Developmental symbiosis is a biological phenomenon in which the normal development of an organism depends on interactions with symbiotic partners, often microbes, that influence gene expression, tissue formation, or physiological function.
Symbiosis is the intimate relationship between one or more organisms of different species. These organisms are referred to as symbionts. Many types of relationships are found in symbiosis; three examples are mutualism, commensalism, and parasitism. As the name suggests, mutualism is a mutual dynamic between the organisms where both can benefit from the relationship. Parasitism, however, is when one organism actively harms the host for their own benefit. Commensalism refers to a relationship where only one organism benefits while the other gains nothing but is also left unharmed. The most common type is the mutualistic relationship and can be viewed as either obligatory or facultative. Developmental symbiosis is the relationship between a developing organism and specific microorganisms.
Microscopic organisms exist all around the globe, even in the human body. They are responsible for the formation of many developmental functions within the body from its earliest stages of life. Microorganisms are prevalent in most somatic tissues and in reproductive germline cells. Certain bacteria allow cells to determine whether other bacteria are either harmful or helpful, building a stronger and sharper immune system. Other bacteria aid in the development of tissue to fully form structures of an organism's body.
Understanding the strong relation between developmental symbiosis and evolution is crucial to understanding how organisms function and adapt over time. This connection emphasizes that many different plants and animals are heavily influenced by the internal and external symbiotic microbes to develop their bodily structures and functions.
History Scientific understanding of symbiosis in development arose in the 19th century, particularly through the study of lichens and legumes.
Lichens Lichens had long been considered a discrete plant organism. In 1866, one researcher, Heinrich Anton de Bary, speculated that gelatinous lichens might actually form as a result of the penetration of an alga by a fungus.Egerton, F. (2015). Frank Egerton's history of ecological sciences. The Quarterly Review of Biology, 96(1), 80–139. https://esajournals.onlinelibrary.wiley.com/doi/10.1890/0012-9623-96.1.80#i0012-9623-96-1-80-b13 The following year, Simon Schwendener advanced the view that all lichens could be considered the product of such an association, notably in which the fungus exploits the algae. Mitchell, M. E. (2002). "Such a strange theory": Anglophone attitudes to the discovery that lichens are composite organisms, 1871–1890. Huntia, 11(2), 193–208. https://www.huntbotanical.org/admin/uploads/04hibd-huntia-11-2-pp193-208.pdf Schwendener received harsh opposition, as the dual nature of lichen challenged the traditional taxonomic view.
In 1873, Albert Frank conducted studies on hypophloedal lichens, which grow beneath the outermost layer of bark. Chardronnet, E. (2024). The politics of symbiosis. The Laboratory Planet, 6. https://www.makery.info/en/2024/12/08/english-the-politics-of-symbiosis/ He proposed that the association between fungi and algae is far more complex than simple parasitism, introducing the term "symbiotismus" to describe this intimate relationship. Two years later, Bary expanded on this concept by formally defining the dual nature of lichens as "Symbiose" in a lecture to German naturalists and physicians at Cassel. Later, in his publication Die Erscheinung der Symbiose, Bary refined the definition of symbiosis, establishing three fundamental criteria: (1) two entities must live together, (2) they must be in physical contact, and (3) they must be of different species.
Legumes While the study of lichens dominated early symbiosis research, parallel discoveries in microbiology soon expanded the concept of symbiosis. Toward the end of the 19th century, Hermann Hellriegel demonstrated that legumes form nodules in response to microbial colonization. Shurtleff, W., & Aoyagi, A. (2018). History of research on nitrogen fixation in soybeans (1887-2018): Extensively annotated bibliography and sourcebook. Soyinfo Center. ISBN 9781948436007. https://www.soyinfocenter.com/pdf/210/NiFi.pdf Using sterilized sand cultures, he showed that bacteria in the soil are responsible for inducing nodule formation, enabling leguminous plants to fix atmospheric nitrogen and grow even in nitrogen-poor soils.Linda Hall Library. (2020, October 21). Hermann Hellriegel. https://www.lindahall.org/about/news/scientist-of-the-day/hermann-hellriegel/ Dutch microbiologist Martinus Beijerinck later successfully isolated the bacteria from legume root nodules and named it Rhizobium.
Types
Obligate symbiosis Obligate symbiosis is a type of symbiotic relationship in which at least one organism, the symbiont, cannot survive independently and requires a host organism to survive and/or reproduce. The symbiont organism can survive by living on or inside of the host organism. A big form of this is endosymbiosis, where one organism lives inside the cells of another in a close, often essential, relationship.
This type of symbiosis can be harmful or beneficial to the host organism. Some obligate symbionts can cause deleterious mutations to the host organism, causing altered gene expression or manipulation of the host's reproductive systems to help reproduce more of the symbiont species. This benefits the symbiont, but in turn, can harm the fitness of the host. To benefit the host, some symbiont organisms provide essential nutrients and/or vitamins that the host would not normally be able to synthesize or consume. Symbionts can also provide priming for the host's immune system, allowing the immune system to fight off future infections more effectively.
Obligate relationships are often referred to as a process known as a "rabbit hole." The "rabbit hole" process is a metaphor for when the obligate organisms become committed to an inherited, mutually dependent symbiotic relationship in which both organisms are affected by unusual genomic evolutions. Changes for the symbionts can involve genome reduction, rapid protein evolution, and codon reassignments. In contrast, the host's changes can involve acquiring bacterial genes to help regulate and support their symbionts. These evolutionary changes are often irreversible and highly complex, leading to continuous and profound transformations in both partners—hence the term "rabbit hole." Once the process becomes permanently established this relationship cannot be broken without causing harm or death to one or both partners.
Facultative symbiosis Facultative symbiosis is a type of symbiotic relationship where organisms do not require one another for survival or reproduction. This means they can live independently, and still participate in symbiotic interactions. Organisms that choose to have facultative symbiosis do so due to the benefits. These benefits can be mutual, commensal or parasitic. Facultative symbiotic relationships and host changes can happen when organisms gain new genes from other species or lose important genes due to mutations, making them more dependent on their partners.
Facultative symbionts can provide protection against environmental stressors and some provide nutritional benefits. An example of this is the relationship between clownfish and sea anemone. The sea anemone provides protection for clownfish through its stinging tentacles, and the anemone benefits from cleaning by water circulation, being provided nitrogen compounds, and luring prey by the bright colors of the clownfish. Some bacteria (Serratia symbiotica) help aphids resist fungal infections and survive extreme temperatures. These relationships are becoming harder to detect or study, meaning that this could lead to biases in research.
In some species, facultative bacteria can be passed down to offspring (vertical transmission), allowing the relationship to continue across generations. As the offspring grows, the facultative relationship between host and symbionts can strengthen. This leads to a deeper physiological integration, where the host relies on the symbionts for nutrient synthesis, defense, and/or environmental adaptation. Meaning over time, the relationship may shift from facultative to obligate symbiosis.When facultative symbiosis becomes essential for survival, the relationship evolves into an obligate relationship. This transition is not always a one-way process. This means that even after becoming obligate, the symbiosis can revert or break down under the right conditions This shift from facultative to obligate is often seen in tightly integrated systems, such as those involving host-microbe interactions or plant-pollinator relationships, especially when consistent mutual benefit leads to genetic or physiological adaptations that make independent survival less practical.
Vertical vs. horizontal transmission As most animals use either vertical or horizontal transmission, there are some animals that use both. As an example, holometabolous insects use both vertical and horizontal transmission of gut bacteria.
Vertical transmission Vertical transmission refers to the transferring of symbionts directly from parent to offspring. This process ensures that each generation of hosts inherit a symbiotic partner. This maintains the symbiotic relation over time. Since the symbionts are consistently passed from parent to offspring, the host and symbiont co-evolve, making it unnecessary for the host to obtain the symbiont from nature each generation. Vertical transmission can be found in all types but is mostly found in obligate symbiosis, where the host is dependent on the symbiont and vice versa. As symbionts live in a protected environment (in the host), they lose the ability to survive on their own.
In aphids, for example, the subpopulation of a single mother's bacteriocyte is transferred to the embryo. Later on, in cellularization, the symbionts that have penetrated the embryo are subdivided again into different bacteriocytes, restarting and reaffirming the symbiotic relationship.
Horizontal transmission Horizontal transmission involves the gaining of symbionts from the external environment or other individuals, rather than from a parent. This transmission process can happen at all stages of life in different ways such as eating, physical contact, environmental exposure, or interactions with other individuals. This approach allows for greater flexibility and adaptability, enabling animals like insects to pick up microbes suited to new diets, habitats, or environmental conditions. Horizontal transmission allows animals to obtain symbionts that are well-suited to handle the current environment or changes in diet, instead of being bound to the symbiont, like in vertical transmission. Because the symbionts gained come from a variety of places, horizontal transmitted host tend to be diverse and allow them to adapt to new environments. It is important that hosts must encounter the right symbionts at the right time, because if the necessary symbionts are not present in the environment, the host may suffer.
Molecular and Genetic Mechanisms Symbiotic organisms produce signals within their hosts to influence certain functions within the host's body. These functions can range from gene expression to immune response to foreign bacteria.
Lipid-derived molecules