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There's a little microbial DNA in all of us, says study

23 March 2015
Appeared in BioNews 795

The human genome contains genes from plants, fungi and various microorganisms that cohabited with our ancestors, research shows.

The study from the University of Cambridge, reveals that humans and other animals carry several 'foreign' genes, acquired via a process called horizontal gene transfer (HGT).

HGT is common in single-celled organisms, such as bacteria or yeast. It involves an exchange of DNA fragments between different species; for example, bacteria often transfer antibiotic resistance genes by HGT.

In this study, the scientists looked at various species' genomes and checked how well each gene corresponded to similar genes in other species, thus establishing how likely they were to be 'foreign'. Further comparisons allowed the researchers to estimate how long ago the genes were acquired.

They frequently found evidence of HGT having occurred in more than a hundred instances in flies (12 species examined), nematode worms (four species) and primates (ten species, including humans).

'Surprisingly, far from being a rare occurrence, it appears that HGT has contributed to the evolution of many, perhaps all, animals and that the process is ongoing, meaning that we may need to re-evaluate how we think about evolution,' said Dr Alastair Crisp, one of the study's lead authors.

The study confirmed 17 previously found foreign genes in humans, and identified 128 new ones. Examples include the famous 'obesity gene' FTO (see BioNews 756), acquired from marine algae; the ABO gene for blood group determination, transferred from bacteria; and genes HAS1-3, transferred from fungi, which are used for production of large sugar molecules that serve as structural support in various body tissues.

Overall, the foreign genes mostly encoded enzymes. Accordingly, the acquired genes were often involved in metabolism, but also in the immune response.

Bacteria and protists were the most frequent gene 'donors' to all species. Up to 50 genes, plausibly acquired from viruses, were additionally found in primates but were rare in flies or worms. These genes were not included in the final analysis, as it was impossible for the scientists to say whether HGT occurred from viruses to primates or vice versa.

The scientists say they have probably underestimated the total incidences of HGT due to the very strict experimental criteria they used.

Yet HGT in evolutionarily advanced species remains controversial. Professor Jonathan Eisen of the University of California, Davis, USA, who was not involved in the study, told The Scientist: 'Many researchers show evidence that is consistent with the occurrence of HGT (which they did here) but few actually explicitly test alternative hypotheses such as gene loss, bad alignments, convergence, divergence, contamination, random noise, and more.'

Dr Crisp told The Scientist he suspects that the study 'won't be universally accepted'.

The research was published in Genome Biology.

SOURCES & REFERENCES
Expression of multiple horizontally acquired genes is a hallmark of both vertebrate and invertebrate genomes
Genome Biology |  13 March 2015
Genetically modified people
The Economist |  14 March 2015
Horizontal Gene Transfer a Hallmark of Animal Genomes?
The Scientist |  12 March 2015
Humans 'may have evolved with genes from plants' study finds
The Telegraph |  13 March 2015
Some genes 'foreign' in origin and not from our ancestors
EurekAlert! (press release) |  12 March 2015
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