Lateral gene transfer, also known as horizontal gene transfer (HGT), describes the movement of genetic material between organisms through means other than parent-to-offspring inheritance. This process stands in contrast to vertical gene transfer, which is the typical transmission of DNA from a parent organism to its offspring during reproduction. The acquisition of foreign DNA through lateral gene transfer can significantly alter an organism’s characteristics and capabilities.
How Genes Move Horizontally
Bacteria primarily employ three distinct mechanisms for lateral gene transfer: transformation, transduction, and conjugation. Transformation involves a bacterium directly taking up free DNA from its surrounding environment, often from dead bacterial cells that have released their genetic contents. This “naked” DNA can then be integrated into the recipient bacterium’s own genome.
Transduction facilitates gene movement through viruses that infect bacteria, known as bacteriophages. During infection, a bacteriophage can accidentally pick up a fragment of its host bacterium’s DNA. When this “transducing particle” subsequently infects another bacterium, it injects the carried bacterial DNA fragment into the new host, where it can integrate into that bacterium’s genome. This mechanism can transfer either chromosomal DNA or plasmids.
Conjugation is a more direct method, involving the transfer of genetic material through cell-to-cell contact between two bacteria. A donor bacterium, often possessing a specific type of plasmid called a conjugative plasmid, forms a physical bridge, or pilus, with a recipient bacterium. Through this bridge, a copy of the plasmid DNA is transferred to the recipient cell. This process is a common way for genetic information, including genes for antibiotic resistance, to spread between different bacterial species.
Why Lateral Gene Transfer Matters
Lateral gene transfer impacts the evolution of organisms by introducing new traits and increasing genetic diversity. This process allows organisms to adapt rapidly to changing environments, bypassing the slower pace of traditional mutation and selection. An example of this is the widespread dissemination of antibiotic resistance genes among bacteria. When one bacterium acquires resistance to an antibiotic, it can transfer those genes to other, previously susceptible, bacterial species through lateral gene transfer mechanisms like conjugation, transformation, or transduction. This rapid sharing of resistance genes presents a significant challenge in medicine, as it enables bacteria to quickly become resistant to various antimicrobial treatments.
Beyond antibiotic resistance, lateral gene transfer also plays a significant role in the evolution of virulence factors in bacteria. Virulence factors are traits that enable bacteria to cause disease, such as toxins or mechanisms for evading the host immune system. The acquisition of these genes through lateral transfer can transform a harmless bacterium into a pathogenic one. For instance, the genes for certain toxins, like those produced by Clostridium botulinum or Corynebacterium diphtheriae, can be transferred via bacteriophages.
The pervasive nature of lateral gene transfer also challenges the traditional “tree of life” concept, which depicts evolution as a branching tree where genetic information flows vertically from ancestors to descendants. Lateral gene transfer introduces “cross-links” or “webs” of genetic exchange between distantly related organisms, suggesting a more complex “mosaic” pattern of evolution. This phenomenon has implications for fields such as medicine, informing strategies for developing gene therapies. In agriculture, concerns exist regarding the potential for genes from genetically modified organisms to transfer to wild plant species, which could lead to unforeseen ecological consequences like the development of “superweeds.”
Where Lateral Gene Transfer Occurs
Lateral gene transfer is prevalent in prokaryotes, such as bacteria and archaea. It is considered a major driving force in their evolution, enabling them to acquire new genetic information and adapt to diverse environments. Studies on Escherichia coli, for instance, suggest that a notable portion of its genome may have originated from other bacterial species through lateral gene transfer. This demonstrates the extensive impact of this process on bacterial genetic makeup.
While less frequent than in prokaryotes, growing evidence indicates that lateral gene transfer also occurs in eukaryotes. The complexity of eukaryotic genomes, with their larger size and abundance of repetitive regions, can make the detection and characterization of transferred genes more challenging. However, instances of gene transfer between bacteria and eukaryotes, including animals, plants, and fungi, have been observed.
For example, mobile genetic elements called transposons have been shown to facilitate gene transfers between species. In plants, there is evidence of genes being transferred from bacteria, archaea, and fungi into the ancestors of land plants, contributing to the development of features like branching roots and vascular structures. Some unicellular eukaryotes, particularly protists with phagotrophic lifestyles (meaning they ingest other cells), appear to experience lateral gene transfer at rates comparable to prokaryotic organisms. This suggests that the ability to engulf other cells may provide opportunities for genetic exchange in these eukaryotic groups.