Horizontal Gene Transfer in Eukaryotes: Not if, but How Much? - review


The journal article "Horizontal Gene Transfer in Eukaryotes: Not if, but How Much?" published in Trends in Genetics in October 2020, challenges a long-held belief in evolutionary biology: the notion that horizontal gene transfer (HGT) is primarily a prokaryotic phenomenon with limited relevance to eukaryotes. The authors, Julia Van Etten and Debashish Bhattacharya, argue that HGT has played a more significant role in the evolution of eukaryotic genomes than previously thought.

What is Horizontal Gene Transfer?

Horizontal gene transfer refers to the movement of genetic material between organisms through mechanisms other than traditional reproduction. In contrast to vertical gene transfer, which occurs from parent to offspring, HGT involves the acquisition of genes from unrelated or distantly related organisms. This process is well-established in prokaryotes (bacteria and archaea) and has been a driving force behind their rapid adaptation and evolution.

Challenging the Traditional View

Traditionally, it was believed that HGT was rare or inconsequential in eukaryotes (organisms with complex cells, including plants, animals, and fungi). Eukaryotic cells possess a nuclear membrane that encloses their DNA, and they reproduce sexually, creating a barrier to the uptake of foreign DNA. However, mounting evidence suggests that HGT has occurred more frequently in eukaryotes than previously assumed.

Evidence for HGT in Eukaryotes

The article highlights several lines of evidence supporting HGT in eukaryotes:

  1. Genomic Studies: Comparative genomic analyses have revealed the presence of genes in eukaryotic genomes that are more closely related to prokaryotic genes than to those of other eukaryotes. These findings suggest that these genes were acquired through HGT.

  2. Experimental Studies: Laboratory experiments have demonstrated the transfer of genes between eukaryotes and prokaryotes, as well as between different eukaryotic species. These studies have shown that HGT can occur through various mechanisms, including viral transfer, endosymbiosis, and direct DNA uptake.

  3. Functional Studies: The identification of functional genes acquired through HGT in eukaryotes further supports the importance of this process in their evolution. These genes have been shown to confer adaptive advantages, such as resistance to antibiotics, toxins, and environmental stresses.

Implications for Evolutionary Biology

The recognition of the widespread occurrence of HGT in eukaryotes has profound implications for evolutionary biology. It challenges the traditional view of the tree of life as a strictly branching structure and highlights the importance of reticulate evolution, where genetic material is exchanged between different lineages.

HGT can lead to the rapid acquisition of new traits and functions, accelerating evolutionary processes. It can also facilitate the adaptation of organisms to new environments and contribute to the emergence of new species. Understanding the extent and mechanisms of HGT in eukaryotes is essential for comprehending the complex patterns of evolution and the diversity of life on Earth.

Future Directions

The article concludes by highlighting the need for further research to determine the frequency and impact of HGT in different eukaryotic groups. The authors emphasize the importance of developing standardized methods for identifying and analyzing HGT events, as well as for assessing their functional significance.

Overall, "Horizontal Gene Transfer in Eukaryotes: Not if, but How Much?" presents a compelling case for the widespread occurrence of HGT in eukaryotes and highlights the need to revise our understanding of eukaryotic evolution. The article serves as a call to action for the scientific community to explore the fascinating world of HGT and its implications for the evolution and diversity of life.

The journal article challenges the traditional understanding of evolution as primarily driven by vertical inheritance (parent to offspring) and gradual mutation. It highlights the growing evidence for horizontal gene transfer (HGT) in eukaryotes, which is the acquisition of genetic material from unrelated organisms. This process, well-documented in prokaryotes, can lead to significant evolutionary leaps in a short time frame, rather than slow, incremental changes.

The article's findings underscore the need to move beyond the Modern Synthesis, a mid-20th century theory that emphasizes gradualism and adaptation through natural selection. The Modern Synthesis is increasingly recognized as incomplete. The Extended Evolutionary Synthesis (EES) provides a broader framework, incorporating mechanisms like HGT, epigenetics, and niche construction, which can influence evolutionary trajectories alongside natural selection.

HGT, in particular, challenges the tree-like model of evolution, where species diverge linearly. Instead, it suggests a more web-like pattern, with genetic material exchanged across different lineages. This has implications for our understanding of biodiversity, adaptation, and the emergence of new traits. The article argues that HGT is not just a rare anomaly but a significant factor in eukaryotic evolution, especially in microorganisms.

The article calls for further research into the frequency, mechanisms, and consequences of HGT in eukaryotes. By integrating HGT into our evolutionary models, we can gain a more comprehensive understanding of the complex processes that shape life on Earth. This shift towards the EES represents a paradigm change in evolutionary biology, acknowledging the diverse mechanisms that contribute to the ongoing saga of life's evolution.


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