Genome Dynamics Reveals the Keys to Long-Term Survival in an Asexual Mite

An international group of researchers at the University of Cologne have explored the asexual reproduction of oribatid mites using genome sequencing methods. Their findings suggest that the key to evolution without sexual reproduction in oribatid mites may lie in the independent evolution of their two chromosome copies, a phenomenon referred to as the “Meselson effect.”

The team identified several mechanisms contributing to the genetic diversity of chromosome sets, potentially enhancing the mite's long-term survival.

Oribatid mites, like humans, have two sets of chromosomes. However, the asexual oribatid mite Platynothrus peltifer reproduces parthenogenetically, meaning females produce offspring from unfertilized eggs, resulting in entirely female populations.

Using single-individual sequencing, researchers explored for the first time how differences accumulate between the mite's chromosome copies and assessed their role in survival. The study, supported by the German Research Foundation (DFG), was published in Science Advances.

Sexual reproduction is traditionally seen as essential for evolution, promoting genetic diversity and helping organisms adapt to environmental changes. Without it, species risk genetic stagnation and eventual extinction—at least according to conventional evolutionary theory.

Yet, Platynothrus peltifer defies this assumption. This species has persisted for over 20 million years without sexual reproduction. Asexual reproduction in these mites leads to female-only populations, with males either entirely absent or so rare that they play no role in the gene pool.

The way offspring inherit genetic material depends on how the diploid chromosome set is restored. Offspring may inherit all or only a subset of the mother’s gene variants (alleles), making them either “full clones” or partial clones of the mother.

In P. peltifer, the two chromosome copies evolve independently, generating new genetic variants while maintaining critical genetic information. The researchers observed significant differences in gene expression—essentially, which genes are active and to what extent. This variability helps the mites adapt to environmental changes, providing a selective advantage.

Horizontal gene transfer (HGT) is another contributor to genetic diversity. HGT involves acquiring genetic material from external sources, bypassing traditional sexual reproduction.

“Horizontal gene transfer can be thought of as adding new tools to an existing toolbox. Some of these genes seem to help the mite digest cell walls, thus expanding its food spectrum,” explained Dr. Hüsna Öztoprak, the study’s first author from the University of Cologne.

Transposable elements (TEs), or "jumping genes," also play a key role. These genetic segments can move within the genome, akin to rearranging chapters in a book to create a new storyline. Interestingly, the researchers found that TEs behave differently across the mite's two chromosome copies. On one copy, they are highly active, driving genetic changes, while on the other, they are relatively dormant.

This study highlights how asexual organisms like P. peltifer maintain genetic diversity and adapt over time. It provides insights into the evolutionary strategies that sustain them in the absence of sexual reproduction.

“In future research projects, we aim to uncover additional mechanisms that could be crucial for evolution without sex,” said Dr. Jens Bast, Emmy Noether Group Leader at the University of Cologne.