Evolution without sex: How mites have survived for millions of years

In collaboration with colleagues from international partner institutions, researchers at the University of Cologne have investigated the asexual reproduction of oribatid mites using genome sequencing techniques. They show that the key to evolution without sex in oribatid mites may lie in the independent evolution of their two chromosome copies – a phenomenon known as the ‘Meselson effect’. The research team identified various mechanisms that may contribute to the genetic diversity of the chromosome sets, potentially enabling the long-term persistence of the mite.

Like humans, oribatid mites possess two sets of chromosomes. However, the asexual oribatid mite Platynothrus peltifer reproduces parthenogenetically: Mothers produce daughters from unfertilized eggs, resulting in a population consisting entirely of females. Using single-individual sequencing, the researchers analysed the accumulated differences between the chromosome copies for the first time and evaluated their significance for the mite’s survival. The study titled ‘Chromosome-scale genome dynamics reveal signatures of independent haplotype evolution in the ancient asexual mite Platynothrus peltifer’, funded by the German Research Foundation (DFG), was published in Science Advances.

Sex is the driving force of evolution: It promotes genetic diversity and helps organisms to adapt more quickly to changing environmental conditions. Without sex, however, organisms risk genetic stagnation and extinction – at least according to prevailing evolutionary theory. Yet, the oribatid mite Platynothrus peltifer challenges this paradigm: It has existed for over 20 million years – entirely without sex. The asexual oribatid mites produce their female offspring from unfertilized eggs without males. Males are absent or extremely rare and do not contribute to the gene pool. Depending on the mechanism restoring the diploid set of chromosomes, offspring can inherit either all or some of the mother’s gene variants (alleles). They can therefore be ‘full clones’ of the mother.

In the oribatid mite, the two copies of the chromosome sets evolve independently of each other, allowing new genetic variants to emerge while also retaining important information. The team observed notable differences in gene expression – in other words, which copies of the genes are active and to what extent. These differences enable rapid responses to environmental changes and provide a selective advantage.

Another mechanism contributing to genetic diversity is horizontal gene transfer (HGT), i.e. the movement of genetic material outside of the confined barriers of 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 to digest cell walls, thus expanding its food spectrum,” explained the study’s first author, Dr Hüsna Öztoprak from the University of Cologne’s Institute of Zoology.

Additionally, transposable elements (TE) or ‘jumping genes’, play an important role. TEs move within the genome like chapters in a book that are rearranged to change the course of the plot. The fact that the activity of these TEs differs between the two chromosome copies is particularly exciting. While they are active on one copy and thus can cause dynamic changes, they tend to remain rather inactive on the other.

The study provides new insights into the survival strategies of asexual organisms. Asexual evolution is supported by various sources of genetic diversity, to which the research team draws attention in the study. “In future research projects, we would like to find out whether there are additional mechanisms that might be important for evolution without sex,” said Dr Jens Bast, Emmy Noether group leader at the University of Cologne.

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