Horizontal gene transfer (HGT) describes the movement of genetic material between organisms not directly related through parent-to-offspring inheritance. While commonly recognized in bacteria, where it drives antibiotic resistance, this process also occurs in more complex life forms known as eukaryotes. HGT in these diverse organisms broadens our understanding of how species acquire new traits and adapt.
Pathways for Gene Transfer in Eukaryotes
Several pathways allow genetic material to move horizontally between eukaryotic species. Viruses often serve as vehicles for genetic information. Retroviruses, for instance, integrate their RNA genome into host cell DNA. If this integration occurs in germline cells, the viral DNA can become a permanent part of the host’s genome, known as an endogenous retrovirus.
Transposable elements, or “jumping genes,” can move within a genome and sometimes transfer between different species. This is particularly observed in plants, where they facilitate gene movement across species boundaries.
Close physical associations between organisms also create opportunities for gene transfer. Parasitism, where one organism lives on or in another, can lead to the exchange of genetic material. Similarly, predation or direct ingestion of one organism by another can expose host cells to foreign DNA, which may be incorporated into the recipient’s genome.
From Microbes to Complex Life
A well-documented instance of HGT from microbes to complex life involves the Wolbachia bacterium and its insect hosts. This intracellular bacterium can transfer genes into the chromosomes of various insects and nematodes. Such transfers can affect host reproduction, sometimes leading to effects like cytoplasmic incompatibility or feminization.
The human genome provides evidence of gene transfer from viruses. Approximately 8% of the human genome consists of sequences derived from endogenous retroviruses (ERVs), remnants of ancient viral infections. The syncytin gene, for example, is derived from an ancient viral envelope gene. Syncytin plays a role in the formation of the placenta, mediating cell fusion for nutrient and gas exchange.
Fungi also exhibit HGT from bacteria, acquiring new capabilities. Certain fungi have obtained genes from bacteria that enable them to produce specific toxins. This acquisition can provide a competitive advantage, allowing the fungi to deter predators or inhibit the growth of competing organisms.
Gene Swapping Between Eukaryotes
Gene swapping between different eukaryotic organisms reveals evolutionary pathways. Certain sea slugs, notably those in the genus Elysia, display a phenomenon called kleptoplasty. These slugs consume algae and retain the algal chloroplasts within their own cells, allowing them to perform photosynthesis for weeks or even months. Studies indicate that some slugs have even incorporated algal genes into their own nuclear genomes.
Among plants, a fern species acquired a gene from a moss. This transferred gene provides the fern with resilience in shady, low-light environments.
The genes responsible for caffeine production in coffee plants were acquired through HGT from soil bacteria. Rather than evolving independently within the coffee plant lineage, these genes were horizontally transferred, suggesting a bacterial origin for this well-known stimulant pathway in plants.
Tardigrades, or water bears, are studied for HGT. Early research suggested a high percentage of foreign DNA in their genomes, prompting discussions about HGT’s extent. While subsequent studies refined these estimates, suggesting some initial figures were overestimated due to contamination, tardigrades still show evidence of HGT.
The Evolutionary Impact of Borrowed Genes
Horizontal gene transfer offers a rapid source of evolutionary innovation, contrasting with the slower process of random mutation and natural selection. Through HGT, organisms can acquire functional genes and traits in a single step, rather than waiting for gradual genetic changes to accumulate. This allows for the swift acquisition of complex abilities, such as toxin resistance or a new metabolic pathway, which would otherwise take long evolutionary time to develop.
The ability to acquire pre-existing genes can accelerate adaptation to new environmental challenges or opportunities. For instance, the shade-tolerant fern gained a beneficial trait for survival in specific conditions, while the syncytin gene, derived from a virus, became integrated into a mammalian reproductive process. The widespread occurrence of HGT suggests that the traditional “Tree of Life” model, depicting solely diverging branches, may be more accurately represented as an interconnected “web of life,” where genetic information flows across lineages.