1.15:
Types of Genetic Transfer Between Organisms
Genetic transfer occurs when genetic information is transferred from one organism to another.
There are two types of genetic transfer: vertical and horizontal.
Vertical gene transfer, the most common of the two, is the transmission of genetic information from one generation to the next.
Vertical gene transfer occurs in sexually reproducing organisms, where they pass roughly half of the DNA onto the offspring, and in asexually reproducing organisms, which produce genetically identical progeny.
Horizontal gene transfer, conversely, occurs when genetic material from one organism is introduced to another organism of the same generation.
In the case of some prokaryotic species, such as bacteria, horizontal gene transfer can occur between different species.
Horizontal gene transfer in prokaryotes occurs via three mechanisms: transformation, transduction, and conjugation.
In transformation, a prokaryotic cell takes up naked DNA from the environment.
During transduction, viruses, bacteriophages in this example, degrade their host’s DNA. Small lengths of the host’s DNA can be packaged into the bacteriophage’s progeny before the host cell lyses.
The viral progeny then go on to infect more bacterial cells, thus becoming inadvertent mediators to transduction.
In conjugation, DNA is directly transferred from one cell to another via a structure called the sex pilus.
While conjugation is a form of ‘mating’ for asexually reproducing organisms, it is not true sexual reproduction because there are no gametes exchanged or offspring produced.
Each of these mechanisms can be observed in nature and have laboratory applications.
For example, conjugation can be used to confer antibiotic resistance from one bacterial strain to another, resulting in strains that can be resistant to multiple antibiotics.
1.15:
Types of Genetic Transfer Between Organisms
Genetic transfer occurs when genetic information is passed from one organism to another. It occurs via two mechanisms: vertical gene transfer and horizontal gene transfer. Vertical gene transfer occurs when genetic information is transferred from one generation to the next, which happens much more frequently than horizontal gene transfer. Both sexual and asexual reproduction are forms of vertical gene transfer, where one or more organisms pass some or all of their genome onto their progeny. Additionally, vertical gene transfer occurs in both prokaryotic and eukaryotic species.
Horizontal gene transfer occurs when genetic information is passed to a member of the same generation and happens most frequently in prokaryotic species. While inter-species horizontal gene transfer is exceedingly rare among eukaryotes, it occurs frequently in prokaryotes. Horizontal gene transfer between different species is a significant source of genetic diversity among prokaryotes.
Most prokaryotic species reproduce asexually. While this allows for faster production of offspring, the offsprings produced possess limited genetic diversity. Horizontal gene transfer, therefore, serves a vital role in introducing genetic diversity to prokaryotes. Through horizontal gene transfer, prokaryotes can share a small fraction of their genome with other organisms, either conspecific (the same species) or allospecific (a different species), in the same generation. Many scientists posit that horizontal gene transfer and mutation are the most significant sources of genetic variation in prokaryotes. Thus, horizontal gene transfer provides some of the raw material upon which natural selection acts.
A prominent example of this is the emergence of antibiotic-resistant strains of bacteria. Genes that confer resistance to an antibiotic can be transferred between different species and strains of bacteria, giving the recipient bacteria a selective advantage, such as penicillin-resistant strains of Neisseria gonorrhoeae, which causes gonorrhea. On an even grander scale, some estimates suggest that at least 18% of E. coli’s genome was acquired via horizontal gene transfer over the course of millions of years of evolution.
Suggested Reading
- Arber W. Horizontal Gene Transfer among Bacteria and Its Role in Biological Evolution. Life (Basel). 2014;4(2):217–224. Published 2014 May 16. doi:10.3390/life4020217
- Puigbò, Pere, Yuri I. Wolf, and Eugene V. Koonin. "Search for a 'Tree of Life’ in the thicket of the phylogenetic forest." Journal of Biology 8.6 (2009): 59.