Comparing the genomes of many animal species, scientists realize genes are transferred between individuals within a generation at a higher rate than expected.
Classically taught evolution says that in sexually reproducing organisms, offspring acquire traits in the form of genes from their parents. This process of passing along characteristics is known as vertical gene transfer (VGT). In addition to this intuitive mechanism of evolution, there is another important process that drives evolution called horizontal gene transfer – and it may be even more widespread than previously thought.
Horizontal gene transfer (HGT) refers to the movement of genetic information (DNA molecules) between individuals within the same generation, rather than from one generation to the next. These individuals may or may not be the same species and HGT is a critical mode of sharing genes that confer antibiotic resistance and pathogenicity in many bacteria. In fact, HGT is a well-characterized method of evolution in microbes, along with some worms and arthropods. However, a paper recently published in Genome Biology explained that this powerful evolutionary mechanism is more prevalent in “higher” organisms, such as humans than is explained in current scientific understanding of evolution.
A research team at the University of Cambridge, led by Prof. Alastair Crisp, examined high quality genomes from 10 primate, 12 fly, and four nematode species. They used phylogenetic analysis and a calculated HGT index to decide whether certain active genes found in these organisms were “foreign” (present in several related species but not in their descendants) or inherited over time as expected by vertical transfer.
Results of the analysis showed that between the 26 species that were studied in depth, hundreds of new potential HGT agents were identified. Specifically, most of the HGT DNA coded for genes involved in metabolism and were ancient horizontal transfers, mostly originating from bacteria and protists. In humans, at least 33 new examples of HGT genes were found along with evidence supporting previously suspected HGT events. Interestingly, many of the HGT events in humans are as old or older than our evolutionary radiation from other primate species.
The researchers went to great lengths to confirm their conclusions based on statistics and previously reported results. For example, they subjected known horizontally transferred genes to their analysis to be sure they obtained a strong positive result (i.e. a known HGT gene should strongly resolve as an HGT gene in their analysis, whereas a known VTG gene should not).
The team concluded that HGT is an evolutionary mechanism likely much more important in complex species than previously thought. Specifically in metazoans they believe that HGT contributed, albeit slowly, to the biochemical diversity found in all animals today. Even more interestingly, the analysis of flies and nematodes showed evidence of recent HGT, meaning that these groups are still acquiring foreign genes that are contributing to their evolution. This is corroborated by findings of known insect-microbe HGT events. Comparatively, humans and other primates have had relatively few HGT events since our split.