Horizontal transmission is the transmission of organisms between biotic and/or abiotic members of an ecosystem that are not in a parent-progeny relationship. Because the evolutionary fate of the agent is not tied to reproductive success of the host, horizontal transmission tends to evolve virulence. It is therefore a critical concept for

evolutionary medicine Evolutionary medicine or Darwinian medicine is the application of modern evolutionary theory to understanding health and disease. Modern biomedical research and practice have focused on the molecular and physiological mechanisms underlying hea ...

Biological

Pathogen transmission

In biological, but not cultural, transmissions the carriers (also known as vectors) may include other species. The two main biological modes of transmission are ''anterior station'' and ''posterior station''. In anterior station, transmission occurs via the bite of an infected organism (the vector), like in

malaria Malaria is a Mosquito-borne disease, mosquito-borne infectious disease that affects vertebrates and ''Anopheles'' mosquitoes. Human malaria causes Signs and symptoms, symptoms that typically include fever, Fatigue (medical), fatigue, vomitin ...

dengue fever Dengue fever is a mosquito-borne disease caused by dengue virus, prevalent in tropical and subtropical areas. Asymptomatic infections are uncommon, mild cases happen frequently; if symptoms appear, they typically begin 3 to 14 days after i ...

, and bubonic plague. Posterior station is transmission via contact with infected feces. Examples are rickettsiae driven diseases (like

typhus Typhus, also known as typhus fever, is a group of infectious diseases that include epidemic typhus, scrub typhus, and murine typhus. Common symptoms include fever, headache, and a rash. Typically these begin one to two weeks after exposu ...

), which are contracted by a body louse's fecal material being scratched into the bloodstream. The vector is not necessarily another species, however. For example, a dog infected with

Rabies Rabies is a viral disease that causes encephalitis in humans and other mammals. It was historically referred to as hydrophobia ("fear of water") because its victims panic when offered liquids to drink. Early symptoms can include fever and abn ...

may infect another dog via anterior station transmission. Moreover, there are other modes of biological transmission, such as generalized bleeding in

ebola Ebola, also known as Ebola virus disease (EVD) and Ebola hemorrhagic fever (EHF), is a viral hemorrhagic fever in humans and other primates, caused by ebolaviruses. Symptoms typically start anywhere between two days and three weeks after in ...

Symbiont transmission

Symbiosis Symbiosis (Ancient Greek : living with, companionship < : together; and ''bíōsis'': living) is any type of a close and long-term biological interaction, between two organisms of different species. The two organisms, termed symbionts, can fo ...

describes a relationship in which at least two organisms are in an intimately integrated state, such that one organism acts a host and the other as the symbiont. There are obligate, those that require the host for survival, and facultative symbionts, those that can survive independently of the host. Symbionts can follow vertical, horizontal, or a mixed mode of transmission to their host. Horizontal, or lateral, transmission describes the acquisition of a facultative symbiont from the environment or from a nearby host. The life cycle of the host includes both symbiotic and aposymbiotic phases. The aposymbiotic phase generally begins in the

germ cell A germ cell is any cell that gives rise to the gametes of an organism that reproduces sexually. In many animals, the germ cells originate in the primitive streak and migrate via the gut of an embryo to the developing gonads. There, they unde ...

s, and during development, the host organism acquires the symbiont and translocates it to a symbiont-housing organ. The host will release the symbiont before reproduction. Determining the transmission mode of a symbiont requires identifying the key aspects that define transmission. For horizontal transmission one would need to check the host at each life stage for the presence of the symbiont and determine whether the symbiont is released from the host into the environment. In hydrothermal vent tubeworms, release of a symbiont upon host death was demonstrated using a symbiont recruitment plates and fluorescence ''in situ'' hybridization (FISH). Each life cycle stage of the tubeworm was sampled and examined also using FISH to determine the aposymbiotic and acquisition life cycle stages as larvae and settled larvae >250μm in length. Implications of horizontal transmission include the critical need for specificity in recognition and acquisition methods and the larger genetic diversity maintained by the symbiont species. Recognition specificity can be achieved through complex signaling through the release of hormones, such as with Rhizobia species and legumes. The release of the symbiont allows it to exchange genetic material with external microbes as well as between the genetically diverse individuals within the host. This maintaining of genetic exchange allows for the symbiont's host range to be extended and the selection for new functionality or adaptations of hosts, symbionts, and holobiont. An example of a horizontally transmitted symbiont with a high specificity recognition and acquisition method is the

Rhizobia Rhizobia are diazotrophic bacteria that fix nitrogen after becoming established inside the root nodules of legumes (Fabaceae). To express genes for nitrogen fixation, rhizobia require a plant host; they cannot independently fix nitrogen. I ...

Legume Legumes are plants in the pea family Fabaceae (or Leguminosae), or the fruit or seeds of such plants. When used as a dry grain for human consumption, the seeds are also called pulses. Legumes are grown agriculturally, primarily for human consum ...

symbiosis. The establishment of the symbiosis begins with the aposymbiotic plant releasing flavinoids that are detected by a specific Rhizobium species and triggers the induction of ''nod'' genes in the bacterium. Nod factors, which are

lipooligosaccharide Lipopolysaccharide (LPS), now more commonly known as endotoxin, is a collective term for components of the outermost membrane of the cell envelope of gram-negative bacteria, such as ''Escherichia coli, E. coli'' and ''Salmonella'' with a common ...

signals, are released as a result of the expression of the ''nod'' genes and when detected by the host plant initiate

root nodule Root nodules are found on the roots of plants, primarily legumes, that form a symbiosis with nitrogen-fixing bacteria. Under nitrogen-limiting conditions, capable plants form a symbiotic relationship with a host-specific strain of bacteria known ...

formation which eventually trap the Rhizobia. The established symbiosis can be further contained in a symbiosome in which the Rhizobia symbionts reside and carry out the nitrogen-fixation.

Anthropological

Cultural transmission may also be horizontal which is explicitly reified in

Dual Inheritance Theory Dual inheritance theory (DIT), also known as gene–culture coevolution or biocultural evolution, was developed in the 1960s through early 1980s to explain how human behavior is a product of two different and interacting evolutionary processes: g ...

. Horizontal transmission is implicit in the meme theory of cultural evolution, where the "meme" has been characterized by

Richard Dawkins Richard Dawkins (born 26 March 1941) is a British evolutionary biology, evolutionary biologist, zoologist, science communicator and author. He is an Oxford fellow, emeritus fellow of New College, Oxford, and was Simonyi Professor for the Publ ...

as a " Virus of the Mind".

References

{{reflist, 30em Infectious diseases

Biological

Pathogen transmission

Symbiont transmission

Anthropological

See also

References