check where gene X is located in the genome of species D. If gene X is found right next to other genes that are already known to belong to species D, that’s a good sign it’s truly a part of D’s genome, rather than just a stray piece of DNA from somewhere else. It also helps to see if gene X has common features of a real gene, like introns (non-coding sequences commonly found in eukaryotic genes). On top of that, RNA sequencing data can show if the gene is actually expressed and used by the cell. HGT and the Tree of Life The phylogenetic conflicts introduced by HGT have led some biologists to question whether Darwin’s tree of life model still fully captures the complexity of evolutionary relationships, especially among prokaryotes [9]. In Darwin’s metaphor, life evolves like a great branching tree, with species diverging from common ancestors over time. HGT adds a new dimension to this picture: It causes some branches to reach sideways, connecting distant twigs in unexpected ways. This lateral exchange creates a more web-like structure, leading some to propose a network model of evolution rather than a strictly branching tree. Yet whether life is ultimately represented as a tree, a network, or a mix of both, one thing remains clear: HGT has played a profound role in shaping the evolution of the “endless forms most beautiful and most wonderful.” to prefer one color over the other, thus retaining the color variation. A more recent example of HGT in eukaryotes involves a newly discovered system similar to CRISPRCas systems [6–7]. CRISPR is a naturally occurring DNA sequence found in prokaryotic genomes, derived from fragments of past invading viral DNA [8]. During subsequent infections, a family of DNA-cutting enzymes (or more technically “endonucleases”) called Cas uses complementary RNA transcribed from CRISPR sequences as a guide to recognize the invader’s DNA, and then destroy matching viral DNA to protect themselves. While researchers wondered whether such RNA-guided systems only exist in prokaryotes, they found the eukaryotic counterpart in fungi. At its core is an RNA-guided endonuclease called Fanzor. Interestingly, scientists discovered that the Fanzor genes may have originated from prokaryotic tnpB genes, which were likely transferred to eukaryotes through HGT. Detecting HGT Events So, how do scientists actually find these examples of HGT? The key lies in looking closely at the gene that was horizontally transferred. Since it originally came from another species, it should still look similar to the version in the original donor species in terms of the amino acid sequence encoded. This idea is the basis for a common method called “phylogenetic conflict [2–3].” In simple terms, scientists compare two kinds of “family trees”: one for the species themselves, and one for the gene in question. If a gene was transferred from one species to another, the gene tree might show two species as closely related, even if the species tree says otherwise. For example, in figure 2, if gene X was transferred from species C to species D, the gene X tree would wrongly suggest that C and D are close relatives as they share a more similar gene X. But a tree mismatch isn’t always enough to prove that HGT happened. There could be other reasons for the odd pattern, such as contamination during DNA sequencing [3]. To be more certain, scientists would Figure 2 Phylogenetic conflict arising from a mismatch between a species tree and a gene tree. If gene X was horizontally transferred from species C to species D, the gene X tree would wrongly suggest that C and D are close relatives as they share a more similar gene X.
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