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How does evolution move from conventional sexual reproduction to asexuality without males? A new study by researchers from the Universities of Lausanne, Lund and Rostock reveals that this remarkable transition can occur gradually, through intermediate evolutionary steps rather than in a single leap.
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 favour 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.
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, the researchers analyzed genomic data from more than 500 wild stick insects which 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.
An adult female of Bacillus rossius on its favourite food source, bramble. © Bart Zijlstra, UNIL
Everything started with one initial hybridization event after the end of the last ice age, around 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 now 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!
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 like 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.
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.”
Dr. Alexander Brandt
University of Rostock
Institute of Biological Sciences
Email: alexander.brandt@uni-rostock.de
The study has been published in Proceedings of the National Academy of Sciences (PNAS): www.pnas.org/doi/10.1073/pnas.2535700123
An adult female of Bacillus rossius on its favourite food source, bramble.
Copyright: Bart Zijlstra, UNIL
Illustration of the mechanisms of parthenogenesis and hybridogenesis in Bacillus and the discovered ...
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An adult female of Bacillus rossius on its favourite food source, bramble.
Copyright: Bart Zijlstra, UNIL
Illustration of the mechanisms of parthenogenesis and hybridogenesis in Bacillus and the discovered ...
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