
  • Gravitaxis (known historically as geotaxis) is the directional movement (along the vector of gravity) to the center of gravity. The planktonic larvae of a king crab, Lithodes aequispinus, combine positive phototaxis (movement towards the light) and negative gravitaxis (upward movement).[16] Also the larvae of a polychaete, Platynereis dumerilii, combine positive phototaxis (movement to the light coming from the water surface) and UV-induced positive gravitaxis (downward movement) to form a ratio-chromatic depth-gauge.[17] Both positive and negative gravitaxes are found in a variety of protozoans (e.g., Loxodes, Remanella and Paramecium).[18]
  • Magnetotaxis is, strictly speaking, the ability to sense a magnetic field and coordinate movement in response. However, the term is commonly applied to bacteria that contain magnets and are physically rotated by the force of Earth's magnetic field. In this case, the "behaviour" has nothing to do with sensation and the bacteria are more accurately described as "magnetic bacteria".[19]
  • Pharotaxis is the movement to a specific location in response to learned or conditioned stimuli, or navigation by means of landmarks.[20][21]
  • Phonotaxis is the movement of an organism in response to sound.
  • Phototaxis is the movement of an organism in response to light: that is, the response to variation in light intensity and direction.[8][22] Negative phototaxis, or movement away from a light source, is demonstrated in some insects, such as cockroaches.[8] Positive phototaxis, or movement towards a light source, is advantageous for phototrophic organisms as they can orient themselves most efficiently to receive light for photosynthesis. Many phytoflagellates, e.g. Euglena, and the chloroplasts of higher plants positively phototactic, moving towards a light source.[8] Two types of positive phototaxis are observed in prokaryotes: scotophobotaxis is observable as the movement of a bacterium out of the area illuminated by a microscope, when entering darkness signals the cell to reverse direction and reenter the light; a second type of positive phototaxis is true phototaxis, which is a directed movement up a gradient to an increasing amount of light. There is a different classification to orientation towards dark areas called scototaxis.
  • Rheotaxis is a response to a current in a fluid. Positive rheotaxis is shown by fish turning to face against the current. In a flowing stream, this behaviour leads them to hold their position in a stream rather than being swept downstream. Some fish will exhibit negative rheotaxis where they will avoid currents.
  • Thermotaxis is a migration along a gradient of temperature. Some slime molds and small nematodes can migrate along amazingly small temperature gradients of less than 0.1 °C/cm.[23] They apparently use this behaviour to move to an optimal level in soil.[24][25]
  • Thigmotaxis is the response of an organism to physical contact or to the proximity of a physical discontinuity in the environment (e.g. rats preferring to swim near the edge of a water maze). Codling moth larvae are believed to use thigmotactic sense to locate fruits to feed on.[26] Mice and rats, when inhabiting human-made structures, tend to stick close to vertical surfaces; this primarily manifests as running along the floor/wall juncture. Whiskers (vibrissae) are often used to detect the presence of a wall or surface in the absence of sufficient light in rodents and felines to aid in thigmotaxis.

See also

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Biology
Different, wider context
  • Taxonomy, science of categorisation or classification

References

[edit]
[edit]
Revisions and contributorsEdit on WikipediaRead on Wikipedia

Taxis

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from Grokipedia
Taxis is the directed, oriented movement of a motile toward (positive taxis) or away from (negative taxis) a specific environmental stimulus, it from undirected random movement known as kinesis. This behavioral response is an innate mechanism observed across various taxa, including , protists, and animals, enabling survival advantages such as , predator avoidance, or mating. Common forms of taxis include phototaxis, the response to light, where organisms like moths exhibit positive phototaxis by moving toward light sources. involves movement in response to chemical gradients, crucial for processes like bacterial infection or immune cell recruitment in multicellular organisms. Other notable types encompass (response to gravity), hydrotaxis (response to ), and rheotaxis (response to water currents), each adapted to specific ecological niches. Taxis can be mediated through different sensory mechanisms, such as klinotaxis, which relies on sequential comparisons of stimulus intensity via side-to-side movements, or tropotaxis, utilizing simultaneous bilateral sensory inputs for direct orientation. In more complex cases, telotaxis and menotaxis allow for angular navigation relative to stimuli, as seen in light-compass reactions. These behaviors are fundamental to understanding ecological interactions, evolutionary adaptations, and applications in fields like and .

Introduction

Definition

Taxis is the directed, oriented movement of a motile or cell toward or away from an external stimulus, enabling purposeful in response to environmental cues. This behavior contrasts with kinesis, which involves non-directional changes in the speed or frequency of movement without orientation to the stimulus source. Key characteristics of taxis include its polarity—positive taxis directs movement toward the stimulus, while negative taxis directs it away—and its dependence on detecting gradients of the stimulus. Common stimuli encompass chemical gradients, light intensity, temperature variations, and gravitational forces, allowing organisms to respond adaptively to their surroundings. In distinction from tropism, which involves growth-oriented bending or turning in sessile organisms such as , taxis relies on active locomotion of the whole motile entity. For instance, motile cells may traverse chemical gradients to reach favorable conditions, underscoring taxis as a fundamental locomotor response in mobile life forms.

Biological Significance

Taxis plays a crucial role in the survival and fitness of motile organisms by enabling directed movement toward beneficial s and away from harmful conditions, such as for s, avoiding predators, facilitating , and establishing symbiotic relationships. In , taxis allows microorganisms to sense and ascend nutrient gradients, optimizing resource acquisition in patchy environments. For predator avoidance, negative taxis directs organisms away from toxic or predatory cues, enhancing evasion and persistence. In mating processes, taxis guides gametes or cells toward chemical signals from potential partners, increasing in like in eukaryotes. Symbiotic interactions rely on taxis for motile microbes to locate and colonize host partners via host-released signals, promoting mutualistic associations. These behaviors collectively boost reproductive fitness by minimizing exposure to stressors and maximizing access to supportive niches. Beyond ecology, taxis has significant applications in microbiology, , and , where it influences dynamics and therapeutic strategies. In and ecology, taxis facilitates microbial dispersal and community structuring by allowing rapid adaptation to environmental gradients. Medically, bacterial taxis contributes to by directing pathogens to sites during infections, accessing host nutrients and exacerbating disease progression. For instance, chemotaxis enables bacteria like and to colonize wounds or mucosal surfaces, complicating treatment. In , eukaryotic cell taxis, such as chemotaxis toward damage signals, promotes tissue repair by recruiting immune cells and fibroblasts to injury sites. These insights inform development targeting chemotaxis pathways and bioengineered therapies mimicking taxis for directed cell delivery. Quantitatively, taxis enhances navigational efficiency in heterogeneous environments, substantially reducing energy expenditure relative to strategies. In , chemotactic run-and-tumble biases movement up gradients, can increase the instantaneous uptake rate by approximately a factor of two compared to non-chemotactic diffusive random walks. This directed bias minimizes time spent in suboptimal areas, conserving energy for growth and reproduction in nutrient-variable habitats. The biological importance of taxis was first recognized through Theodor Wilhelm Engelmann's 1881 observations of bacterial phototaxis, where aerobic bacteria accumulated near oxygen-producing regions of illuminated algae filaments, linking taxis to photosynthetic and establishing its role in microbial ecology.

Classification

By Stimulus Type

Taxis in are classified according to the type of environmental stimulus that elicits the directed movement, with specific terms derived from Greek roots to denote the combined with "taxis," meaning "" or "order." This reflects the organized response of organisms to gradients or directional cues, such as chemicals, , or physical forces, enabling toward favorable conditions or away from harm. The classification emphasizes the sensory input rather than the or movement orientation, though responses can be positive (toward the stimulus) or negative (away from it). Chemotaxis refers to the movement of organisms in response to chemical gradients, where attractants such as nutrients draw cells toward higher concentrations, while repellents like toxins prompt avoidance. The term "chemotaxis" was coined in 1884 by botanist Wilhelm Pfeffer. Derived from the Greek "chemo-" (relating to chemistry, from χημεία, khemeía) and "taxis." This response is fundamental for foraging and survival, as seen in bacteria navigating toward sugars or away from poisons through temporal sensing of concentration changes. Phototaxis describes organismal movement directed by light stimuli, including variations in intensity or wavelength, with photosynthetic organisms often exhibiting positive phototaxis to optimize energy capture. The prefix "photo-" stems from the Greek φῶς (phōs), meaning "light," highlighting the role of photoreceptors in detecting directional light cues. This behavior aids in photic zone positioning for algae and other light-dependent microbes. Thermotaxis involves directed movement along gradients, allowing organisms to seek optimal environments for metabolic activity. Derived from "thermo-" (θέρμη, thermē, "heat"), this taxis is crucial for in varied habitats. Additional types include electrotaxis, or galvanotaxis, which is movement in response to ; gravitaxis (formerly geotaxis), directed by ; rheotaxis, alignment with fluid flow; and thigmotaxis, triggered by mechanical touch or contact. These terms incorporate Greek roots such as "electro-" for , "gravis" for heavy (Latin influence but rooted in Greek concepts), "rheo-" for flow (ῥέω, rheō), and "thigmo-" for touch (θιγμός, thigmos). Across these taxis types, detection generally occurs through stimulus binding to specialized receptors on the cell surface or within the , which modulates intracellular signaling to produce a biased —alternating straight runs and reorientations that net directional progress along the . This principle underlies efficient without requiring constant measurement, relying on temporal comparisons of stimulus levels during movement.

By Directional Response

Taxis can be classified based on the direction of movement relative to the stimulus . Positive taxis refers to directed movement toward the source of the stimulus, effectively up the , while negative taxis involves movement away from the source, or down the . These directional responses are further modified by the sensory mechanisms used for detection and orientation. Klinotaxis (or clinotaxis) describes an organism's wavering, side-to-side movement as it detects a stimulus, involving alternating receptor signals to compare stimulus intensity sequentially during movement. This enables directional guidance by turning toward the side detecting a stronger signal, allowing the organism to adjust its path progressively toward or away from the source. Klinotaxis is common in organisms with simpler sensory systems lacking paired receptors, in contrast to tropotaxis which uses paired receptors for simultaneous direct comparison of stimulus intensity. For example, an organism sensing a chemical gradient may turn its head left and right to detect differences in concentration, then direct its movement toward the side with the stronger signal. Telotaxis entails direct orientation toward a perceived stimulus direction using specialized receptors that provide a fixed bearing, without relying on comparisons. Tropotaxis, in contrast, achieves orientation through simultaneous bilateral sensing, where differences in stimulus intensity between two sides of the body enable immediate adjustments. In some systems, particularly prokaryotic ones, taxis is realized through behavioral models like , where periods of straight-line swimming (runs) alternate with random reorientations (tumbles); modulation of tumble frequency in response to the creates a net bias in movement direction, achieving effective taxis without precise steering. This contrasts with kinesis, an undirected response where stimulus intensity alters the rate or speed of random movement but does not produce oriented displacement toward or away from the source. Such distinctions highlight that taxis requires sensory-directed navigation, whereas kinesis reflects non-specific changes in locomotor activity.

Mechanisms

Stimulus Detection

In taxis, organisms detect environmental stimuli through specialized receptors that initiate the sensory process. Chemoreceptors, such as methyl-accepting chemotaxis proteins (MCPs) in , are transmembrane proteins that bind specific ligands like sugars or , triggering conformational changes that propagate signals intracellularly. Photoreceptors, exemplified by rhodopsins in microorganisms, detect light wavelengths via retinal isomerization upon absorption, enabling phototactic responses. Gradient sensing in taxis occurs via two primary mechanisms: temporal sensing, where cells compare stimulus concentrations over time as they move, and spatial sensing, where differences are detected across the cell body simultaneously. Bacteria predominantly employ temporal sensing due to their small size, while larger eukaryotic cells often rely on spatial sensing to resolve shallow gradients efficiently. The choice between these mechanisms depends on cell dimensions, motility speed, and signaling kinetics, optimizing detection in varying environments. Upon stimulus detection, begins with or binding to receptors, activating intracellular pathways. In prokaryotes, this modulates the activity of kinases like CheA, altering cascades that relay information to response regulators. In eukaryotes, binding often elevates second messengers such as cyclic AMP (cAMP), which activates and subsequent kinase cascades to amplify the signal. Receptor occupancy, a key quantitative aspect, follows the model: θ=[S]Kd+[S]\theta = \frac{[S]}{K_d + [S]} where θ\theta represents the fraction of bound receptors, [S][S] is the stimulus concentration, and KdK_d is the dissociation constant, describing saturation at high concentrations. Adaptation ensures sustained sensitivity by desensitizing receptors to constant stimuli, preventing saturation. In bacterial chemoreceptors, this involves reversible methylation of glutamate residues by CheR methyltransferase and demethylation by CheB methylesterase, adjusting receptor activity to baseline levels. This feedback mechanism allows cells to detect changes in dynamic gradients over wide concentration ranges, maintaining responsiveness without external modulation.

Locomotor Response

In taxis, detected signals are transduced into directed locomotion through specialized motility structures that generate propulsive force. Prokaryotes primarily employ flagella, which function as rotary motors that propel the cell by rotating a helical filament, achieving speeds up to approximately 35 body lengths per second in bacteria like Escherichia coli . In eukaryotes, cilia provide motility via coordinated beating, where dynein motors slide microtubules to produce asymmetric waveforms that drive forward motion, as seen in protists such as Chlamydomonas . Amoeboid crawling, common in leukocytes and other eukaryotic cells, relies on actin-myosin contractility to extend pseudopods and deform the cell body, enabling navigation through complex environments without rigid appendages . Locomotor responses are modulated by altering the probability and of directional changes in response to signal gradients. In bacterial , cells alternate between straight "runs" of smooth and random "tumbles" that reorient the body; favorable signals bias this by suppressing tumbling rates, increasing run persistence toward the stimulus . This modulation ensures efficient navigation without precise steering, with tumble frequency adjusting from approximately 1 Hz in neutral conditions to near zero in attractant gradients . Signal integration occurs through temporal summation, where cells compare current and past stimulus levels over seconds to minutes, computing net directionality from cumulative changes. This process involves feedback loops that adapt sensitivity, such as integral feedback in bacterial pathways, which corrects deviations by resetting baseline activity and maintaining orientation accuracy amid noise . These loops enable robust trajectory adjustments, preventing overshoot in varying gradients . Motility is powered by ATP-driven molecular motors, with flagellar rotation in generating via proton motive force coupled to stator-rotor interactions. The E. coli flagellar motor produces approximately 1300 pN·nm of at stall, sufficient to drive viscous at rotational speeds exceeding 100 Hz . This energy efficiency underpins sustained taxis over extended distances. The overall directed movement can be modeled as a biased , where the net displacement velocity vv is given by v=v0(pforward−pbackward),v = v_0 (p_\text{forward} - p_\text{backward}), with v0v_0 as the baseline speed and pforwardp_\text{forward}, pbackwardp_\text{backward} as the probabilities of forward and backward turns modulated by the signal . This formulation captures how subtle biases in turning probabilities yield macroscopic drift toward or away from stimuli .

Examples

In Prokaryotes

In prokaryotes, taxis manifests through simple yet efficient mechanisms adapted to unicellular life, primarily in bacteria and archaea. A paradigmatic example is bacterial chemotaxis in Escherichia coli, where cells navigate chemical gradients using a two-component signaling system involving methyl-accepting chemotaxis proteins (MCPs) and Che proteins. MCPs, such as Tar and Tsr, span the inner membrane and detect attractants or repellents in the periplasm, undergoing conformational changes that modulate the autophosphorylation activity of the histidine kinase CheA. CheA transfers its phosphate to the response regulator CheY via the adaptor protein CheW; phosphorylated CheY (CheY-P) binds to the flagellar motor's FliM switch, promoting clockwise rotation and tumbling, which reorients the cell randomly. In favorable gradients, reduced CheA activity lowers CheY phosphorylation, favoring counterclockwise flagellar rotation for smooth "runs" toward the stimulus. This run-and-tumble bias enables net migration without direct spatial gradient detection. Phototaxis in exemplifies light-directed movement, where these photosynthetic prokaryotes exhibit positive phototaxis to optimize light capture for energy production. Unicellular cyanobacteria, such as Synechococcus elongatus, use bacteriophytochrome photoreceptors like TaxD1, which bind and detect red/far-red light (around 660–750 nm), triggering two-component signaling systems similar to pathways that modulate type IV pili motility for directed movement toward light sources. Unlike retinal-based rhodopsins in halobacteria, these phytochromes couple to CheA/CheY-like systems, adjusting reversal frequencies for biased or . Other notable prokaryotic taxis include aerotaxis and magnetotaxis. In aerobic bacteria like and E. coli, aerotaxis directs cells toward optimal oxygen concentrations (typically around 0.5% for E. coli) via energy taxis, where oxygen gradients are sensed indirectly through respiratory chain components like oxidases, which influence CheA activity and tumbling rates. This ensures positioning in oxic microenvironments for efficient ATP production. Magnetotaxis, observed in such as Magnetospirillum magnetotacticum, relies on —intracellular chains of membrane-bound (Fe₃O₄) or (Fe₃S₄) nanocrystals that impart a permanent . These organelles align cells passively with Earth's geomagnetic field, coupling with flagellar motility to guide vertical migration along oxygen gradients in aquatic sediments, preventing exposure to lethal anoxia or . Experimental observations of prokaryotic taxis highlight its kinetics. In E. coli, three-dimensional tracking revealed that smooth runs last approximately 1–2 seconds, covering 20–40 μm at speeds of 15–25 μm/s, while tumbles endure about 0.1 seconds, randomizing direction for unbiased exploration. These parameters yield a biased , with run lengths extending in attractant gradients to achieve net displacement. Prokaryotes' small size (typically 1–5 μm) constrains spatial sensing, as chemical gradients across the cell body are negligible compared to timescales, necessitating temporal comparisons of stimulus levels during . Cells thus detect changes by modulating signaling over time, adapting via /demethylation of MCPs (catalyzed by and ) to reset sensitivity and prevent saturation. This temporal strategy suits their rapid, diffusive environments, enabling efficient navigation despite physical limitations.

In Eukaryotes

In eukaryotes, taxis manifests in more complex cellular architectures compared to prokaryotes, often involving multicellular coordination and developmental es. A prominent example is in leukocytes, where neutrophils migrate toward infection sites by sensing gradients of such as interleukin-8 (IL-8, also known as CXCL8). This is mediated by G-protein-coupled receptors like CXCR1 and CXCR2, which trigger intracellular signaling cascades leading to polymerization and directed , enabling rapid recruitment to inflammatory sites. Phototaxis in unicellular algae like exemplifies light-directed movement, where the eyespot (stigma) acts as a photoreceptor to detect light intensity and direction, coordinating asymmetric flagellar beating for navigation toward optimal conditions. The eyespot's position relative to the flagella allows spatial comparison of across the cell, resulting in helical swimming paths that adjust based on light gradients. Additional instances include , in which mammalian spermatozoa navigate toward the via gradients of attractants like progesterone released from the cumulus cells surrounding the , facilitating fertilization through flagellar reorientation and increased beat frequency. In social amoebae such as Dictyostelium discoideum, drives aggregation during starvation, with cells responding to propagating waves of cyclic AMP (cAMP) that serve as chemoattractant signals, forming multicellular mounds for fruiting body development. Eukaryotic cells exhibit unique adaptations for taxis, such as reliance on actin-myosin cytoskeletal dynamics to form pseudopods for crawling , contrasting with prokaryotic flagellar rotation. Their larger size facilitates spatial gradient sensing, where receptors on the cell surface detect concentration differences across the , amplifying signals through localized phosphoinositide and calcium fluxes to polarize the . A key study demonstrated electrotaxis in fibroblasts, showing their directed migration in endogenous generated at sites, which guides closure and tissue repair by orienting lamellipodia toward the .

Ecological and Evolutionary Aspects

Ecological Roles

Bacterial plays a crucial role in nutrient acquisition within soil microbiomes by directing motile toward organic carbon sources, such as exudates and decaying material, thereby accelerating processes. In the , enables to exploit nutrient gradients, with studies showing that it can increase carbon and nitrogen uptake by up to fourfold in heterotrophic communities metabolizing dissolved . This targeted movement enhances the breakdown of complex polymers into bioavailable forms, supporting broader nutrient cycling and microbiome diversity in terrestrial ecosystems. In aquatic environments, negative phototaxis in , such as species, influences predator-prey dynamics by prompting vertical migration away from (UV) light exposure near the surface. This behavior reduces vulnerability to UV-induced damage and predation by sight-dependent , as histamine-mediated signaling in the drives descent into deeper, safer waters during daylight hours. Such taxis-mediated adjustments help maintain zooplankton population stability and regulate trophic interactions in freshwater and marine food webs. Chemotaxis facilitates symbiotic relationships between root nodule bacteria, like Rhizobium species, and by guiding bacteria toward host-derived exuded from roots. These compounds act as potent chemoattractants, promoting directed and initial attachment at infection sites, which is essential for nodule formation and subsequent . For instance, such as induce bacterial nod gene expression while enhancing chemotactic responses, ensuring efficient symbiont recruitment and mutualistic colonization. In pathogenic contexts, taxis contributes to the spread of infections through dynamics, as exemplified by swarming motility. This chemotaxis-regulated process allows coordinated surface migration toward nutrient-rich host tissues, facilitating establishment in lungs and wounds, which enhances persistence and resistance during chronic infections like . Swarming, modulated by chemosensory pathways, promotes rapid colonization and dispersion, amplifying dissemination within host communities. Taxis-driven aggregation in microbial mats structures communities by responding to environmental gradients, particularly oxygen, influencing spatial organization and metabolic interactions. Aerotaxis directs , such as , to oxygen-replete interfaces, forming biofilms that generate and maintain steep O₂ gradients, with high levels near the surface (>50%) supporting aerobic metabolizers while fostering anaerobic niches below. In cyanobacterial mats, positive phototaxis leads to filament aggregation under , optimizing and oxygen production, which in turn shapes layered community architectures and biogeochemical cycling.

Evolutionary Origins

The evolutionary origins of taxis are rooted in the early prokaryotic phase of life's history, with the emergence of motile behaviors coinciding with the appearance of the first free-living prokaryotes approximately 3.5 to 4 billion years ago. Fossil evidence from and genomic reconstructions indicate that swimming motility, a prerequisite for taxis, was present in the last common bacterial ancestor shortly after the divergence from the (LUCA). The system, a key mechanism for directed movement, is widely distributed across prokaryotic genomes, present in about 68% of bacterial and 47% of archaeal , suggesting its ancient establishment in prokaryotic lineages through vertical and occasional lateral transfer. Genomic analyses reveal conserved components of the taxis machinery, such as methyl-accepting chemotaxis proteins (MCPs), which function as primary sensory receptors in prokaryotes and show across and domains. These studies demonstrate that the core signaling elements evolved from simpler two-component regulatory systems, which are ubiquitous in prokaryotes and provided the foundational architecture for sensing and locomotor bias. In , the system exhibits structural conservation, including identical hexagonal arrays of chemoreceptors, indicating minimal divergence since its introduction via horizontal transfer from over 3.5 billion years ago. The expansion of taxis into eukaryotic lineages occurred through endosymbiotic events around 2 billion years ago, when an archaeal host acquired bacterial endosymbionts, enabling the development of more intricate structures like flagella and associated sensory pathways. While prokaryotic taxis relies on two-component systems for , eukaryotic versions show analogous but distinct mechanisms, such as G-protein-coupled receptors that transduce stimuli into cytoskeletal rearrangements, reflecting evolutionary rather than direct homology. This transition allowed for taxis responses to diverse stimuli beyond chemicals, including and . A major phase of for taxis followed the approximately 541 million years ago, as bilaterian animals diversified and integrated multiple sensory modalities into coordinated behaviors. Phylogenetic reconstructions highlight how conserved prokaryotic elements influenced the evolution of complex neural circuits in metazoans, enabling taxis to evolve into higher-order tropisms that supported exploitation. Overall, the phylogenetic history of taxis underscores its role as a foundational , conserved and diversified across life's domains through incremental genetic and structural innovations.

References

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