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Trees, vines and nets – microbial evolution changes its face methods and different research groups are remarkably consistent with each other, projecting the same story. We used these trees as the scaffold of the net, on which we looked for the evidence of horizontally transferred genes,” explains Victor. To get a grip on horizontal gene transfer, they used a method called GeneTrace, previously developed by Victor and Christos. GeneTrace infers horizontal transfer from the patchy presence of a gene family in distantly related organisms. The data generated by GeneTrace allowed them to draw ‘vines’, representing horizontal-gene- transfer events, connecting branches on the evolutionary tree. In all, more than 600,000 vertical transfers are observed, coupled with 90,000 gene loss events and A bird's-eye view of the tree of life, showing the vines in red and the tree's branches in grey (Bacteria) and green (Archaea). The last universal approximately 40,000 horizontal gene transfers. Thus, common ancestor is shown as a yellow sphere. although the distribution of most of the gene families present today can be explained by the classical theory of Hinxton, July 1, 2005 – EBI researchers have changed evolution by descent, anomalies of these patterns are our view of 4 billion years of microbial evolution. Christos revealed by the ‘minority report’ of horizontal exchange. Ouzounis and colleagues have gained intriguing To understand the influence of horizontal gene transfer on quantitative insights into how gene families are the microbial tree of life, they focused on the network of transferred, not only ‘vertically’ through passage from one vines coursing through the tree’s branches. This behaves organism to its progeny, but also ‘horizontally’ through the in a ‘scale-free’ manner, a term used to describe networks exchange of genetic material between distantly related that have an uneven distribution of connectivity and a organisms. This new view of the tree of life could help us small number of hubs that are far more connected than to better understand how disease-causing bacteria other nodes. manage to stay one step ahead of us in our battle to tackle antibiotic resistance. One property of scale-free networks is their ‘small-world’ nature: travelling from one node to any other is very fast. Since the time of Darwin, the evolutionary relationships (Other small-world networks include social networks, the between organisms have been represented as a tree, with internet and air-travel connections.) These characteristics the common ancestors at the base of the trunk and the allow the hubs to serve as bacterial ‘gene banks’, most recently evolved species at the tips of the branches. providing a medium to acquire and redistribute genes in Microbiologists have argued for a long time that this microbial communities. “This has important implications representation doesn’t really hold true for microbes, which for our understanding of horizontal gene transfer because, often exchange genes among different species. Their in small-world networks, the shortest path between any claim has been that the evolution of these organisms is two network nodes is relatively small: in other words, a better represented by a net. Unfortunately, no-one knew gene can rapidly be disseminated from organism to exactly where to draw the horizontal lines in this net. organism through very few horizontal gene transfer Victor Kunin, previously a PhD student in Christos events,” explains Christos Ouzounis. A few species, Ouzounis's group, and their colleagues have now including beneficial nitrogen-fixing soil bacteria, appear to constructed a map of microbial evolution, going back be ‘champions’ of horizontal gene transfer; “it's entirely billions of years to the last universal common ancestor, possible that apparently harmless organisms are quietly that includes these horizontal lines. “Reassuringly, spreading antibiotic resistance under our feet,” concludes evolutionary trees constructed by many independent Christos Ouzounis.

Source Article: V. Kunin, L. Goldovsky, N. Darzentas, and C. A. Ouzounis. The net of life: Reconstructing the microbial phylogenetic network. Genome Res. 1 July 2005.

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