Symbiosis

What is symbiosis?

An intimate and sustained association of individuals of different species, often (but not always) for mutual benefit.

What are some example of symbiosis we’ve seen thus far?

Squi-brio, Wolbachia, and some of the human microbiome

What are requirements to be a symbiont?

1. Does the microbe live consistently with the host, often inside tissues, rather than just passing through? - yes.

2. Does the microbe persist for a meaningful part of the host’s life cycle? - yes.

3. Does the microbe influence host physiology, reproduction, or ecology (even if it’s bad for the host)? - yes.

All three requirements must be met to be considered a symbiont.

What are other things to consider for determining if a relationship is symbiont?

Predicting the persistence of the relationship, coevolutionary relationships, and molecular integration.

Lichen is a highly evolved form of _____.

Mutualism.

Describe the symbiont relationships of lichen.

Lichens consist of an intimate symbiosis between a fungus and an alga or cyanobacterium (sometimes both).

-Algae perform photosynthesis.

-Fungus provides protected environment, water, and nutrients absorbed from the environment.

-The lichen can persist in environments where neither would be able to persist alone.

Give an example of host-microbe symbiosis in water.

Many invertebrate animals acquire endosymbiotic algal symbionts, most commonly dinoflagellates. These endosymbionts are called zooxanthellae.

Algae:

Provide: photosynthetic products

Receive: protection from predators

The Animal (Coral, Clam, Anemone):

Provide: protection from predators

Receive: photosynthetic products

What is the cause of coral bleaching?

Coral bleaching occurs when environmental stress (rising water temperatures) causes corals to expel their endosymbiotic zooxanthellae, stripping the coral of both its color and its primary energy source.

Explain an example of microbial symbiosis on land.

Leafcutter ants do not actually eat the leaves they cut. Instead, they chop plant material into tiny pieces, bring it underground, and use it as compost to nourish their fungal gardens.

Because these dense, warm underground gardens are prime targets for parasitic competitor molds (like Escovopsis), the ants actively tend the garden and secrete specialized antimicrobials to keep pathogens at bay.

The Fungus

Provides: Specialized nutrient-rich food structures for the ants to eat

Receives: A steady supply of freshly harvested leaf substrate, constant weeding, and protection from parasitic molds via antimicrobial secretions

The Ants

Provide: Labor, plant food material, garden maintenance, and chemical defense against competitor fungi

Receive: A dependable, highly nutritious food source produced specifically by the fungus for the colony

Give an example of symbiosis in insects.

Bed bugs feed exclusively on vertebrate blood, which is a rich source of proteins and lipids but severely deficient in B-vitamins. Without Wolbachia synthesizing these missing nutrients, bed bugs cannot develop, reproduce, or survive.

Through HGT this specific strain of Wolbachia acquired the necessary metabolic operon to synthesize B-vitamins, locking in this nutritional partnership.

The Bacterium (Wolbachia)

Provides: Essential B-vitamins synthesized via horizontally acquired operons to supplement the host's blood diet

Receives: A protected intracellular home inside specialized host cells (bacteriocytes) and transmission to the next generation

The Host (Bed Bug)

Provides: A steady supply of host-derived resources, protection

Receives: Complete metabolic viability on a nutritionally restricted vertebrate blood diet

Give an example of a symbiosis in insects (specifically the smallest bacterial genomes).

Carsonella ruddii possesses a tiny genome of roughly 160 kb containing only about 182 predicted genes. Over millions of years of living exclusively inside host cells, it has shed genes for nearly all basic metabolic functions.

Because Carsonella depends so heavily on its host for survival and has lost the ability to live independently, many evolutionary biologists view it as a modern-day snapshot of how mitochondria and chloroplasts originated. It is actively losing genetic independence and transferring or outsourcing its essential functions to the host cell.

The Bacterium (Carsonella ruddii)

Provides: A few essential amino acid synthesis pathways that are missing from the host's nutrient-poor plant sap diet

Receives: Nucleotides, ATP, metabolic cofactors, components to build its own cell membrane, and a stable intracellular home

The Host (Insect)

Provides: Almost all basic metabolic machinery, energy (ATP), structural membrane components, and vertical transmission into offspring

Receives: Essential nutrients (amino acids) required to survive on a strict phloem sap diet

Explain the symbiotic relationship within Planococcus citri.

Planococcus citri has specialized host cells called bacteriocytes. Inside the bacteriocyte cytoplasm lives the primary endosymbiont, the β-proteobacterium Tremblaya princeps. Inside Tremblaya's cytoplasm lives a second endosymbiont, the γ-proteobacterium Moranella endobia.

Over evolutionary time, genes from diverse bacterial lineages (such as Alphaproteobacteria, Gammaproteobacteria, and Bacteroidetes) were transferred via HGT into the mealybug’s host genome. These host-encoded bacterial genes produce enzymes that fill in missing steps in the symbionts' truncated pathways.

Neither the insect nor either bacterium can produce essential amino acids on their own. Instead, synthesis pathways (such as for tryptophan, phenylalanine, and tyrosine) are shared across all three genomes in a circular, multi-organism assembly line.

Explain how lichen hit all the points to be considered symbiont.

Lives consistently with/inside tissues: Yes, because zooxanthellae reside permanently and intracellularly within the host's gastrodermal cells.

Persists for a meaningful part of the life cycle: Yes, because the algae stay with the host continuously throughout its adult life.

Influences physiology, reproduction, or ecology: Yes, because they supply up to 90% of the host's daily energy through photosynthesis and drive coral reef ecology.

Explain how ants hit all the points to be considered a symbiont.

Lives consistently with/inside tissues: Yes, because the fungus is maintained continuously within specialized underground garden chambers built by the colony.

Persists for a meaningful part of the life cycle: Yes, because the fungal cultivar persists across the entire 10–15 year lifespan of the ant colony.

Influences physiology, reproduction, or ecology: Yes, because it digests plant matter into specialized food structures (gongylidia) that nourish the entire colony and drive foraging behavior.

Explain how Wolbachia hit all the points to be a symbiont.

Lives consistently with/inside tissues: Yes, because Wolbachia lives intracellularly inside specialized host organs called bacteriocytes.

Persists for a meaningful part of the life cycle: Yes, because the bacteria persist through all nymphal instars and adult stages, being passed vertically via eggs.

Influences physiology, reproduction, or ecology: Yes, because they synthesize essential B-vitamins that are completely missing from the host's blood diet, making them vital for survival.

Explain how Carsonella hit all the points to be a symbiont.

Lives consistently with/inside tissues: Yes, because Carsonella permanently resides intracellularly inside the host's bacteriocytes.

Persists for a meaningful part of the life cycle: Yes, because it is retained continuously across all developmental stages and inherited vertically by offspring.

Influences physiology, reproduction, or ecology: Yes, because it provides essential amino acid pathways that enable the insect to survive on a nutrient-poor plant sap diet.

Explain how Planococcus citri hit all the points to be a symbiont.

Lives consistently with/inside tissues: Yes, because Tremblaya lives inside host bacteriocytes and Moranella lives inside Tremblaya.

Persists for a meaningful part of the life cycle: Yes, because both bacterial symbionts persist continuously across the mealybug's entire life cycle.

Influences physiology, reproduction, or ecology: Yes, because they form an integrated three-way metabolic network that synthesizes essential amino acids required for host growth and reproduction.

A _____ of one host may be a _____ of another, and give an example.

Parasite; mutualist.

In Humans: Vibrio cholerae is famously known as a pathogen that causes the severe waterborne disease cholera.

In Marine Ecosystems: V. cholerae acts as a mutualist to copepods.

What are the evolutionary origins of proteobacterial mutualists?

• Mutualism evolved independently 34–39 separate times within Proteobacteria.

• Beneficial host traits aren't just a rare, one-off event; frequently.

• Mutualistic Proteobacteria are much more likely to have evolved from parasitic ancestors than from free-living ones.

• A pathogen/parasite already possesses host-entry and cell-interaction machinery, making it easier to transition into a beneficial partner.

• Once a proteobacterial lineage becomes a mutualist, it rarely reverts back to being a parasite or a free-living bacterium.

• Extreme genome reduction and host dependency lock the bacteria into mutualism long-term.

What is Rhizobial symbiosis?

Associations between legumes (such as peas and beans) and Rhizobia bacteria (comprising multiple genera within Alphaproteobacteria). They induce legume roots to form specialized nitrogen-fixing nodules.

Explain the symbiosis: how it began, how do they recognize one another, and what keeps them from “cheating.”

1. The legume root releases plant secondary metabolites called flavonoids into the rhizosphere.

2. Rhizobia detect these flavonoids, which bind to NodD, a bacterial transcriptional regulator

3. Upon binding flavonoids, NodD activates the expression of other bacterial nodulation (nod) genes.

4. Different Rhizobia strains carry distinct NodD variants that recognize specific flavonoids (e.g., pea-nodulating Rhizobia respond to one flavonoid profile, while soybean-nodulating Rhizobia respond to another).

5. Activated nod genes cause Rhizobia to secrete Nod factors.

6. Host plant cell receptors recognize specific Nod factor chemical structures.

7. Nod factors trigger the root hair to curl around the bacterial cell, trapping it.

8. Host receptors only recognize compatible Nod factors, providing a second layer of species specificity. nod mutants that cannot synthesize Nod factors fail to induce root hair curling or nodule development

The bacteria induce plant cells to construct an infection thread (an invagination of the cell wall) through which Rhizobia travel into the root cortex.

Bacteria stimulate plant cortical cells to divide, forming the physical root nodule. Inside nodule cells, the bacteria differentiate into specialized, non-dividing forms called bacteroides that actively fix nitrogen.

Explain the nutrient exchange in nodule physiology and nitrogen fixation.

1. The plant supplies bacteroids with dicarboxylic acids/photosynthetic catabolites (malate, succinate, fumarate) that feed directly into the bacterial TCA cycle to produce energy (ATP and NADH).

2. In exchange, the bacteroid reduces atmospheric dinitrogen into ammonium or amino acids and exports them to the host plant.

3. Dinitrogen reduction is catalyzed by the bacterial enzyme nitrogenase.

4. Nitrogenase requires substantial energy (ATP/NADH from aerobic cellular respiration), but oxygen irreversibly inactivates (poisons) the nitrogenase enzyme complex.

5. To resolve this paradox, the host plant produces leghemoglobin (an oxygen-binding heme protein).

6. Leghemoglobin tightly binds free oxygen to keep free O2 levels extremely low (protecting nitrogenase) while steadily delivering micro-concentrations of oxygen to the bacterial electron transport chain for ATP production.

Explain the partner choice hypothesis.

The plant can detect cheater rhizobia and only allow nodule formation from noncheater bacteria.

Explain the Sanctions hypothesis.

Plants can’t prevent nodule formation from cheater strains, so they impose “sanctions” (reduced carbon and oxygen supply) on nodules that aren’t producing enough nitrogen.

• Experimental support: Studied soybeans, replaced N2 with Argon, which stops nitrogen fixation. N2 fixing rhizobia consistently grew to larger numbers than non-fixing rhizobia in nodules. Decrease in O2 supply may be the primary mechanism.