Symbiosis

Clean, symbiosis of an algae and a fungus

The term symbiosis (from the Greek: σύν, syn, 'together'; and βίωσις, biosis, to live) is applied to biological interaction, to the relationship or intimate association of organisms of different species to benefit each other in their vital development.

The German biologist Albert Bernhard Frank, while studying lichens, coined the term to describe the close relationship between organisms of different types. The German botanist Anton de Bary, in 1879 defined symbiosis as "the life in conjunction of two dissimilar organisms, usually in close association, and usually with beneficial effects to at least one of them". The definition of symbiosis is under debate, and the term has been applied to a wide range range of biological interactions. Other sources define it more narrowly, as those persistent relationships in which both organisms obtain benefits, in which case it would be synonymous with mutualism.

Symbiosis is often identified as long-term mutualistic relationships that end in coevolution. By analogy, in sociology, symbiosis can refer to societies and groups based on collectivity and solidarity.

Concepts

The term symbiosis can have the following definitions:

• Wide symbiosis: Symbiosis Lato sensu it is defined as any biological interaction between species, whether they affect positive or negatively among them, such as mutualism, comensalism and parasiticism. After 130 years of debate, the books of biology and ecology in English prefer this broad definition that coincides with the last concept that De Bary used.
• Symbiosis as a synonym for mutualism: It is the most widely used definition, where the relationship between two species is beneficial for both.
• Symbiosis in the strict sense: Other treaties even differentiate symbiosis from mutualism, since in both cases the relationship between species is beneficial, but in mutualism this is convenient but not indispensable for survival, instead in symbiosis the relationship is indispensable or obligatory.

Types of symbiosis

Symbiosis can be classified:

• In view of the spatial relationship between participating agencies: ectosimbiosis and endosymbiosis. In ectosimbionosis, the symbiote lives on the body, on the outside of the host organism, included inside the surface of the digestive tract or the canal of the exocrine glands. In endosymbionosis, the symbiote lives either within the host cells, or in the space between them.

• From a perspective on the costs and benefits of each participant, symbiotic relationships in nature can be classified as mutualism, comensalism and parasitism. In mutualism both species benefit; in comensalism the relationship is beneficial to one of them and indifferent to the other; and in parasiticism the relationship is positive for one, although harmful to the other.

• Whether these are optional or mandatory, or also distinguishing whether they are permanent or temporary.

• Similarly, it can be distinguished between symbiosis of vertical transmission, in which there is a direct transfer of the infection from host organisms to its progeny, and symbiosis of horizontal transmission, in which symbiote is acquired from the environment in each generation.

The continuum between parasitism, commensalism and mutualism

The theoretical criteria for assigning the labels of parasitism, commensalism, or mutualism is the net effect on the host's inclusive fitness.

Parasitism is widespread in nature. In some cases it may be the first stage of a continuous process that would lead to mutualism.

Many species of arthropods have endosimbionts of hereditary transmission. Because the persistence of these hereditary symbionts depends intimately on the survival of their hosts, it has generally been assumed that the microorganisms that, with great efficiency, are transmitted from parents to children for generations of hosts should evolve over time until they become beneficial to their hosts. It is becoming increasingly clear that hereditary symbionts are very common in arthropods. Many of these symbionts are beneficial, but a considerable part of them are spellers and, rather than being beneficial to their hosts, they have antagonistic relationships with some of them.

Michael E. N. Majerus, Simbionts who cause speller effects in arthropods.

In this stage, the parasite must attenuate its virulence against its host and, among other adaptations, it emerges from a typical characteristic of all organisms, its tendency to reproduce geometrically, self-regulating this tendency; in parallel, the host must react by neutralizing the deleterious effects of its parasite. "At any moment those associations can dissolve, their members can change and even destroy each other, or simply lose their symbiote."

This first symbiotic stage is the most unstable, since «the success of parasitism [success of the parasite and also of the symbiosis] lies in accommodation and survival. The parasite's success is measured not by the disorders it causes its host, but by its ability to adapt and integrate into the latter's internal environment." Very high integration stages could be reached in which, before reaching a mutualistic relationship could already be taking place transfer of genetic material.

It has also been postulated that there is a balance between competition and mutualism interactions depending on whether environmental conditions are favorable or not, this explains the hypothesis or paradigm of continuous parasitism mutualism.

Degrees of integration in symbiotic processes

American biologist Ivan Wallin tried to explain how symbiotic relationships might get started. In 1927, in his book Symbionticism and the Origin of Species he used the term "prototaxis" to explain the initiation of symbiotic relationships; To explain this beginning, he resorted to the innate response of cells and, in general, of organisms in the presence of other organisms. Wallis used as examples the tendency of the mouse to flee from the cat, the tendency of the shark to swallow the fish, the fly to lay eggs in the bloody tissues of a wild boar. He called these positive or negative tendencies «prototaxis». «The prototactic tendency of heterotrophs to absorb the products of photosynthesis, or else to ingest the photosynthetic organisms themselves —and the resistance of these organisms to being ingested— would be prototactic reactions that promote the proliferation of photosynthetic eukaryotes. Algae, lichens, green worms, brown corals, and green hydras constitute a small part of the result of these symbiotic relationships." Based on Wallin's prototaxis, it could be said that the tension produced by the different reactions of organisms in the presence of other organisms—tendency to "approach" and "move away"—would explain the beginning of symbiotic relationships.

Once established, the relationship between symbionts could reach different degrees of integration:

• The degree of less integration would be the one in which symbionts establish a relationship of "comportation": they would live together with each other and both would have learned to benefit from their mutual presence. The Portuguese frigate and pastoral fish or anemones and clown fish are examples of these subtle symbiotic behaviors.
• Another degree that can achieve symbiotic relations would be the "metabolic": "Frequently the metabolic product, the exudate or the residue of one of the members of the association becomes food for the other. Probably all the green animals that have been studied (such as the flat or silvery worm) Convoluta roscoffensisor Hydra viridis of the ponds), as well as all lichens, are integrated at this level."
• A greater degree of integration means that in which, for example, the proteins of one of the members of symbiosis become essential to the other: "In the beans and peas plants we find an excellent example of this kind of integration. At the roots of a clover, a cloak or a bean plant you can see a small rosacea protuberances. It is nitrogen fixing nodules, in which inside it medra certain type of bacteria, rhizobes. Another swimmer-shaped bacteria, all of them have ended up becoming bacteroid swollen. These oversized bacteroids, full of holes, can no longer be divided or grown."
• The maximum degree of integration and the most radical would be that in which these unions lead to the transfer of genetic material and the consequent fusion of symbionts, forming from it a new individual. Genetic material of one of the symbionts becomes integrated into the genome of the other, emerging a new individual that integrates his symbionts. This state is known as "simbiogenesis". The most transcendental cases of this type of extreme symbiosis were the symbiogenetic processes that originated the eukaryotes. The ability to breathe oxygen as a result of the acquisition of mitochondria caused the origin of the animals, and the photosynthetic capacity subsequently acquired with chloroplasts originated the plant kingdom. In both cases, mitochondria and chloroplasts have their origin in free living bacteria. The descendants of these bacteria are still among us.

Symbiotic processes, plausibly, would follow these steps: initially, an individual would come into contact with another individual or group of individuals, in principle that relationship could be parasitic, but over time both individuals could reach a relationship mutualist, the host would find advantages in the characteristics and specialties of the hosted. If this point is not reached, natural selection would not favor this relationship, gradually decreasing the number of these individuals in the population as a whole; on the contrary, a fruitful relationship would be favored and the individuals involved would proliferate.

Symbiosis from an evolutionary point of view could be considered as a process in which symbionts strengthen their relationship. Depending on the characteristics of the symbiosis and the symbionts that make it up, this relationship could reach its maximum degree of integration: symbiogenesis.

Examples of symbiosis in nature

Lichens are characteristic symbiotic organisms, in their case the product of symbiosis between a fungus and an alga or cyanobacteria. Numerous structural types of lichens can be distinguished: from the simplest, where fungus and algae come together by chance, to the most complex, where their symbionts give rise to a thallus that is morphologically very different from the one they form separately, and where the alga is found forming a layer under the protection of the fungus. As a product of symbiosis, they are exceptionally resistant to adverse environmental conditions and are therefore capable of colonizing very diverse ecosystems, finding them in ecological niches in extreme conditions.

Mycorrhizae are symbiotic relationships that sometimes include more than two types of interactions.

Many corals, as well as other groups of cnidarians such as Aiptasia (a genus of sea anemones), form a symbiotic relationship with a class of algae, zooxanthellae, of the genus Symbiodinium, a dinoflagellate.Aiptasia, a well-known pest among coral aquarium hobbyists, serve as a valuable model organism in the study of cnidarian-algal symbiosis. Typically, each polyp harbors a species of algae. Through photosynthesis, these provide the coral with energy, and aid in calcification. Up to 30% of a polyp's tissue can be plant material.

Many organisms exhibit symbiotic associations with chemosynthesis bacteria, with the first discoveries in the 1980s being giant tubeworms from deep ocean hydrothermal vents.

Many organisms have symbiotic relationships with viruses. For example, parasitoid wasps (such as Braconidae and Ichneumonidae) insert viruses into their hosts to disable host defenses that would destroy the parasitoid larvae. Some aphids inject viruses into host plants to disable their defenses.

Importance of symbiosis in nature

Symbiosis, the system in which members of different species live in physical contact, is an archaic concept, a specialized biological term that surprises us. This is because of the unaware that we are of its abundance. It is not only our eyelashes and intestines that are crowded with animal and bacterial symbionts; if you look in your garden or in the neighborhood park the symbionts may not be obvious but are omnipresent. The clover and the vine, two common herbs, have buns on their roots. They are nitrogen-fixing bacteria essential for their healthy growth in poor soils in this element. Let's take the trees, maple, oak and the American walnut, interwoven in their roots, there are three hundred different symbiont fungi: the mycorrizas that we can observe in the form of mushrooms. Or we count a dog, usually unable to notice the symbiotic worms that live in their intestines.

Lynn Margulis, Symbiotic Planet.

An example of mutual symbiosis between a clown fish that nothing among the tentacles of Anémona. That fish protects its territory from other fish eaters of anemone and in return the tentacles of anemone protect you from other predators.

A typical example of "behavioral symbiosis" is the relationship between the sea anemone and hermit crab: the crab "offers" displacement to the anemone and the anemone offers protection with its venomous tentacles. Another example is that of the Luther's goby, a fish, and a blind shrimp. The shrimp digs a burrow with its strong legs and allows the fish to occupy it as well. In return, he acts as a guide, guiding the shrimp in the search for food. The shrimp touches the fish's tail with its antennae and it moves it when it detects some danger: in this case, the two retreat towards the burrow.

Mycorrhiza is also important as a symbiotic association.

Different degrees of symbiotic integration are represented by termites and the communities of bacteria housed in their digestive system and that allow them to digest wood. In different species, the degree of genetic integration is also different. Ruminants also have communities of microorganisms that allow them to digest the cellulose of grasses. We, the human species, are made up of numerous communities of bacteria; 10% of our dry weight corresponds to those microorganisms that maintain different symbiotic relationships with us. Also, 250 of our genes correspond to genetic material from bacteria.

Symbiosis and biological novelty

There is biological novelty when an individual acquires new characteristics that in turn are inherited by their descendants.

The mutual adaptation of the symbionts supposes a transformation of both that alters their characteristics and can be observed stable past generations. Those in which it can be proven that they are hereditary should be considered biological novelties.

Also, symbiotic processes would be a direct source of biological novelty in those cases in which genetic transfer occurs; genes or sets of genes are transmitted horizontally between symbionts, giving them new characteristics that would be hereditary. Symbiogenesis would be the most radical source of biological novelty through this horizontal transfer of genes, resulting in a new individual with the symbionts forming part of the new individuality.

It is impossible for us to witness the process followed by symbiotic relationships in nature, they are processes that could span tens of thousands of years. Kwang Jeon, from the Department of Zoology at the University of Tennessee (United States) was able, by chance, to reproduce, in part, one of these processes.

During an experiment with amoebas, he observed how in one of the batches the amoebas were getting sick and dying. Observed under the microscope he could see that they were infected by bacillus-shaped bacteria. A small proportion managed to survive, they were fragile amoebas, very sensitive to environmental changes. For five years, Jeon cared for these infected amoebas, allowing a proportion of them to survive and reproduce. After ten years, the infected amoebas lived and reproduced normally. At this point, through various experiments, he was able to observe that the amoebas could no longer survive without their bacteria. In the process, the community of bacteria in each amoeba, which had originally numbered about 100,000, had self-regulated and dropped to 40,000, and "Jeon's amoebas were killed by penicillin, which attached to the wall." cells of the bacteria that they had inside and destroyed the interdependent population that is the cell. The pact between bacteria and amoebas has become so intimate and strong that the death of one of the members of the alliance means the death of both".

This laboratory work could be considered as proof that symbiosis generates biological novelty. Another laboratory work, in this case that of Theodore Dobzhansky with Drosophila (fruit fly) subjected two groups of Drosophila to different environments. After two years of reproducing intensely, being subjected to these different environments, a differential in the number of their bacterial symbionts was promoted from one group to another. The result showed that inter-group fertility decreased. Individuals were fully fertile with individuals from the same group, but fertility decreased among individuals from different groups. Margulis argues that such a decrease in fertility expresses the beginning of genetic isolation and, therefore, a beginning of speciation.

The transition from prokaryotes to eukaryotes has meant one of the most important milestones in biological evolution. The hatching of eukaryotes, with the newly acquired complexity, enabled their evolution towards very diverse and complex forms that today constitute four of the five kingdoms in which life is classified: protists, fungi, animals and plants are eukaryotes (the kingdom is not eukaryotic). including it is made up of bacteria, origin of prokaryotes). This step would not have been possible if different bacteria had not entered into a symbiotic relationship. Margulis describes the origin of eukaryotes through successive symbiotic relationships between different bacteria that led to the most extreme symbiotic relationship: symbiogenesis.

Symbiogenesis

The most impressive cases of symbiogenesis in evolution, and the most documented, are those that describe the origin of eukaryotic cells. If this milestone in evolution had not occurred, neither protists, nor fungi, nor animals, nor plants would exist; life today would probably be limited to a conglomerate of bacteria.

In 1883, the German biologist Andreas Schimper proposed that the photosynthetic capacity of plant cells could come from cyanobacteria still present in nature and with similar capacities. At the beginning of the 20th century, the Russian school (it was Konstantin Merezhkousky who coined the term Symbiogenesis) and later the French biologist Paul Portier and the American Ivan Wallin proposed that the origin of eukaryotes was found in symbiotic processes. Margulis, rescuing these forgotten and undervalued works, described this step through a succession of these symbiotic processes. Although the first step (the acquisition of spirochetes as responsible for the motility of these cells) is still discussed today, he was able to demonstrate that mitochondria (responsible for their aerobic capacity and origin from the animal kingdom) and chloroplasts (origin of photosynthetic capacity and from the plant kingdom) came from free-living bacteria involved in symbiotic processes.

Symbiogenetic hypothesis

However, Margulis went beyond the process of eukaryotic cell emergence and also posited more controversial hypotheses.

Lynn Margulis, after formulating in 1967 the theory of serial endosymbiosis in which the origin of eukaryotes is described through successive symbiogenetic processes, once the action of symbiogenesis in this origin has been demonstrated, defends that these processes are widespread in nature, being the promoter of the symbiogenetic theory that highlights the role of symbiogenesis in evolution, considering symbigenetic processes the main source of biological novelty: «Symbiosis, the union of different organisms to form new groups, has turned out to be the most important force for change on Earth."

To defend his hypothesis, Margulis indicated that since the end of the 19th century for the Russian school (Konstantín Merezhkovski, Andrey Faminstyn and Borís Kozo-Polianski) «symbiogenesis was considered crucial for the generation of biological novelty. The Russian bibliography, interpreted by the historian of science Liya N. Khakhina, was not available in English until 1922. It took two generations of scholars to summarize the vast bibliography of Russian botanists. It seems today as if this bibliography was ignored for this very reason. Old literature written by Russian botanists lacks appeal for the Anglophone market."

The presence of 250 genes in our DNA, genes in which its bacterial origin can be identified, could be the vestiges of recent symbiotic processes that culminated in genetic transfer and, consequently, would mean biological novelty. Likewise, the multiple communities of microorganisms that constitute us could lead to future symbiogenetic processes, passing the genetic information of these microorganisms to form part of our genome.

Among the numerous quotes from Margulis about his hypothesis, the following stand out:

Living beings challenge a precise definition. They fight, feed, dance, mate and die. On the basis of creativity of all forms of large family life, symbiosis generates newness. Gather different life forms, always for some reason. Hunger often binds the predator to the prey, or to the mouth with the photosynthetic bacteria or the algal victim. Symbiogenesis brings together different individuals to create larger and more complex entities. Symbiogenetic forms of life would be even more improbable than their inverossimiles "progenitors". The "individues" permanently merge and regulate their reproduction. They generate new populations that become new multi-unit symbiotic individuals, which become "new individuals" at broader and inclusive levels of integration. Symbiosis is not a marginal or rare phenomenon. It's natural and common. We have a symbiotic world.

Lynn Margulis, Symbiotic Planet.

The creative force of symbiogenesis produced eukaryotic cells from bacteria. Therefore, all higher organisms—protoctists, fungi, animals and plants— originated symbiogenetically. However, the creation of novelty through symbiosis did not end the evolution of the first nucleated cells, but the symbiosis remains everywhere. There are numerous examples of evolution by symbiosis that aspire to its beauty.

Margulis, Sagan, Captando genomas.

Another example of recent research on symbiosis indicates that the transition from green algae to terrestrial plants was made from the union of genomes (genetic material) of a fungus with some ancestor of green algae. Lichens are very well known symbiosis products. They are all fungi in symbiosis with cyanobacteria or fungi in symbiosis with green algae. The two types of life—photo-synthetic and heterotrophic— are intermingled to form a new plant-like organism that can achieve great longevity: lichen.

Lynn Margulis, Dorion Sagan, Microcosmos.

Scientists have discovered that bacteria, besides being the basic structural units of life, are also found in all other beings that exist on Earth, for which they are indispensable. Without them, we would not have air to breathe, our food would lack nitrogen and there would be no soils to cultivate our crops. Without the microorganisms, the essential processes for life would slow down and the Earth would be as sterile as Venus and Mars. The microorganisms have not been left behind in the evolutionary scale; on the contrary, they surround us everywhere and form part of us. In addition, the new knowledge of biology alters the vision that shows evolution as a continuous and bloodthirsty competition between individuals and species. Life did not conquer the planet through combat, but through cooperation. Life forms multiplied and became more complex by associating others, not by killing them.

Lynn Margulis, A revolution in evolution.

Every living being must be contemplated as a microcosm, a small universe formed by a multitude of inconceivably tiny organisms, capable of spreading themselves, as numerous as the stars in the sky."

Margulis maintained that life does not resemble, as postulated from Darwinism, what one would expect from a zero-sum game in which one wins at the cost of what the other loses. The wedges metaphor described by Darwin, to describe this relationship between organisms, exemplifies this model: Nature is represented by a limited surface completely occupied by wedges inserted into it; when hitting on one of the wedges and making it insert more, another wedge comes out displaced to the outside. Symbiosis contradicts these models, it is not assimilated to a zero-sum game in which one wins and the other loses, in the case of symbiosis both win; nor do these relationships necessarily have to prosper at the expense of other individuals (in the In the case of eukaryotes with the acquisition of mitochondria capable of metabolizing oxygen, they did not prosper at the expense of the rest of the bacteria, for example, at the expense of sulfur-metabolizing bacteria.The number of bacteria continued to prosper despite or favored by the great expansion of eukaryotes), symbiotic relationships are synergistic relationships in which individuals who learn to live together benefit from a multiplier effect.

The lichen, according to Margulis, would be another well-established example of symbiogenesis. As he postulates, its characteristics allow its symbiogenetic origin to be perfectly recognized: the respective sizes of what its symbionts were are not excessively discrepant and observed under a microscope, these symbionts that were involved in the fusion can be recognized.

The lichens provide a characteristic example of symbiogenesis. Moreover, the individual lichen is something different from his two components. It's not a green algae or a cyanobacteria, not a fungus. It's a lichen. The lichens, evolutionary novelties arising through the acquisition of alga genomes or cyanobacteria, took their own path and exhibit features other than those of their ancestors. Although traditionally studied within botany, lichens have been fundamental to the concepts of symbiosis and symbiosis in evolutionary thinking, despite which their symbiotic nature has made them considered as marginal evolutionary phenomena. They may have been accepted as an example of the power of symbiogenesis to generate evolutionary novelty, only because both partners are of the same size. Both algae and fungi can be easily observed, simply with the help of a microscope of few increases, so it is not possible to study one without simultaneously studying the other. On the other hand, in some green animals (as in the case of the flat worm species) Convoluta roscoffensis) the respective sizes of the components differ enormously. The worm measures inches, while the tiny photosynthetic organisms — algae — are microscopic. Such discrepancies in size make both symbiosis and the corresponding symbiogenesis less evident.

Lynn Margulis, Dorion Sagan, Captando genomas.

Lichens, in cases with a high degree of integration of their symbionts, could become a "highly illustrative" model of the origin of biological novelty through symbiogenesis.

Initially considered as an evolutionary mechanism with little transcendence, since Lynn Margulis discovered the symbiotic origin of the eukaryant cell, the role of symbiosis as a mechanism for generating biological novelties has been acquiring a growing role, to the point that for many scientists, including Margulis, the symbiogenesis is the source of a large number of elutiveses.

Rafael García Alonso

Criticism of the symbiogenetic hypothesis

However, from the scientific community, which currently accepts the theory of modern evolutionary synthesis, according to which errors in DNA replication are the main cause of biological novelty, these cases of symbiogenesis are considered as cases sporadic and insignificant. Although it is difficult to find published criticism of Lynn Margulis's symbiogenetic proposal, this proposal is rejected by numerous specialists in the field of evolution who today consider the current theory of modern evolutionary synthesis (neo-Darwinism) to be satisfactory.).

The most recent ecogenomic works have found evidence that selection by competition is not the main force in the selection of genomes of natural communities, on the contrary, various works have been published on symbiogenesis and its role in the construction of niches and extragenetic mechanisms (epigenetics, extragenetic adaptation, phenotypic plasticity)—these phenomena also modify the selective pressures of organisms' genomes and are more common than previously thought. positive interactions such as mutualism and the assembly of communities. Therefore, controversy has been unleashed in having to restructure the modern evolutionary synthesis incorporating null models of evolution and niche construction.

The most advanced studies of mutualism and coevolution were led by biologist Peter Kropotkin, a Russian scientist and anarchist in opposition to Darwin's idea of survival of the fittest which has historically been socially misinterpreted putting competition and selection as the main engine of evolution, for which reason his works were discarded for a long time by Western science in times of the USSR that supported the idea of community (socialism) and mutual support, later with the genomic era his works are being resumed that complement the symbiogenetic theory of Margulis as the hypothesis of the Red King.