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Vertical transmission of
symbionts Symbiosis (from Greek , , "living together", from , , "together", and , bíōsis, "living") is any type of a close and long-term biological interaction between two different biological organisms, be it mutualistic, commensalistic, or parasit ...
is the transfer of a microbial symbiont from the parent directly to the offspring.  Many metazoan species carry
symbiotic bacteria Symbiotic bacteria are bacteria living in symbiosis with another organism or each other. For example, rhizobia living in root nodules of legumes provide nitrogen fixing activity for these plants. Symbiosis was first defined by Marko de Bary in 1869 ...
which play a mutualistic, commensal, or parasitic role.  A symbiont is acquired by a host via horizontal, vertical, or mixed transmission.


Implications

Complex interdependence occurs between host and symbiont. The genetic pool of the symbiont is generally smaller and more subject to
genetic drift Genetic drift, also known as allelic drift or the Wright effect, is the change in the frequency of an existing gene variant (allele) in a population due to random chance. Genetic drift may cause gene variants to disappear completely and there ...
. In true vertical transmission, the evolutionary outcomes of the host and symbiont are linked. If there is mixed transmission, new genetic material may be introduced. Generally, symbionts settle into specific niches and can even transfer part of their genome into the host nucleus.


Evolutionary consequences


Benefits

The mechanism promotes tightly coupled evolutionary pressure, which causes the host and symbiont to function as a holobiont.


Disadvantages

Evolutionary bottlenecks lead to less symbiont diversity, and thus
resilience Resilience, resilient, resiliency, or ''variation'', may refer to: Science Ecology * Ecological resilience, the capacity of an ecosystem to recover from perturbations ** Climate resilience, the ability of systems to recover from climate change * ...
.  Similarly, this greatly reduces the effective population size. Ultimately, without an influx of new genetic material, the population becomes clonal
Mutation In biology, a mutation is an alteration in the nucleic acid sequence of the genome of an organism, virus, or extrachromosomal DNA. Viral genomes contain either DNA or RNA. Mutations result from errors during DNA or viral replication, m ...
s tend to persist in symbionts and build up over time.


Transmission Modes


Matrilineal


Germline

Since the egg contributes the
organelle In cell biology, an organelle is a specialized subunit, usually within a cell, that has a specific function. The name ''organelle'' comes from the idea that these structures are parts of cells, as organs are to the body, hence ''organelle,'' th ...
s and has more space and opportunity for intracellular symbionts to be passed to subsequent generations, it is a very common method of vertical transmission.  Intracellular symbionts can migrate from the bacteriocyte to the ovaries and become incorporated in germ cells.


Live birth

Human infants acquire their
microbiome A microbiome () is the community of microorganisms that can usually be found living together in any given habitat. It was defined more precisely in 1988 by Whipps ''et al.'' as "a characteristic microbial community occupying a reasonably wel ...
from their mothers, from every sphere where there is contact.  This includes potentially the mother's
vagina In mammals, the vagina is the elastic, muscular part of the female genital tract. In humans, it extends from the vestibule to the cervix. The outer vaginal opening is normally partly covered by a thin layer of mucosal tissue called the hymen ...
,
gastrointestinal tract The gastrointestinal tract (GI tract, digestive tract, alimentary canal) is the tract or passageway of the digestive system that leads from the mouth to the anus. The GI tract contains all the major organs of the digestive system, in humans and ...
,
skin Skin is the layer of usually soft, flexible outer tissue covering the body of a vertebrate animal, with three main functions: protection, regulation, and sensation. Other animal coverings, such as the arthropod exoskeleton, have different ...
,
mouth In animal anatomy, the mouth, also known as the oral cavity, or in Latin cavum oris, is the opening through which many animals take in food and issue vocal sounds. It is also the cavity lying at the upper end of the alimentary canal, bounded on ...
and
breastmilk Breast milk (sometimes spelled as breastmilk) or mother's milk is milk produced by mammary glands located in the breast of a human female. Breast milk is the primary source of nutrition for newborns, containing fat, protein, carbohydrates ( lacto ...
. These routes are typical if the delivery is a vaginal birth and the infant is nursed. When other actions, such as Caesarian delivery, bottle feeding, or maternal
antibiotic An antibiotic is a type of antimicrobial substance active against bacteria. It is the most important type of antibacterial agent for fighting bacterial infections, and antibiotic medications are widely used in the treatment and prevention ...
s during nursing occur, these modes of vertical transmission are disrupted.


Patrilineal

Though extremely rare,
Rickettsia ''Rickettsia'' is a genus of nonmotile, gram-negative, nonspore-forming, highly pleomorphic bacteria that may occur in the forms of cocci (0.1 μm in diameter), bacilli (1–4 μm long), or threads (up to about 10 μm long). The term "ricke ...
is transmitted to ''Nephotettix cincticep'' through the paternal line in the
sperm Sperm is the male reproductive cell, or gamete, in anisogamous forms of sexual reproduction (forms in which there is a larger, female reproductive cell and a smaller, male one). Animals produce motile sperm with a tail known as a flagellum, ...
.


Aposymbiotic

Earthworm An earthworm is a terrestrial invertebrate that belongs to the phylum Annelida. They exhibit a tube-within-a-tube body plan; they are externally segmented with corresponding internal segmentation; and they usually have setae on all segments. T ...
s (Eisenia) have an extracellular symbiont, '' Verminephrobacter''. Rather than being passed through the egg in the germline, the young are aposymbiotic when still in the egg capsule; however, they acquire ''Verminephrobacter'' before the egg capsule ruptures, so it is still vertical transmission.


Well studied archetypes


Pea aphids and ''Buchnera''

Pea Aphids do not get all of the necessary amino acids from their diet.  '' Buchnera'', synthesize the needed ones in an obligate relationship.   


Head lice and ''Candidatus'' ''Riesia pediculicola''

The head louse (''
Pediculus humanus ''Pediculus humanus'' is a species of louse that infects humans. It comprises two subspecies: *''Pediculus humanus humanus'' Linnaeus, 1758 – body louse The body louse (''Pediculus humanus humanus'', also known as ''Pediculus humanus cor ...
)''  has an obligate symbiotic relationship with ''Candidatus'' Riesia pediculicola.  The louse provides shelter and protection while bacteria provides essential B vitamins. ''C. riesia'' lives in the bacteriocyte but move to the ovaries to be transmitted to the next generation.Human DNA Extracted From Nits on Ancient Mummies Sheds Light on South American Ancestry
SciTechDaily, December 28, 2021. Source: University of Reading.


References

{{Reflist Symbiosis