Vertical transmission of symbionts is the transfer of a microbial symbiont from the parent directly to the offspring. Many metazoan species carry symbiotic bacteria 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 f ...

, or parasitic role. A symbiont is acquired by a host via horizontal, vertical, or mixed transmission.

Fitness benefits

Vertical transmission, passage of symbiotic microflora from parents to offspring, is common in species of animals which have

parental care Parental care is a behavioural and evolutionary strategy adopted by some animals, involving a parental investment being made to the evolutionary fitness of offspring. Patterns of parental care are widespread and highly diverse across the animal k ...

. There are fitness benefits in providing youths with established microorganism community early on. # Immune system development: parents microbes prime young immune system. # Disease resistance: because skin is already colonized by parental microbes, pathogen flora has a harder time to establish itself. # Digestive help: parental microbes might help with digestion, as a result, the young ones can survive on a diet which would not meet their nutritious needs otherwise. # Environmental adaptation: microflora might help to cope with environmental stress. # Increased social cohesion: the microbiome may produce neurological or chemical signals that alter social behavior.

Evolutionary consequences

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 random genetic drift, allelic drift or the Wright effect, is the change in the Allele frequency, frequency of an existing gene variant (allele) in a population due to random chance. Genetic drift may cause gene va ...

. 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 large biomolecules that are crucial in all cells and viruses. They are composed of nucleotides, which are the monomer components: a 5-carbon sugar, a phosphate group and a nitrogenous base. The two main classes of nucleic aci ...

may be introduced. Generally, symbionts settle into specific niches and can even transfer part of their genome into the host nucleus.

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 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 the size of an idealised population that would experience the same rate of genetic drift as the real population. Idealised populations are those following simple one- locus models that comply with ass ...

. 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, ...

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 (biology), cell, that has a specific function. The name ''organelle'' comes from the idea that these structures are parts of cells, as Organ (anatomy), organs are to th ...

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 to the ovaries and become incorporated in germ cells. In plants, vertical transmission of microbial

endophyte An endophyte is an endosymbiont, often a bacterium or fungus, that lives within a plant for at least part of its life cycle without causing apparent disease. Endophytes are ubiquitous and have been found in all species of plants studied to date; ...

s through germline can occur matrilineally via seed. There are several mechanisms by which a seed can matrilineally become infected with endophytes. The mother plant can produce vascular connections from its somatic microbiomes to the

endosperm The endosperm is a tissue produced inside the seeds of most of the flowering plants following double fertilization. It is triploid (meaning three chromosome sets per nucleus) in most species, which may be auxin-driven. It surrounds the Embryo#Pla ...

. Alternatively, endophytes can be transmitted directly when reproductive organs are developing in the shoot apical

meristem In cell biology, the meristem is a structure composed of specialized tissue found in plants, consisting of stem cells, known as meristematic cells, which are undifferentiated cells capable of continuous cellular division. These meristematic c ...

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 and other animals, the vagina (: vaginas or vaginae) is the elastic, muscular sex organ, reproductive organ of the female genital tract. In humans, it extends from the vulval vestibule to the cervix (neck of the uterus). The #Vag ...

gastrointestinal tract The gastrointestinal tract (GI tract, digestive tract, alimentary canal) is the tract or passageway of the Digestion, digestive system that leads from the mouth to the anus. The tract is the largest of the body's systems, after the cardiovascula ...

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 A mouth also referred to as the oral is the body orifice through which many animals ingest food and animal communication#Auditory, vocalize. The body cavity immediately behind the mouth opening, known as the oral cavity (or in Latin), is also t ...

and breastmilk. 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 pathogenic bacteria, bacterial infections, and antibiotic medications are widely used in the therapy ...

s during nursing occur, these modes of vertical transmission are disrupted.

Patrilineal

Though extremely rare, Rickettsia is transmitted to ''Nephotettix cincticep'' through the paternal line in the

sperm Sperm (: sperm or sperms) is the male reproductive Cell (biology), 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 ...

. In plants, vertical transmission of microbial

s through germline can occur patrilineally via

pollen Pollen is a powdery substance produced by most types of flowers of seed plants for the purpose of sexual reproduction. It consists of pollen grains (highly reduced Gametophyte#Heterospory, microgametophytes), which produce male gametes (sperm ...

. Patrilineal transmission has been hypothesized to be a common mechanism for fungal endophyte transmission, as well as bacteria.

Parental care

Microbes can be transmitted through the actions of parents caring for their offspring, such as the cultivation of gut microbes through regurgitation feeding. This type of vertical transmission does not always occur via the behavior of the genetic parent; instead, other members of a social or family groups may transmit the microbial community, resulting in

kin selection Kin selection is a process whereby natural selection favours a trait due to its positive effects on the reproductive success of an organism's relatives, even when at a cost to the organism's own survival and reproduction. Kin selection can lead ...

Aposymbiotic

Earthworm An earthworm is a soil-dwelling terrestrial invertebrate that belongs to the phylum Annelida. The term is the common name for the largest members of the class (or subclass, depending on the author) Oligochaeta. In classical systems, they we ...

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.

Examples

Invertebrates

Vertical transmission of endosymbiotic bacteria is very common in

insect Insects (from Latin ') are Hexapoda, hexapod invertebrates of the class (biology), class Insecta. They are the largest group within the arthropod phylum. Insects have a chitinous exoskeleton, a three-part body (Insect morphology#Head, head, ...

Wolbachia

It's estimated that about 70% of all insects carry the bacteria Wolbachia, which can be transmitted vertically as well as horizontally. Depending on the host species, it may function as mutualist or a pathogen. In order to maintain the infection within a host species, it must enter the forming egg cell and be transmitted through the germline. To improve the rate of vertical transmission, Wolbachia can alter its host's reproductive system in a diverse array of mechanisms, such as induced

parthenogenesis Parthenogenesis (; from the Greek + ) is a natural form of asexual reproduction in which the embryo develops directly from an egg without need for fertilization. In animals, parthenogenesis means the development of an embryo from an unfertiliz ...

, male killing, or feminization. All of these increase the ratio of infected females, which is beneficial to a matrilineally-spread infection.

Pea aphids and ''Buchnera''

Pea Aphids do not get all of the necessary

amino acid Amino acids are organic compounds that contain both amino and carboxylic acid functional groups. Although over 500 amino acids exist in nature, by far the most important are the 22 α-amino acids incorporated into proteins. Only these 22 a ...

s from their diet. Their obligate symbiont, '' Buchnera'', synthesize the remainder.

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

The head louse ('' Pediculus humanus)'' 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 (biology), cell metabolism and synthesis of red blood cells. They are a chemically diverse class of compounds. Dietary supplements containing all eight are referr ...

. ''C. riesia'' lives in the bacteriocyte but move to the ovaries to be transmitted to the next generation.

Tsetse flies and ''Wigglesworthia glossinidia''

Tsetse flies have a fascinating life cycle. Tsetse gives life birth, which is extremely rare among insects. The fly fertilized one egg at the time and for the first three larval stages the single offspring developed inside the mother's uterus feeding on milk substance coming from milk glands in the uterus. Through the "milk" the youngsters receive parent microflora including '' Wigglesworthia glossinidia'', the bacteria providing host with vitamins B scarce in the tsetse fly's blood-only diet.

Social spiders

Social spiders '' Stegodyphus dumicola'' live in Namibia and Botswana. The majority of females in the colony are virgins but participate in offspring care for reproducing females. Offspring hatch symbiont-free, and bacterial symbionts are transmitted vertically across generations by social interactions with the onset of regurgitation feeding by (foster) mothers early in the development.

Vertebrates

Caecililans

Caecilian Caecilians (; ) are a group of limbless, vermiform (worm-shaped) or serpentine (snake-shaped) amphibians with small or sometimes nonexistent eyes. They mostly live hidden in soil or in streambeds, and this cryptic lifestyle renders caecilians ...

s feed youngsters by mother skin, passing to them the microflora which colonize youngster's skin and gut. The mother's skin is adapted for this purpose, thickening beforehand and regenerating quickly after being consumed to continue providing for her young. She repeats the process several times during early development without significant harm to herself. The repeated nature of skin feeding means that juveniles are exposed to their mother

several times, enhancing the likelihood of microbial gut and skin successful colonization.

Bornean foam‑nesting frogs

Bornean foam‑nesting frogs Leptomantis harrissoni tadpoles receive microbes from both their parents (vertically) and environment (horizontally). Initially they have microbiomes resembling their parents and the exterior of the foam nest, but after one week in the pond tadpoles pick up new microbes from the pond environment.

Imitator dart frog

A '' Ranitomeya imitator'' dart frog feeds tadpoles with unfertilized trophic eggs. Anaerobic parabasalian protists are passed to the tadpoles via vertical transmission. In the gut, these protists express digestive enzymes Proteinases. By doing so, they help youngsters to have the ability to digest fat and protein in the mother egg versus plant debris in the mini pond they live in. Genes that code for Proteinases are not present in the Ranitomeya genome. The symbiosis allows ''Ranitpomeya imitator'' to expand into the new ecological niche and tadpoles to grow more robustly. Another mechanism of vertical transmission via parental care occurs when the father carries a tadpole on its back from the egg to the breeding pool, which allows the tadpole an opportunity to receive microflora patrilinealy.

References

{{Reflist Symbiosis