The researchers say their findings show that unlike Darwin's finches or Mendel's peas, genetic variation isn't always small, incremental, and predictable.

In addition to the long reads, the researchers validated junctions between integrated bacteria genes and the host fruit fly genome. To determine if the bacteria genes were functional and not just DNA fossils, the researchers sequenced the RNA from fruit flies specifically looking for copies of RNA that were created from templates of the inserted bacterial DNA. They showed the bacteria genes were encoded into RNA and were edited and rearranged into newly modified sequences indicating that the genetic material is functional.

An analysis of the unique sequences revealed that the bacteria DNA integrated into the fruit fly genome in the last 8,000 years—exclusively within chromosome 4—expanding the chromosome size by making up about 20 percent of chromosome 4. Whole bacterial genome integration supports a DNA-based rather than an RNA-based mechanism of integration.

They also found nearly a complete second genome and much more with almost 10 copies of some bacterial genome regions.

Wolbachia is an intracellular bacteria that infects numerous types of insects. Wolbachia transmits its genes maternally through female egg cells. Some research has showed that these infections are more mutualistic than parasitic, giving insects advantages, such as resistance to certain viruses.

Eric S. Tvedte et al, Accumulation of endosymbiont genomes in an insect autosome followed by endosymbiont replacement, Current Biology (2022). DOI: 10.1016/j.cub.2022.05.024