Last week, we looked at a new study of the origin of complex cells, one that showed that our ancestors’ genomes were pieced together from bits and pieces of multiple species. It put a spotlight on a phenomenon called horizontal gene transfer, in which a gene from one species is incorporated into the genome of a distantly related species. The frequency of horizontal gene transfer means that, in addition to the neatly branching trees that relate species by common descent, there are small threads connecting distant branches of the tree of life.

It’s easy to see why horizontal gene transfer would be common among microbes. They often live in complex communities that are likely awash in the DNA of dead and damaged cells. Plus, 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 transfers are remarkably common even in multicellular animals. And it does so by examining the genomes of multiple cockroach species, which have had bits of bacterial DNA for millions of years.

Going horizontal

Neither bacteria nor archaea keep their DNA in a structure like the nucleus. As a result, any DNA that finds its way inside the cell has the potential to become intermingled with the genome and be incorporated permanently. That permanent incorporation is often aided by the DNA damage repair enzymes, which sometimes “fix” damage by inserting any DNA they come across in a cell.

Another reason horizontal gene transfer is a big factor among microbes is that they lack dedicated germ cells. If foreign DNA gets incorporated into the genome of any cell, it will be inherited by any descendants of that cell. In contrast, in multicellular animals, any foreign DNA incorporated into the genome of a liver cell will not be inherited by anything. So, you not only have to get the foreign DNA into the nucleus, but it also needs to get into the nucleus of the right cell.