An international team of researchers has uncovered several methods in asexual mites that promote genetic diversity, which is essential for their survival.
Researchers from the University of Cologne, in collaboration with partners from various global institutions, have explored the asexual reproduction of oribatid mites through genome sequencing techniques. Their findings suggest that the secret to evolution occurring without sexual reproduction in oribatid mites may be attributed to the separate evolution of their chromosome pairs – a process referred to as the ‘Meselson effect’. The team highlighted several mechanisms that could enhance genetic diversity within these chromosome sets, potentially aiding the mites in their long-term survival.
Similar to humans, oribatid mites have two sets of chromosomes. However, the asexual oribatid mite Platynothrus peltifer reproduces through parthenogenesis, allowing mothers to produce daughters from unfertilized eggs, resulting in a population made up entirely of females. For the first time, researchers utilized single-individual sequencing to analyze the genetic variations between the chromosome copies and assess their importance for the survival of the mite. Funded by the German Research Foundation (DFG), this study, titled ‘Chromosome-scale genome dynamics reveal signatures of independent haplotype evolution in the ancient asexual mite Platynothrus peltifer‘, was published in Science Advances.
Sexual reproduction is generally seen as a major driver of evolution, fostering genetic diversity and enabling organisms to adapt swiftly to environmental changes. However, organisms that reproduce asexually face potential genetic stagnation and the threat of extinction, according to traditional evolutionary theories. The case of the oribatid mite Platynothrus peltifer challenges this view, as it has thrived for over 20 million years without sexual reproduction. These asexual mites give rise to female offspring solely from unfertilized eggs, with males being either absent or exceedingly rare, thus contributing little to the genetic diversity. Depending on how the chromosome diploid set is restored, the offspring can inherit all or some of their mother’s genetic variants (alleles), resulting in ‘full clones’ of the mother.
In the oribatid mite, the two chromosome copies evolve independently, leading to the emergence of new genetic variants while maintaining crucial genetic information. The research team noticed significant variations in gene expression, meaning that different copies of genes are active to varying degrees. This genetic interplay allows for rapid adaptations to environmental shifts, offering a competitive edge.
Another source of genetic diversity is horizontal gene transfer (HGT), which involves the exchange of genetic material beyond the limitations of sexual reproduction. “You can think of horizontal gene transfer as adding new instruments to a musician’s toolkit. Some of these genes appear to assist the mite in breaking down cell walls, broadening its dietary possibilities,” commented Dr. Hüsna Öztoprak, the first author of the study from the University of Cologne’s Institute of Zoology.
Moreover, transposable elements (TE), often referred to as ‘jumping genes,’ are crucial in this process. TEs shift within the genome like rearranging chapters in a book, altering the narrative. Notably, the activity of these TEs is different between the two chromosome copies. They may be active on one copy, leading to dynamic changes, while remaining largely inactive on the other.
This study sheds light on the survival mechanisms of asexual organisms. It emphasizes that asexual evolution benefits from multiple avenues of genetic diversity. “In upcoming research projects, we aim to uncover whether there are other mechanisms that could play a significant role in evolution without sexual reproduction,” stated Dr. Jens Bast, the Emmy Noether group leader at the University of Cologne.
