Last week, we looked at a new study of the origins of complex cells, which revealed that the genomes of our ancestors were cobbled together from fragments from multiple species. This shed light on a phenomenon called horizontal gene transfer, in which genes from one species are incorporated into the genome of a distantly related species. The frequency of horizontal gene transfer means that, in addition to neatly branching trees relating species by common ancestry, there are also smaller threads connecting distant branches of the tree of life.
It is easy to see why horizontal gene transfer would be common between microbes. They often live in complex communities that are probably filled with DNA from dead and damaged cells. Furthermore, bacteria and archaea lack a membrane between their DNA and the rest of the cell, making it easier for environmental DNA to find its way to the genome.
However, a new study this week shows that horizontal gene transfer is remarkably common even in multicellular animals. And it does this by examining the genomes of several cockroach species, which have contained fragments of bacterial DNA dating back millions of years.
going horizontal
Neither bacteria nor archaea contain their DNA in a structure like a nucleus. As a result, any DNA that finds its way inside the cell has the potential to mix with and become permanently incorporated into the genome. That permanent incorporation is often facilitated by DNA damage repair enzymes, which sometimes “fix” the damage by incorporating any DNA they find into the cell.
Another reason why horizontal gene transfer between microbes is a big factor is that they lack dedicated germ cells. If foreign DNA is incorporated into a cell’s genome, it will be inherited by any descendants of that cell. In contrast, in multicellular animals, any foreign DNA incorporated into the liver cell’s genome will not be inherited by anything. So, you not only have to get the foreign DNA into the nucleus, but you also have to get it into the nucleus of the right cell.
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