An international research team has discovered various mechanisms that generate genetic diversity in asexual mites, ensuring their survival. This study was published in Science Advances.
Researchers from the University of Cologne, in collaboration with international partner institutions, investigated asexual reproduction in horn mites using advanced genome sequencing techniques. They found that the key to evolution without sex in horn mites lies in the independent development of their two chromosome copies, a phenomenon known as the 'Meselson effect.' The team identified several mechanisms contributing to genetic diversity in chromosome sets, thereby ensuring the evolution of the mite.
Like humans, horn mites possess a diploid chromosome set. However, unlike humans, the asexual horn mite Platynothrus peltifer reproduces parthenogenetically: mothers produce daughters from unfertilized eggs, resulting in an all-female society. Through genomic analyses of individual mites, researchers were able to examine the accumulated differences between chromosome copies and analyze their significance for the mite's survival. The results of this study, funded by the German Research Foundation (DFG), were published under the title 'Chromosome-scale genome dynamics reveal signatures of independent haplotype evolution in the ancient asexual mite Platynothrus peltifer' in the journal Science Advances.
Sex is a central driver of evolution, providing genetic diversity and helping organisms adapt to changing environmental conditions. In contrast, the absence of sex can lead to genetic stagnation and extinction, according to prevailing evolutionary theory. However, Platynothrus peltifer contradicts these rules, having existed for over 20 million years entirely without sex. Asexual horn mites produce their female offspring from unfertilized eggs with no contribution from males. Depending on the mechanism restoring the diploid chromosome set, offspring can inherit all or part of the mother's genetic variants (alleles), potentially becoming 'complete clones' of the mother.
In horn mites, the two copies of the chromosome set develop independently, creating an experimental space for new genetic variants to emerge while preserving crucial information. A particularly notable difference is observed in gene expression, indicating which gene copies are more active. These differences facilitate rapid responses to environmental changes, providing a selective advantage.
Additionally, Horizontal Gene Transfer (HGT) plays a role, involving the transfer or acquisition of genetic material outside of sexual reproduction. 'Horizontal gene transfer, where genes can even be transferred from distantly related organisms, functions like adding new tools to an existing toolbox. Some of these genes seem to help the mite digest cell walls, thereby expanding its dietary range,' explains the study's first author, Dr. Hüsna Öztoprak from the Institute of Zoology at the University of Cologne.
Transposable elements (TEs), also known as 'jumping genes,' are significant in this context. TEs move within the genome like chapters being inserted into a new story, potentially altering the narrative. Notably, the activity of these TEs differs between the two chromosome copies. While they are active on one copy and can induce dynamic changes, they remain largely inactive on the other.
This study provides new insights into the survival strategies of asexual organisms. The asexual evolution is supported by various sources of genetic diversity, which the research team highlights. 'In future research projects, we aim to discover whether there are additional mechanisms significant for evolution without sex,' states Dr. Jens Bast, Emmy Noether group leader at the University of Cologne.