The Drosophila genome is not only made up of Drosophila DNA—at least of one species of Drosophila. New research from the University of Maryland School of Medicine’s Institute of Genomic Sciences (IGS) (UMSOM) shows that a single species of Drosophila contains the complete genomes of a type of bacteria, making this finding the largest transfer of genetic material from bacteria to an animal ever. Discover. The new research also sheds light on how this might happen.
IGS researchers, led by Julie Dunning Hottop, PhD, professor of microbiology and immunology at UMSOM and IGS, used a new long-genome read sequencing technology to show how genes from Wolbachia bacteria integrated themselves into the genome of the fly as many as 8,000 years ago.
The researchers say their findings show that unlike Darwin’s finches or Mendelian peas, genetic variation is not always small, gradual, and predictable.
Scientist Barbara McClintock first identified “jumping genes” in the 1940s as those that could move within the genomes of other species or be passed on to the genomes of other species. However, researchers continue to discover its importance in evolution and health.
We previously did not have the technology to unambiguously demonstrate these intragenomes, showing such extensive lateral gene transfer from bacteria to the fly. We used up-to-date and required genetic sequencing to reach this important discovery.”
Julie Dunning Hottop, Professor of Microbiology and Immunology, University of Maryland School of Medicine
The new research was published in the June issue of current biology.
In the past, researchers had to split DNA into short segments in order to arrange their sequence. Then they needed to put it together, like a jigsaw puzzle, to look at a gene or a piece of DNA. However, the long read sequencing allows for more than 100,000 DNA letters to be sequenced, turning a million-piece puzzle into a puzzle made for young children.
In addition to the long reads, the researchers validated the connections between the genes of the integrated bacteria and the genome of the Drosophila host. To determine whether bacteria genes are functional and not just DNA fossils, the researchers sequenced RNA from fruit flies for RNA copies created from the inserted bacterial DNA templates. They showed that the bacteria’s genes were encoded in RNA, edited and rearranged into newly modified sequences that indicated that the genetic material was functional.
An analysis of these unique sequences revealed that the bacteria’s DNA had been integrated into the Drosophila genome in the last 8000 years – exclusively within chromosome 4; Expand chromosome size by making about 20 percent of chromosome 4. Integration of the entire bacterial genome supports DNA-based integration mechanism rather than RNA.
Dunning Hottop and colleagues found a complete bacterial genome of the common Wolbachia bacterium transmitted to the genome of the fruit fly Drosophila ananassae. They also found nearly a second complete genome and much more with nearly 10 copies from some regions of the bacterial genome.
“There has always been some skepticism about lateral gene transfer, but our research clearly demonstrates for the first time the mechanism of integrating Wolbachia DNA into the Drosophila genome,” said Dr Dunning Hottop.
“This new research shows basic science at its best,” said Dean E. Albert Rees, MD, PhD, MBA, who is also executive vice president for medical affairs, UM Baltimore, and the John Z and Aiko Powers Distinguished Professor, and Dean of the College of Medicine. University of Maryland. “It will contribute to our understanding of evolution and may prove to help us understand how microbes contribute to human health.”
Wolbachia is an intracellular bacterium that infects many types of insects. Wolbachia transfers its genes from the mother through the female’s egg cells. Some research has shown that these infections are more mutualistic than parasitic, which gives insects advantages, such as resistance to some viruses.
Fruit flies have been used only three years before the human genome, and have long been used in genomic research due to the abundance of genetic similarities between flies and humans. In fact, 75 percent of human disease-causing genes can also be found in Drosophila.
Among the authors from the University of Maryland School of Medicine’s Institute of Genomic Sciences, at the time of writing, Eric S. Mark Gasser; Xuechu Zhao, Laboratory Research Specialist; luke c. Talon, Executive Scientific Director, Maryland Genomics; Lisa Sadzwicz, Executive Director, Genome Management, Maryland; Robin Bromley, Laboratory Research Supervisor; Matthew Chung, John Matic, PostDoc, and Benjamin C. Sparklin.
Eric S. Tvedte is currently an NCBI affiliated with the National Institutes of Health, Bethesda, MD; Mark Gasser is currently associated with the Applied Physics Laboratory, Johns Hopkins University, Laurel, MD; Matthew Chung is currently a member of the National Institute of Allergy and Infectious Diseases at the National Institutes of Health, Bethesda, MD; and Benjamin C. Sparkin is currently affiliated with AstraZeneca, Rockville, MD.
This work was supported by National Institute of Allergy and Infectious Diseases grant U19AI110820 and National Institutes of Health grant R01CA206188.