Gene Flow (Plants, Animals, Humans)- Definition, Types, Barriers

Gene flow is the transfer of genetic material from one population to another or simply a gene migration.

Natural selection, genetic drift, and gene flow all work together to create population structure or the pattern of genetic variation among populations.

Gene flow is a fundamental evolutionary process based on the dispersal of genes between two populations of the same species. Gene flow entails not only dispersal but also the successful integration of the immigrant genotypes into the new population. It involves the active or passive movement of individual plants, animals, gametes, or seeds.

Gene flow within a population can increase genetic diversity within that population, whereas gene flow between populations that are genetically distinct can lessen genetic diversity between the populations. Lack of gene flow allows speciation, whereas gene flow helps to homogenize linked populations.

Types of Gene Flow

Gene flow can take place between two populations of the same species through migration and is achieved by two mechanisms: 

1. Vertical gene transfer
2. Horizontal gene transfer (HGT) 

Vertical gene transfer

• It is the process through which genes are passed from parents to offspring through cell or germline division.
• It ensures the preservation of species identification and can be found in both prokaryotic and eukaryotic organisms.
• Vertical gene transfer can occur through sexual reproduction or asexual reproduction.

Horizontal gene transfer (HGT)

• It is the transfer of genes between individuals of the same or different species and occurs most frequently in prokaryotic species like viruses and bacteria.
• It is also known as lateral gene transfer.
• It is crucial to the dissemination of multiple genetic features.
• Viruses can transfer genes between species. 
• Bacterial genes are involved in photosynthesis, and bacterioviruses capture toxins.
• Genes essential for antiviral immunity are frequently taken from eukaryotic cells by eukaryoviruses.
• Bacteria can traverse species boundaries to exchange plasmids and genes with living and dead bacteria. This has led to the emergence of bacterial strains resistant to antibiotics.
• Agrobacterium, Rhizobium, and Escherichia coli are three of the most common bacteria that may transfer genes from bacterial to eukaryotic cells.

Gene Flow Examples in Plants

1. Selfing is the primary method used to generate seeds in wheat and barley. Because there is no gene flow during the fertilization of selfed seed, it is characterized by extremely little gene flow.
2. Oaks and pines, frequently outcrossed and wind-pollinated, have substantially higher gene flow.
3. Low gene flow will be seen in plants pollinated by bees who visit numerous flowers on one plant before moving on to an adjacent plant.
4. A hybridization procedure (gene flow) was used to create modern grains like durum wheat.
5. The lager beer yeast (Saccharomyces pastorianus) is a hybrid of two different yeasts, one more tolerant to cold temperatures than the other.

Gene Flow Examples in Animals

1. A Maine coon cat, when mates with wild tabby cats, some of the kittens will have bushy tails and tufted ears as a result of gene flow.
2. Red parrots, when brought to a remote section of the jungle with only blue parrots, introduce color variation into the gene pool of jungle parrots.
3. Brown beetles entering a group of only green beetles produce progeny with a wider range of colors.
4. A population of moths with many white alleles swoops into one with more dark coloration. White moths are consequently produced in increasing numbers over time.
5. After a few generations, a larger population of tigers with better eyesight is born when tigers with enhanced night vision breed a group of tigers with less sensitive eyes.
6. Whether the hummingbirds defend tiny territories or are “trapliners,” flying great distances between successive pollinations, the gene flow mediated by different species of hummingbirds might be low or high.
7. Despite having high gene flow, the North Atlantic blue mussel, Mytilus edulis, displays an abrupt genetic boundary. According to a genomic data-based estimate of gene flow, blue mussels exchange several individuals among populations each generation.

Gene Flow Examples in Humans

1. At least 13 significant genetic regions in non-African humans are descended from Neanderthals. 
   These genetic regions must have undergone gene flow from Neanderthals into modern humans because they are missing in people of African descent.
2. Due to the prevalence of malaria in West Africa, where the Duffy antigen confers some disease resistance, this allele is found in almost all of the region’s inhabitants. 
   In contrast, due to the nearly nonexistent prevalence of malaria, Europeans either bear the genotype Fy^a or Fy^b. The allele frequencies in each community were found to be mixed because of the movement of individuals.
3. Asian women are found to be responsible for the spread of Asian genes into Southeast Asia’s islands more than Asian men.
4. Sweden native with blue eyes, when breed with a Mexican native with brown eyes, results in offspring with blue eyes due to gene flow.

Barriers to Gene Flow

1. Physical barriers separating the populations, such as impassable mountain ranges, oceans, or large deserts, can restrict gene flow as physical proximity of the populations can facilitate gene flow.
2. Individuals hinder gene flow in populations that have incompatible reproductive behaviors.
3. It is often gender-biased and restricted to specific stages of the life cycle.
4. It may accelerate under specific climatic conditions that frequently happen over many years or erratically.
5. Introgressive hybridization is caused by the exchange of genes across species.
6. In the future, genetically deficient groups will prosper from the relocation of individuals from safer areas. Unfortunately, introducing pathogens that could affect the target population or entirely unrelated species involves hazards with such genetic improvement.
7. In the vulnerable population of conservation concern, outbreeding depression may occur due to the transfer of individuals from genetically distinct populations. Thus, gene flow can potentially weaken the genetic components of adaptation to the environment.
8. Previously continuous populations may become fragmented, disrupting historical dispersal patterns and gene flow that could affect population sustainability.

For instance, if habitat damage in the neighboring countryside prevents young female chimpanzees from leaving their natal social group, the isolated population they were born into will experience more inbreeding.

Gene Flow Mitigation

It is necessary to use gene flow mitigation techniques, typically through unintentional cross-pollination and cross-breeding, to stop “genetic pollution” or the genetic modification of cultivated genetically modified (GM) plants or livestock, from spreading to other conventionally hybridized or wild native plant and animal populations.

Biosafety concerns or agricultural coexistence, in which GM and non-GM farming systems coexist, are possible reasons for restricting gene flow.

Many significant research projects are investigating ways to control plant gene flow. Among these programs are:

1. Transcontainer: It investigates methods for biocontainment.
2. SIGMEA: It stands for Sustainable Introduction of GM crops into European Agriculture. It focuses on the biosafety of genetically modified plants.
3. Co-Extra: It studies the co-existence of GM and non-GM product chains.

References

1. Cox M.P. & Hammer M.F. (2010). A question of scale: Human migrations writ large and small. BMC Biology. 8(98). https://doi.org/10.1186/1741-7007-8-98
2. Examples of Gene Flow in Plants and Animals. 
3. Accessed from: https://examples.yourdictionary.com/examples-of-gene-flow.html. Accessed on 16.09.2022
4. Gene flow. Accessed from: https://www.bionity.com/en/encyclopedia/Gene_flow.html. Accessed on: 16.09.2022
5. Jove. Types of Genetic Transfer Between Organisms. 
6. Accessed from: https://www.jove.com/science-education/11487/types-of-genetic-transfer-between-organisms. Accessed on: 15.09.2022
7. Lacroix B. & Citovsky V. (2016). Transfer of DNA from Bacteria to Eukaryotes. mBio, 7(4), e00863-16. https://doi.org/10.1128/mBio.00863-16
8. Lorenzo-Díaz F., Fernández-López C., Lurz R., Bravo A., & Espinosa M. (2017). Crosstalk between vertical and horizontal gene transfer: plasmid replication control by a conjugative relaxase. Nucleic acids research, 45(13), 7774–7785. https://doi.org/10.1093/nar/gkx450
9. Mitton J. B. (2013). Gene Flow. In Brenner’s Encyclopedia of Genetics. Second Edition. Elsevier Inc., pg. 192-196. ISBN 9780080961569. https://doi.org/10.1016/B978-0-12-374984-0.00589-1
10. Choudhuri S. (2014). Fundamentals of Molecular Evolution. In Bioinformatics for Beginners: Genes, Genomes, Molecular Evolution, Databases and Analytical Tools. Academic Press, pg 27-53. ISBN 9780124104716. https://doi.org/10.1016/B978-0-12-410471-6.00002-5.
11. Woodruff D.S. (2001). Populations, Species, and Conservation Genetics. In Encyclopedia of Biodiversity. Elsevier Inc., pg. 811-829. ISBN 9780122268656. https://doi.org/10.1016/B0-12-226865-2/00355-2.