For a decade Prof. Cassandra Extavour pitched a project to students to understand whether horizontal gene transfer — the process of passing genes between organisms without sexual reproduction — might be responsible for part of the makeup of a gene, known as oskar, that plays a critical role in the creation of germ cells in some insects. PhD candidate Leo Blondel (MCO, Extavour Lab) rose to the challenge. Blondel, a first-year student in the Molecules, Cells, and Organisms (MCO) program, was able to provide the strongest suggestive evidence yet that at least part of oskar actually came from bacterial genomes. The findings are described in a paper published in the journal eLife.
Oskar appeared suddenly in evolution and in 480 million years became one of the most essential genes in the reproduction of insects. Blondel performed a phylogenetic analysis to find from where the gene came. His results show part of oksar's sequence is closely related to a sequence found in bacteria. Animals and bacteria are thought to not reproduce with each other, so Extavour determined it must occur through horizontal gene transfer. Extavour and Blondel measured codon use in oskar four different ways. “We noticed that many of the sequences that appear to be closely related to the [oskar] domain came from bacteria that are sometimes endosymbionts — bacteria that live inside insects,” Extavour said. “There are many insect endosymbionts that don’t just live inside the animal, they live in the cytoplasm of the cells … and not just any cells, but germ cells." This suggests horizontal gene transfer from bacteria living inside the cell that contains the DNA to the next generation's genome. The Harvard Gazette