One of the most persistent questions in evolutionary biology is how the genes for new traits arise. One potential process for evolving novel traits is by the incorporation of genetic information from outside sources into an existing genome, a process known as horizontal gene transfer. Horizontal gene transfer occurs widely in bacteria, but very few examples in animals have been described. One class of genes, called transposable elements, can easily move between genes within a cell; perhaps they might also move between organisms. Because the larvae of parasitic wasps develop in very intimate contact with their fruitfly hosts, a pilot study was undertaken to determine whether transposable elements might be able to move between fruitfly and wasp genomes: the PIs found abundant evidence of such horizontal gene transfer. The PIs will conduct experiments designed to assess the direction of transposable element transfer between flies and wasps and to determine the evolutionary fate of these elements in their novel genomes. This project will help to develop the Drosophila-wasp interaction as a model system for the study of horizontal gene transfer between higher organisms. It will reveal characteristics affecting the ability of transposable elements to invade novel genomes.
The seemingly bizarre life histories of these organisms pique student interest and facilitate teaching fundamental biological concepts. This study's findings will be used to further expand the system's uses in both classrooms and research labs. Co-PI Milan will continue to use this project as an avenue to engage in discussions about research science and its value with diverse, lay audiences that otherwise may not be exposed to evolutionary biology and genetics.
In eukaryotes, genomes are thought to be inherited relatively faithfully from parents to offspring, unlike prokaryotes which frequently obtain new genomic information from their environments. However, a lack of genome sequences from closely interacting eukaryotes presumably limits our ability to detect eukaryote gene swapping. Some of the most intimate ecological relationships in nature occur between arthropod hosts and endoparasitoid wasps, which lay their eggs within the bodies of their hosts where they can potentially be transformed by surrounding host tissues. Given that several wasp species use the model fruitfly Drosophila melanogaster as a primary host in nature, we hypothesized that D. melanogaster genes might horizontally transfer into these wasp genomes. Using the canonical transposable element sequences from the D. melanogaster genome, we found evidence of up to 63 cases of horizontal transfer in D. melanogaster specialists, involving 35 of the 43 transposable element families tested, but virtually no evidence of horizontal transfer using other Drosophila endoparasitoids. The disjunct distribution of horizontally transferred transposable elements across wasp lineages confirms the direction of transfer is from flies to wasps. Thus, endoparasitoid wasps regularly incorporate genetic information from their hosts into their genomes, despite being more than 300 million years diverged. Our findings demonstrate that the content and evolutionary potential of eukaryotic genomes can be strongly influenced by the closely interacting eukaryotic species around them. This project helped develop a novel model system for the study of horizontal gene transfer between eukaryotes and for characteristics affecting the ability of TEs to invade novel genomes. We provided our information on the sequences, frequencies and distributions of fly-derived TEs in parasitic wasp genomes to publicly accessible databases, and a manuscript on our data is currently in review. We also used this system as a model for the education and training of undergraduate and high school students in the Atlanta area, through lectures and hands-on laboratories to illustrate basic biological and evolutionary concepts. The uniqueness of these organisms and their seemingly bizarre life histories made them easily accessible to lay audiences, allowing numerous opportunities to discuss basic research science and its applied value.
