What is an endosymbiont, and what is the typical nature of its relationship with its host?

An endosymbiont is an organism that lives within the body or cells of another organism. Typically, this relationship is mutualistic, meaning both the endosymbiont and the host organism benefit from the association.

Can you provide some common examples of endosymbionts and their hosts?

Yes, common examples include nitrogen-fixing bacteria called rhizobia that reside in the root nodules of legumes, single-celled algae living within reef-building corals, and bacterial endosymbionts that supply essential nutrients to insects.

What significant role did endosymbiosis play in the early evolution of life on Earth?

Endosymbiosis was crucial in the development of eukaryotes and plants. For instance, it's theorized that approximately 2.3 billion years ago, a bacterium was engulfed by another organism and eventually evolved into the mitochondria, the powerhouses of eukaryotic cells. Later, about a billion years ago, some of these cells absorbed cyanobacteria, which evolved into chloroplasts, enabling photosynthesis.

Are there any more recent examples of endosymbiotic events leading to organelle-like structures?

Yes, approximately 100 million years ago, a lineage of amoeba called *Paulinella* independently engulfed a cyanobacterium that evolved into a structure functionally similar to chloroplasts, known as a chromatophore. Also, about 100 million years ago, a nitrogen-fixing bacterium, UCYN-A, became an endosymbiont of the marine alga *Braarudosphaera bigelowii*, evolving into a nitroplast that fixes nitrogen.

How are endosymbionts classified based on their dependency on the host?

Endosymbionts are classified as either obligate, meaning they require their host to survive, or facultative, meaning they can survive independently of their host.

What is the etymological origin of the term 'endosymbiont'?

The term 'endosymbiont' originates from the Greek words 'endon' meaning 'within', 'syn' meaning 'together', and 'biosis' meaning 'living', reflecting its meaning of an organism living within another.

What is the core concept of the symbiogenesis theory regarding the origin of eukaryotes?

The symbiogenesis theory posits that eukaryotic cells evolved through the absorption of prokaryotes. In this process, one organism engulfs another, and they develop a mutualistic relationship, with the engulfed organism eventually living exclusively within the host cells, leading to the development of organelles.

What typically happens to the genome of an endosymbiont as it integrates more deeply with its host?

As an endosymbiont becomes more integrated, its genome typically shrinks. This occurs as it discards genes whose functions are now performed by the host cell, effectively becoming dependent on the host for those functions.

What are the three primary modes of symbiont transmission from one generation to the next?

The three main pathways for symbiont transfer are horizontal transmission (acquiring from the environment or other hosts), vertical transmission (direct transfer from parent to offspring), and mixed-mode transmission (a combination of both).

How does the Rhizobia-Legume symbiosis initiate, and what is its significance?

The Rhizobia-Legume symbiosis begins when the legume host releases flavonoids, which trigger the rhizobia (endosymbionts) to activate their *Nod* genes. These genes produce signals that lead to the formation of root nodules, a process vital for nitrogen fixation in plants and contributing to novel functions through genetic exchange between the organisms.

What defines vertical symbiont transmission?

Vertical transmission occurs when a symbiont is passed directly from a parent organism to its offspring. This is in contrast to horizontal transmission, where symbionts are acquired from the environment.

What are the consequences for an endosymbiont that is transmitted vertically?

When symbionts are transmitted vertically, they often do not need to survive independently, which can lead to a reduction in their genome size. They may lose genes for essential molecules, relying on the host for these, while in return providing specific benefits like synthesizing essential amino acids or vitamins.

A holobiont is a unit formed by a host organism and its symbiotic microorganisms, particularly when the symbionts are highly dependent on the host, resembling cellular organelles.

What are primary (P-endosymbionts) and secondary endosymbionts in insects?

Primary endosymbionts (P-endosymbionts) have co-evolved with their insect hosts for millions of years, forming obligate associations and often displaying cospeciation. Secondary endosymbionts are more recently associated with their hosts, may be acquired horizontally, are not obligate, and often reside in the insect's hemolymph rather than specialized cells.

What are some well-studied examples of primary endosymbionts in insects and their functions?

Key examples include *Buchnera* in pea aphids, which synthesizes essential amino acids; *Wigglesworthia* in tsetse flies, which provides essential vitamins; and symbiotic protists in lower termites, which aid in digesting lignocellulosic materials. These symbioses are typically obligate for the insect's health and survival.

How are primary endosymbionts transmitted within insect hosts?

Primary endosymbionts are often transmitted maternally. For example, *Buchnera* is transmitted via the egg, while *Wigglesworthia* is transmitted through milk to the embryo. In termites, endosymbionts in the hindgut are passed through trophallaxis (food sharing) within the colony.

What is cospeciation, and in which type of endosymbiosis is it commonly observed?

Cospeciation is a process where parasites or endosymbionts speciate or branch in parallel with their hosts. This phenomenon is more frequently observed in hosts that have primary endosymbionts, indicating a long-term, co-evolutionary relationship.

What are some examples of secondary endosymbionts in insects and their roles?

The pea aphid harbors secondary endosymbionts like *Hamiltonella defensa*, which defends the aphid against parasitoid wasps by producing toxins. *Spiroplasma poulsonii* is another example, known to defend *Drosophila* flies against nematode parasites using ribosome-inactivating proteins.

How can attacking obligate bacterial endosymbionts be a strategy for controlling insect pests or disease vectors?

By targeting the obligate endosymbionts that are essential for the survival or health of pest insects (like aphids) or disease vectors (like the tsetse fly), it's possible to disrupt their life cycles and control their populations. This approach leverages the dependency of the host on its symbiont.

What are some examples of endosymbiosis in marine organisms?

Endosymbiosis is found in marine environments, such as extracellular endosymbionts in echinoderms, obligate chemoautotrophic bacteria in marine oligochaetes like *Olavius algarvensis*, and photosynthetic algae like *Vaucheria litorea* in the sea slug *Elysia chlorotica*. Jellyfish like *Mastigias* also host algae.

What are zooxanthellae, and where are they commonly found?

Zooxanthellae are dinoflagellate endosymbionts of the genus *Symbiodinium*. They are commonly found in symbiotic relationships with corals, mollusks (especially giant clams), sponges, and foraminifera, providing energy through photosynthesis.

How does the distribution of *Symbiodinium* relate to coral reef ecology and coral bleaching?

The distribution of *Symbiodinium* clades among different coral hosts can be influenced by environmental conditions. When reefs experience stress, shifts in these symbiont populations are linked to coral bleaching events, making their ecology crucial for understanding reef health.

What is the significance of endosymbiotic cyanobacteria in phytoplankton, particularly in nutrient-poor oceans?

In nutrient-poor ocean regions, endosymbiotic cyanobacteria, such as *Richelia intracellularis* found in diatoms like *Hemiaulus* and *Rhizosolenia*, fix nitrogen for their hosts. This process is vital for the phytoplankton's growth and contributes significantly to global carbon cycling.

What are some examples of endosymbiosis involving protists?

Examples include the protozoan *Mixotricha paradoxa*, which lacks mitochondria but hosts spherical bacteria that perform this function. The ciliate *Paramecium bursaria* hosts green algae (*Zoochlorella*) in its cytoplasm, and *Paulinella chromatophora* is an amoeboid with a cyanobacterial endosymbiont.

What is notable about the endosymbiotic relationship in *Paulinella chromatophora*?

*Paulinella chromatophora* is a freshwater amoeboid that harbors a cyanobacterial endosymbiont. This specific endosymbiont evolved independently to become functionally equivalent to traditional chloroplasts, demonstrating a unique case of organelle evolution.

How does the endosymbiosis between *Amoeba proteus* and bacteria illustrate acquired interdependence?

In a study, bacteria infected *Amoeba proteus*, initially causing mortality. However, after many host generations, the amoeba and bacteria became mutually dependent, suggesting that genetic exchange occurred, leading to a stable, integrated relationship where the amoeba required the bacteria to survive.

What is the role of endophytes in vascular plants?

Endophytes, which include bacteria, fungi, viruses, protozoa, and microalgae, reside within vascular plants. They assist in crucial processes such as growth, development, nutrient uptake, and defense against environmental stresses like drought, salinity, heat, and herbivores.

How are plant endosymbionts categorized, and what are their general interactions?

Plant endosymbionts can be categorized as epiphytic (living on the surface), endophytic (living within tissues), or mycorrhizal (associated with roots). These relationships can be beneficial, mutualistic, neutral, or pathogenic, and they can enhance plant productivity by providing resources or producing defensive compounds.

What is the origin of chloroplasts in plants, according to endosymbiotic theory?

Chloroplasts, essential organelles for photosynthesis in plants, are believed to have originated from a cyanobacterial endosymbiosis that occurred over a billion years ago. A free-living cyanobacterium was engulfed by a protist and eventually evolved into the chloroplast organelle.

What are Arbuscular Mycorrhizal Fungi (AMF), and what is their relationship with plants?

AMF are a diverse group of plant microbial endosymbionts that colonize the roots of almost all vascular plants. They help plants acquire soil nutrients, like nitrogen, in exchange for absorbing organic carbon produced by the plant's photosynthesis. AMF generally promote plant health and resilience to abiotic stresses.

How do plants attract AMF endosymbionts?

Plants attract AMF by releasing chemical signals from their roots, particularly flavonoids and strigolactones. These compounds act as attractants, guiding the AMF towards the plant roots to establish the symbiotic relationship.

What are endophytic fungi, and how do they benefit plants?

Endophytic fungi live within plant tissues, often in mutualistic relationships. They can directly benefit their host plants by enhancing growth, improving nutrient uptake, and providing defense against herbivores and environmental stresses. Some also produce compounds that affect the food chain, impacting predators of the plant.

What are endophytic bacteria, and what are some common genera found in plants?

Endophytic bacteria are a diverse group of microorganisms that colonize plant tissues without causing disease. Common genera include *Pseudomonas*, *Bacillus*, *Acinetobacter*, *Actinobacteria*, and *Sphingomonas*. They can promote plant growth by producing hormones like gibberellins.

What is the difference between 'passenger' endophytic bacteria and 'true' endophytes?

'Passenger' endophytic bacteria colonize plant tissues through random events, while 'true' endophytes possess specific adaptive traits that allow them to live in a consistent association with plants, often playing essential roles in host survival.

What role do archaea play as endosymbionts in plants?

While less studied than bacteria and fungi, archaea are found in plant-associated ecosystems. They can contribute to plant health, growth, and survival under stress, although their specific symbiotic roles are still under investigation.

What are endogenous viral elements, and where are they found?

Endogenous viral elements, such as endogenous retroviruses, are sequences found within the genome of an organism that resemble and are derived from viruses. The human genome, for example, contains thousands of these elements organized into various families.

What is the significance of genome reduction in endosymbionts like *Buchnera*?

Genome reduction in endosymbionts like *Buchnera* occurs because they live in a stable, protected environment within the host and rely on the host for certain nutrients or metabolic functions. This process leads to smaller genomes with lost genes, making them highly specialized to their symbiotic lifestyle.

How can endosymbionts influence the evolution of their hosts?

Endosymbionts can profoundly influence host evolution by providing new metabolic capabilities, such as nutrient synthesis or detoxification, or by altering host physiology and behavior. This can lead to the development of new niches, adaptations, and even the emergence of new species over time.

What is the proposed role of *Hamiltonella defensa* in pea aphids?

*Hamiltonella defensa* is a secondary endosymbiont found in pea aphids that provides a defense mechanism against parasitoid wasps. It achieves this by producing toxins that harm the wasp larvae, thereby protecting the aphid host.

What is the significance of *Spiroplasma poulsonii* as a defensive symbiont?

*Spiroplasma poulsonii* is a bacterium that can act as a defensive symbiont for *Drosophila* flies. It protects the flies from nematode parasites by producing toxins that target the parasites' cellular machinery, representing an early example of a mechanistically understood defensive symbiosis.

What are nitroplasts, and how did they originate?

Nitroplasts are organelles derived from cyanobacteria that are capable of fixing nitrogen. They originated from the endosymbiotic event where the bacterium UCYN-A became integrated into the marine alga *Braarudosphaera bigelowii*.

What is the relationship between *Wigglesworthia* and the tsetse fly?

*Wigglesworthia* is an obligate primary endosymbiont of the tsetse fly. It is vertically transmitted and primarily functions by synthesizing essential vitamins that the tsetse fly cannot obtain from its diet of blood.

How do the endosymbiotic protists in lower termites contribute to their diet?

The endosymbiotic protists residing in the hindguts of lower termites play a crucial role in digesting lignocellulosic materials, which form a significant part of the termites' diet. This symbiotic relationship allows termites to break down tough plant matter.

What is the significance of the *Hemialus*-*Richelia* symbiosis for phytoplankton?

The symbiosis between the diatom *Hemialus* and the cyanobacterium *Richelia intracellularis* is important because *Richelia* fixes nitrogen for its host, which is particularly beneficial in nitrogen-poor ocean waters. This partnership enhances the growth of both organisms.

What is the difference between obligate and facultative symbionts in the context of transmission?

Obligate symbionts absolutely require their host to survive and are typically transmitted vertically from parent to offspring. Facultative symbionts can survive independently and are often transmitted horizontally, acquired from the environment.

What is the role of *Cardinium* endosymbionts in arthropods?

*Cardinium* is a group of bacterial endosymbionts that are often vertically transmitted in arthropods. They frequently manipulate the host's reproductive biology, typically favoring females that transmit the endosymbiont to their offspring.

What is the significance of genome reduction in the evolution of organelles like mitochondria and chloroplasts?

Genome reduction is a key process in the evolution of organelles from endosymbiotic bacteria. As these bacteria became integrated into host cells, they lost many genes, either because the host provided those functions or because the reduced population size made natural selection less efficient at purging mutations, leading to streamlined genomes.

What is the potential application of studying insect endosymbionts?

Studying insect endosymbionts can provide insights into the fundamental processes of symbiosis, which can be applied to understanding similar relationships in other organisms. Furthermore, targeting essential endosymbionts could offer novel strategies for controlling insect pests and disease vectors.

What is the role of *Blochmannia* in ants?

*Blochmannia* bacteria are primary endosymbionts of *Camponotus* ants. They are thought to provide essential nutrients or aid in the metabolism of waste products for their host ants.

What is the significance of fungal endophytes in plant adaptation to polluted environments?

Fungal endophytes can help plants adapt to challenging environments, including those contaminated with toxic metals. They can achieve this by producing metabolites that detoxify pollutants or by enhancing the plant's overall resilience and growth under stress.

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Defining Endosymbionts

Living Within

An endosymbiont, or endobiont, is an organism that resides within the body or cells of another organism. Typically, this relationship is characterized by mutualism, where both the host and the endosymbiont benefit from the association.¹

Essential Partnerships

Examples of these vital partnerships include nitrogen-fixing bacteria, known as rhizobia, which inhabit the root nodules of legumes. Single-celled algae also live within reef-building corals, and various bacterial endosymbionts provide crucial nutrients to insects.²³

Ubiquitous Interactions

These intricate biological arrangements are not confined to specific niches; they are fundamental to many life forms, influencing ecological dynamics and evolutionary trajectories across diverse environments.

Evolutionary Milestones

Origin of Eukaryotes

Endosymbiosis played a pivotal role in the development of eukaryotic cells and plants. Approximately 2.3 billion years ago, a primitive archaeon is believed to have engulfed a bacterium, which eventually evolved into the mitochondria, the powerhouses of nearly all eukaryotic cells.⁴

Photosynthetic Powerhouses

Around one billion years ago, some of these early eukaryotic cells further incorporated cyanobacteria. These captured cyanobacteria transformed into chloroplasts, the organelles responsible for photosynthesis, enabling plants and algae to harness solar energy.⁵

Independent Engulfment

More recently, about 100 million years ago, a lineage of amoeba genus Paulinella independently engulfed a cyanobacterium. This symbiont evolved into a functional equivalent of chloroplasts, termed chromatophores, demonstrating recurring evolutionary pathways.⁶

Nitrogen Fixation

Similarly, approximately 100 million years ago, the nitrogen-fixing bacterium UCYN-A became an endosymbiont of the marine alga Braarudosphaera bigelowii, eventually developing into a nitroplast, an organelle specialized for nitrogen fixation.⁷ Diatoms in the family Rhopalodiaceae also host cyanobacterial endosymbionts, known as spheroid bodies or diazoplasts, which are considered early stages of organelle evolution.⁸⁹

Modes of Transmission

Horizontal Transfer

Horizontal symbiont transfer occurs when a host acquires a facultative symbiont from its external environment or from another host organism. The symbiotic relationship between Rhizobia bacteria and legumes, crucial for nitrogen fixation in plants, exemplifies this mode of transmission.²²

Vertical Transfer

Vertical transmission involves the direct transfer of symbionts from a parent organism to its offspring. This is characteristic of obligate endosymbionts, such as the bacteria Wigglesworthia in tsetse flies, which are transmitted via maternal milk, or Buchnera in aphids, passed through the egg.²⁹²³

Mixed-Mode Transmission

Some symbionts utilize a mixed-mode transmission strategy, alternating between horizontal acquisition from the environment and vertical transmission across generations. This flexibility can enhance their adaptability and persistence within host populations.²⁷²⁸

Diverse Hosts

Invertebrates

Insects

Insects host a vast array of endosymbionts, classified as primary (obligate, ancient associations, often cospeciating) and secondary (facultative, more recent associations). Primary symbionts like Buchnera in aphids provide essential amino acids, while others like Wigglesworthia in tsetse flies synthesize vitamins. Secondary symbionts, such as Hamiltonella defensa, can offer defense against parasitoids.³¹³²

Marine Life

Marine organisms exhibit remarkable endosymbiotic relationships. Corals host photosynthetic dinoflagellates (zooxanthellae), sea slugs like Elysia chlorotica retain functional chloroplasts from ingested algae, and diatoms such as Hemiaulus host nitrogen-fixing cyanobacteria (Richelia intracellularis), contributing significantly to oceanic nutrient cycles.⁵⁵⁶⁰

Protists

Cellular Partners

Protists showcase diverse endosymbioses. Mixotricha paradoxa harbors bacteria that function as mitochondria. Paramecium bursaria hosts photosynthetic algae (Zoochlorella), while amoeboid Paulinella chromatophora contains a cyanobacterial endosymbiont that has evolved into a chromatophore. Some amoebas have even undergone genetic exchange with their bacterial partners.⁶⁶⁷⁵

Plants

Plant Symbionts

All vascular plants harbor endosymbionts, including bacteria and fungi (endophytes). Arbuscular mycorrhizal fungi (AMF) form mutualistic relationships, aiding plants in nutrient uptake (especially nitrogen) in exchange for carbon.⁸⁸ Endophytic bacteria, such as Bacillus amyloliquefaciens, can produce plant hormones to promote growth.⁹⁹ The origin of plant chloroplasts itself is a prime example of ancient endosymbiosis with cyanobacteria.⁸⁶

Vertebrates

A Glimpse in Vertebrates

While less extensively documented than in invertebrates or plants, endosymbiosis also occurs in vertebrates. The spotted salamander, Ambystoma maculatum, engages in a relationship with the alga Oophila amblystomatis, which grows within its egg casings.⁷⁹

Key Concepts

Genome Reduction

Over evolutionary time, endosymbionts often undergo significant genome reduction. They shed genes whose functions are redundant or provided by the host, leading to smaller, specialized genomes that reflect their intimate dependence.²⁰

Cospeciation

Cospeciation describes the process where the evolutionary history of the endosymbiont mirrors that of its host. This pattern suggests a long-standing, obligate relationship where speciation events in one partner are tightly linked to speciation events in the other.^Diagram

Obligate vs. Facultative

Symbionts can be classified as obligate, meaning they cannot survive independently of their host, or facultative, capable of surviving both within and outside the host. Organelles like mitochondria and chloroplasts represent extreme cases of obligate endosymbiosis.¹⁰

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References

Aksoy, S., Pourhosseini, A. & Chow, A. 1995. Mycetome endosymbionts of tsetse flies constitute a distinct lineage related to Enterobacteriaceae. Insect Mol Biol. 4, 15â22.

K. Jeon, "Amoeba and X-bacteria: Symbiont Acquisition and Possible Species Change," in: L. Margulis and R. Fester, eds., Symbiosis as a Source of Evolutionary Innovation (Cambridge, Mass.: MIT Press), c. 9.

Von Dohlen, Carol D., Shawn Kohler, Skylar T. Alsop, and William R. McManus. "Mealybug Î²-proteobacterial endosymbionts contain Î³-proteobacterial symbionts." Nature 412, no. 6845 (2001): 433â436.

A full list of references for this article are available at the Endosymbiont Wikipedia page

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Symbiotic Souls

💡 Dive in with Flashcard Learning! 💡

Defining Endosymbionts 🤝

🏠 Living Within

🌱 Essential Partnerships

🌍 Ubiquitous Interactions

Evolutionary Milestones 🧬

✨ Origin of Eukaryotes

☀️ Photosynthetic Powerhouses

🔄 Independent Engulfment

💧 Nitrogen Fixation

Modes of Transmission ➡️

🌐 Horizontal Transfer

👨‍👩‍👧‍👦 Vertical Transfer

🔄 Mixed-Mode Transmission

Diverse Hosts 🌍

Invertebrates

🐜 Insects

🌊 Marine Life

Protists

🦠 Cellular Partners

Plants

🌳 Plant Symbionts

Vertebrates

🦎 A Glimpse in Vertebrates

Key Concepts 💡

📉 Genome Reduction

👯 Cospeciation

⚖️ Obligate vs. Facultative

Teacher's Corner 🧑‍🏫

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📜 Important Notice

Dive in with Flashcard Learning!

Defining Endosymbionts

Living Within

Essential Partnerships

Ubiquitous Interactions

Evolutionary Milestones

Origin of Eukaryotes

Photosynthetic Powerhouses

Independent Engulfment

Nitrogen Fixation

Modes of Transmission

Horizontal Transfer

Vertical Transfer

Mixed-Mode Transmission

Diverse Hosts

Insects

Marine Life

Cellular Partners

Plant Symbionts

A Glimpse in Vertebrates

Key Concepts

Genome Reduction

Cospeciation

Obligate vs. Facultative

Teacher's Corner

True or False?

Test Your Knowledge!

Gamer's Corner

Discover other topics to study!

References

Feedback & Support

Disclaimer

Important Notice