Vertical transmission, passage of symbioticmicroflora from parents to offspring, is common in species of animals which haveparental care. There are fitness benefits in providing youths with established microorganism community early on.[3]
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.[4]
Complex interdependence occurs between host and symbiont.[5] The genetic pool of the symbiont is generally smaller and more subject togenetic drift.[6] In true vertical transmission, theevolutionary outcomes of the host and symbiont are linked.[7] If there is mixed transmission, newgenetic material may be introduced.[8] Generally, symbionts settle into specificniches and can eventransfer part of their genome into the host nucleus.
Since theegg contributes theorganelles and has more space and opportunity forintracellular symbionts to be passed to subsequent generations, it is a very common method of vertical transmission.[1] Intracellular symbionts can migrate from thebacteriocyte to the ovaries and become incorporated in germ cells.[11]
In plants, vertical transmission of microbialendophytes through germline can occur matrilineally via seed.[12] 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 theendosperm.[12] Alternatively, endophytes can be transmitted directly when reproductive organs are developing in the shoot apicalmeristem.[12]
Human infants acquire theirmicrobiome from their mothers, from every sphere where there is contact. This includes potentially the mother'svagina,gastrointestinal tract,skin,mouth andbreastmilk.[13] These routes are typical if the delivery is a vaginal birth and the infant is nursed. When other actions, such asCaesarian delivery, bottle feeding, or maternalantibiotics during nursing occur, these modes of vertical transmission are disrupted.[14][15]
Human infants are generally protected from mostviruses,bacteria and otherpathogens while in theuterus as theplacenta forms a barrier.[16] However, some viruses (e.g.Zika[17]) and bacteria (e.g.listeria[18]) can still pass through the placenta to the fetus through vertical transmission, which is why some foods are recommended to be avoided duringpregnancy.
Though extremely rare,Rickettsia is transmitted toNephotettix cincticep through the paternal line in thesperm.[19]
In plants, vertical transmission of microbialendophytes through germline can occur patrilineally viapollen.[12] Patrilineal transmission has been hypothesized to be a common mechanism for fungal endophyte transmission,[12] as well as bacteria.[20]
Microbes can be transmitted through the actions of parents caring for their offspring, such as the cultivation of gut microbes throughregurgitation feeding.[21] 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 inkin selection.[4][22]
Earthworms (Eisenia) have an extracellular symbiont,Verminephrobacter. Rather than being passed through the egg in the germline, the young areaposymbiotic when still in the egg capsule; however, they acquireVerminephrobacter before the egg capsule ruptures, so it is still vertical transmission.[23]
It's estimated that about 70% of all insects carry the bacteriaWolbachia, which can be transmitted vertically as well ashorizontally.[25] Depending on the host species, it may function as mutualist or a pathogen.[26] 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[26] in a diverse array of mechanisms, such as inducedparthenogenesis, male killing, orfeminization.[27] All of these increase the ratio of infected females, which is beneficial to a matrilineally-spread infection.
The head louse (Pediculus humanus) has an obligate symbiotic relationship withCandidatus Riesia pediculicola. The louse provides shelter and protection while bacteria provides essentialB vitamins.C. riesia lives in thebacteriocyte but move to the ovaries to be transmitted to the next generation.[29][30]
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.[31][32][33][34] Through the "milk" the youngsters receive parentmicroflora includingWigglesworthia glossinidia, the bacteria providing host with vitamins B scarce in the tsetse fly's blood-only diet.[35][36]
Social spidersStegodyphus dumicola live in Namibia and Botswana. The majority of females in the colony are virgins but participate in offspring care for reproducing females.[37] Offspring hatch symbiont-free, and bacterialsymbionts are transmitted vertically across generations by social interactions with the onset of regurgitation feeding by (foster) mothers early in the development.[22]
Caecilians feed youngsters by mother skin, passing to them themicroflora which colonize youngster's skin and gut.[38] 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 mothermicrobiome several times, enhancing the likelihood of microbial gut and skin successful colonization.
Bornean foam‑nesting frogsLeptomantis harrissoni tadpoles receive microbes from both their parents (vertically) and environment (horizontally).[3] 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.
ARanitomeya imitator dart frog feeds tadpoles with unfertilized trophic eggs. Anaerobicparabasalian protists are passed to the tadpoles via vertical transmission. In the gut, these protists express digestive enzymes Proteinases.[39] 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 allowsRanitpomeya imitator to expand into the new ecological niche and tadpoles to grow more robustly.[39] 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.[40]
^Vautrin E, Vavre F (March 2009). "Interactions between vertically transmitted symbionts: cooperation or conflict?".Trends in Microbiology.17 (3):95–99.doi:10.1016/j.tim.2008.12.002.PMID19230673.
^Smith NH, Gordon SV, de la Rua-Domenech R, Clifton-Hadley RS, Hewinson RG (September 2006). "Bottlenecks and broomsticks: the molecular evolution of Mycobacterium bovis".Nature Reviews. Microbiology.4 (9):670–681.doi:10.1038/nrmicro1472.PMID16912712.S2CID2015074.
^Douglas AE (January 1998). "Nutritional interactions in insect-microbial symbioses: aphids and their symbiotic bacteria Buchnera".Annual Review of Entomology.43 (1):17–37.doi:10.1146/annurev.ento.43.1.17.PMID15012383.
^Aksoy S (October 1995). "Wigglesworthia gen. nov. and Wigglesworthia glossinidia sp. nov., taxa consisting of the mycetocyte-associated, primary endosymbionts of tsetse flies".International Journal of Systematic Bacteriology.45 (4):848–851.doi:10.1099/00207713-45-4-848.PMID7547309.
