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 Animals are multicellular, eukaryotic organisms in the biological kingdom Animalia. With few exceptions, animals Heterotroph, consume organic material, Cellular respiration#Aerobic respiration, breathe oxygen, are Motility, able to move, ca ...

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 186 ...

which play a mutualistic,

commensal Commensalism is a long-term biological interaction (symbiosis) in which members of one species gain benefits while those of the other species neither benefit nor are harmed. This is in contrast with mutualism, in which both organisms benefit fro ...

, or

parasitic Parasitism is a close relationship between species, where one organism, the parasite, lives on or inside another organism, the host, causing it some harm, and is adapted structurally to this way of life. The entomologist E. O. Wilson has c ...

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 Nucleic acids are biopolymers, macromolecules, essential to all known forms of life. They are composed of nucleotides, which are the monomers made of three components: a 5-carbon sugar, a phosphate group and a nitrogenous base. The two main cla ...

may be introduced. Generally, symbionts settle into specific niches and can even

transfer Transfer may refer to: Arts and media * ''Transfer'' (2010 film), a German science-fiction movie directed by Damir Lukacevic and starring Zana Marjanović * ''Transfer'' (1966 film), a short film * ''Transfer'' (journal), in management studies ...

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 A holobiont is an assemblage of a host and the many other species living in or around it, which together form a discrete ecological unit through symbiosis, though there is controversy over this discreteness. The components of a holobiont are in ...

Disadvantages

Evolutionary bottlenecks lead to less symbiont

diversity Diversity, diversify, or diverse may refer to: Business *Diversity (business), the inclusion of people of different identities (ethnicity, gender, age) in the workforce *Diversity marketing, marketing communication targeting diverse customers * ...

, and thus resilience. Similarly, this greatly reduces the

effective population size The effective population size (''N'e'') is a number that, in some simplified scenarios, corresponds to the number of breeding individuals in the population. More generally, ''N'e'' is the number of individuals that an idealised population wo ...

. 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, mi ...

s tend to persist in symbionts and build up over time.

Transmission Modes

Matrilineal

Germline

Since the

egg An egg is an organic vessel grown by an animal to carry a possibly fertilized egg cell (a zygote) and to incubate from it an embryo within the egg until the embryo has become an animal fetus that can survive on its own, at which point the a ...

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,'' the ...

s and has more space and opportunity for

intracellular This glossary of biology terms is a list of definitions of fundamental terms and concepts used in biology, the study of life and of living organisms. It is intended as introductory material for novices; for more specific and technical definitions ...

symbionts to be passed to subsequent generations, it is a very common method of vertical transmission. Intracellular symbionts can migrate from the

bacteriocyte A bacteriocyte (Greek for ''bacteria cell''), also known as a mycetocyte, is a specialized adipocyte found primarily in certain insect groups such as aphids, tsetse flies, German cockroaches, weevils. These cells contain endosymbiotic organisms su ...

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 well ...

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 organ (biology), organs of the digestive syste ...

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 cuticle, animal coverings, such as the arthropod exoskeleton, have diffe ...

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 (lactos ...

. 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 of ...

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 "rickett ...

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, whi ...

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. Th ...

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 *''Pediculus humanus capitis'' De Geer, 1767 – head louse The head louse (''Pediculu ...

)'' has an obligate symbiotic relationship with ''Candidatus'' Riesia pediculicola. The louse provides shelter and protection while bacteria provides essential

B vitamins B vitamins are a class of water-soluble vitamins that play important roles in cell metabolism and synthesis of red blood cells. Though these vitamins share similar names (B1, B2, B3, etc.), they are chemically distinct compounds that often coexist ...

. ''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