This ancient species has existed for 10 million years without sexual reproduction

Oribatid mites, minuscule soil-dwelling creatures, have been decomposing organic matter for millions of years. With over 10,000 known species, they were among the earliest arthropods to colonize land, appearing as far back as the Devonian period. Despite their tiny size—ranging from just 150 to 1,400 micrometers—some of these mites have defied fundamental rules of evolution.

A striking feature of oribatid mites is their ability to thrive without sexual reproduction. Roughly 10% of species reproduce parthenogenetically, meaning females generate offspring without males. Unlike most asexual organisms, which struggle to maintain genetic diversity, these mites have successfully diversified across multiple evolutionary lineages.

This raises a perplexing question: How do they sustain genetic variation and adapt over time without the mixing of genes that sexual reproduction provides? Evolutionary theory predicts that asexual lineages should eventually accumulate harmful mutations and decline. Yet, oribatid mites appear to defy this expectation.

One species, Platynothrus peltifer, exemplifies this evolutionary anomaly. These mites have likely been asexual for tens of millions of years, dating back to before Europe and North America separated. Instead of traditional sexual reproduction, they rely on a process called automictic thelytoky, which produces only female offspring.

This form of reproduction limits genetic recombination to the ends of chromosomes. While this might seem restrictive, it has an unexpected advantage: It preserves genetic heterozygosity, effectively allowing the species to maintain diversity without mating. In laboratory studies, males of this species are rare, and when they do appear, they are infertile. The persistence of such a reproductive system challenges conventional evolutionary models.

To understand how Platynothrus peltifer has survived for so long without sexual reproduction, researchers at the University of Cologne collaborated with international teams to analyze its genetic makeup. Using advanced genome sequencing, they created a phased reference genome, allowing them to study the species' evolutionary history in unprecedented detail.

Their research, published in Science Advances, revealed a surprising genetic phenomenon known as the Meselson effect. This occurs when an asexual organism's two chromosome copies evolve independently, accumulating genetic differences over time. Rather than suffering from genetic stagnation, these mites seem to maintain diversity through this process.

For comparison, humans also have two sets of chromosomes, but they undergo recombination during sexual reproduction. Oribatid mites, however, pass down genetic material differently. By studying variations between their chromosome copies, scientists uncovered mechanisms that may explain how these creatures continue to adapt despite their unconventional reproductive strategy.

These differences in genetic expression, or the activity of specific genes, allow the mites to adapt rapidly to changing environmental conditions. This adaptability challenges traditional views of evolution, which emphasize the necessity of sex for long-term survival and genetic diversity.

One surprising contributor to the mites’ genetic diversity is horizontal gene transfer (HGT). This process, common in bacteria, involves the exchange of genetic material between unrelated species, bypassing traditional reproductive methods.

In Platynothrus peltifer, HGT introduces new genetic tools, such as genes that help digest plant cell walls. These new abilities expand the mites’ dietary options, providing a distinct ecological advantage.

Another factor at play is the activity of transposable elements (TE), often referred to as "jumping genes." These genetic sequences can move within the genome, reshuffling genetic material much like reorganizing chapters in a book. Interestingly, the activity of these elements differs between the two chromosome copies.

On one copy, TEs are active, driving dynamic genetic changes. On the other, they remain largely inactive, preserving genetic stability. This balance between change and stability is crucial for the mites' long-term survival.

The researchers also noted differences in the activity levels of specific genes between the chromosome copies. These variations enable the mites to quickly respond to environmental challenges, providing them with a selective advantage.

For example, one chromosome copy may activate genes that confer resistance to pathogens or enable efficient nutrient extraction, while the other remains unaltered, safeguarding essential genetic information.

The findings from this study shed light on how asexual organisms like Platynothrus peltifer defy the odds of extinction. By maintaining genetic diversity through independent chromosome evolution, HGT, and transposable elements, these mites have persisted for over 20 million years. This challenges the traditional view that sex is indispensable for evolutionary success.

Dr. Hüsna Öztoprak, the study’s lead author, highlighted the significance of these mechanisms: “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.”

Such insights not only deepen our understanding of asexual reproduction but also open new avenues for exploring alternative evolutionary strategies in other species.

Moreover, the study provides a framework for future research. Dr. Jens Bast, an Emmy Noether group leader at the University of Cologne, expressed interest in uncovering additional mechanisms that might drive evolution without sex. “In future research projects, we would like to find out whether there are additional mechanisms that might be important for evolution without sex,” he said.

Understanding the unique evolutionary strategies of asexual organisms has far-reaching implications. It can help scientists unravel the adaptive value and constraints of sexual reproduction and offer insights into genetic conservation.

Furthermore, the study of asexual species like Platynothrus peltifer could inform biotechnological applications, such as developing crops with enhanced resistance to pests or environmental stresses.

The persistence of these mites serves as a testament to nature’s ability to adapt and thrive in unexpected ways. By exploring the genomic and evolutionary dynamics of Platynothrus peltifer, researchers not only challenge long-held assumptions about the necessity of sex but also pave the way for new discoveries in evolutionary biology.

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