From hybrids to 'virgin birth,' stick insects reveal stepwise loss of sex

From hybrids to 'virgin birth,' stick insects reveal stepwise loss of sex Lisa Lock Scientific Editor Robert Egan Associate Editor The evolution of sex remains one of biology's greatest puzzles. While sexual reproduction dominates across the animal kingdom, scientists still debate why it persists despite its high costs. Even more mysterious is the loss of sex in favor of asexual reproduction whereby females give birth to copies of themselves without any contribution from males. Nearly all animals carry two copies of the genome, one from the mother and one from the father. In sexual reproduction the two genomes are mixed up and only one copy is transmitted, which contains both maternal and paternal sequences. In parthenogenesis, also called "virgin birth," a form of asexual reproduction common among animals, both copies can be clonally inherited from mother to offspring—without any mixing or other changes involved. Stick insects as a model system Because evolutionary transitions between sex and parthenogenesis are rarely observed, understanding how sex is lost is very challenging. To investigate this, the researchers turned to one of nature's most extraordinary masters of disguise: stick insects. They are known for their ability to mimic twigs and leaves with astonishing precision, thereby blending seamlessly into their surroundings in order to avoid hungry predators. Yet beyond their spectacular camouflage lies another, far less visible form of diversity—the many different ways they reproduce. This reproductive diversity is especially striking among Mediterranean stick insects of the genus Bacillus where even very closely related species utilize different ways to produce offspring. To uncover their history of reproduction, researchers from the Universities of Lausanne, Lund and Rostock analyzed genomic data from more than 500 wild stick insects that they collected over several years of field work across Sicily, mainland Italy and France. In the end, their work paid off and they disentangled a surprisingly complex sequence of evolutionary transitions. The paper is published in the Proceedings of the National Academy of Sciences. A hybrid origin to the story Everything started with one initial hybridization event after the end of the last ice age, about 8,000 years ago. Hybridization describes the process where two species mate and produce offspring with one genome copy inherited from the maternal and the other from the paternal species. In many cases the offspring is not fertile, like the mule, the offspring of a female horse and a male donkey. However, in Bacillus the hybrids' reproduction was "rescued"—so it seems—by a switch to a very rare and unusual reproductive mode called hybridogenesis. In hybridogenesis only one of the two parental genomes (here the maternal one) is clonally inherited. The paternal genome is somehow recognized and removed from the developing eggs every generation. It is then reintroduced—thereby recovering the hybrid state—by mating with males of the paternal species whose sperm is in a way "parasitized on" by the hybridogenetic lineage. Many generations later, hybridogenesis gave rise to parthenogenesis. From then on, both parental genomes were inherited clonally. This was even topped: After some time, the parthenogenetic lineage added a third genome by mating with another parental species, forming parthenogenetic lineages combining three different genomes. Recreating evolution in the laboratory The team then recreated several of these transitions experimentally in laboratory crosses, showing that the inferred evolutionary pathway is biologically plausible. A remarkable effort, given that Bacillus stick insects have a generation time of one year and the experimental lab crosses took several years to complete. "What makes this system so fascinating is that we were able to reconstruct every single transition in the lab which we inferred from the evolutionary histories of the wild stick insects," says co–first author Guillaume Lavanchy of Lund University, Sweden. "Each evolutionary transition appears to have prepared the ground in a way that enabled the next." People often think of asexual reproduction as an evolutionary dead end because there are many disadvantages associated with it, such as the accumulation of deleterious mutations and the reduced potential for adaptation. Instead, the new results suggest that hybridization and the loss of sex can create opportunities for further innovation, possibly helping asexuals persist despite the negative consequences without sex. Open questions and future directions Tanja Schwander, last author and professor at the Department of Ecology and Evolution at the University of Lausanne, Switzerland, where the study was conducted, concludes, "The stick insects impressively demonstrate how a single genomic perturbation in the form of a hybridization event and the loss of sex can act as a catalyst to fuel a whole series of evolutionary innovations." Alexander Brandt, corresponding author from the University of Rostock, Germany, adds, "The proximate, molecular causes underlying the different transitions, however, still remain elusive. For us researchers, the work elucidating the evolution of asexuality has just begun." Publication details Alexander Brandt et al, Hybridogenesis as an intermediate step between sexual reproduction and parthenogenesis in stick insects, Proceedings of the National Academy of Sciences (2026). DOI: 10.1073/pnas.2535700123 Journal information: Proceedings of the National Academy of Sciences Provided by University of Rostock